The roles of marginal lagoons in the maintenance of genetic diversity in the Brazilian migratory fishes Prochilodus argenteus and P. costatus

Bruno F. Melo Yoshimi Sato Fausto Foresti Claudio Oliveira About the authors

Abstracts

The rio São Francisco basin contains many endemic species, such as Prochilodus argenteus and P. costatus, which have great commercial importance. However, information about the main recruitment sites and genetic studies containing extensive sampling of these species are scarce. To investigate the roles of the marginal lagoons in the maintenance of genetic variability and in the population structure, we analyzed six microsatellite loci in nine sampling groups of P. argenteus and five sampling groups of P. costatus. Our results showed high levels of genetic variability and low values of genetic differentiation for P. argenteus (FST = 0.008, P < 0.05) and for P. costatus (FST = 0.031, P < 0.05). In addition, high values of gene flow combined with a small genetic distance suggest the presence of a single population for each species in the middle rio São Francisco basin. Moreover, putative migration routes involving marginal lagoons during the reproductive season could be detected, confirming the importance of these nurseries in the lifecycle of these species. Our results also indicate the necessity of adequate management of the fish resources and the conservation of the floodplains in the rio São Francisco basin.

Conservation genetics; Fish migration; Microsatellite; Nursery area; Rio São Francisco basin


A bacia do rio São Francisco contém muitas espécies endêmicas, tais como Prochilodus argenteus e P. costatus, os quais têm grande importância comercial. Entretanto, informações sobre as principais áreas de recrutamento e estudos genéticos contendo uma extensa amostragem dessas espécies no rio São Francisco são escassas. Para investigar o papel das lagoas marginais na manutenção da variabilidade genética e na estruturação populacional dessas espécies, nós analisamos seis loci microssatélites em nove grupos amostrais de P. argenteus e cinco grupos amostrais de P. costatus. Nossos resultados revelaram altos níveis de variabilidade genética para ambas as espécies e valores baixos de diferenciação genética para P. argenteus (FST = 0.008, P < 0.05) e P. costatus (FST = 0.031, P < 0.05). Adicionalmente, valores altos de fluxo gênico combinados com a distância genética baixa sugerem a presença de uma única população para cada espécie no médio rio São Francisco. Possíveis rotas migratórias envolvendo lagoas marginais durante o período reprodutivo puderam ser detectadas, confirmando a importância das lagoas marginais no ciclo de vida dessas espécies. Nossos resultados também indicaram a necessidade de um manejo adequado dos recursos pesqueiros e a conservação das várzeas na bacia do rio São Francisco.


Introduction

The rio São Francisco basin constitutes an area that covers 619,543 km², ca. 7.5% of Brazil, with ecological domains ranging from Atlantic rainforest to Cerrado and Caatinga. In recent years, human impact, including the destruction of wetlands and marginal lagoons for agriculture and the construction of large dams, such as the Três Marias dam (TMD), for hydroelectric power generation, has affected the basin (Sato et al., 1987Sato, Y., E. L. Cardoso & J. C. C. Amorim. 1987. Peixes das lagoas marginais do rio São Francisco a montante da represa de Três Marias, Minas Gerais. Brasília, CODEVASF.; Menezes, 1996Menezes, N. A. 1996. Methods for assessing freshwaters fish diversity. Pp. 289-295. In: Bicudo, C. E. M. & N. A. Menezes (Eds.). Biodiversity in Brazil: a first approach. São Paulo, CNPq.). The central portion of the rio São Francisco basin, including Três Marias region, has greater investment in the fishery (Camargo & Petrere, 2001Camargo, S. A. F. & M. Petrere. 2001. Social and financial aspects of the artisanal fisheries of Middle São Francisco River, Minas Gerais, Brazil. Fisheries Management and Ecology, 8: 163-171.) and it is characterized by the presence of many floodplains and marginal lagoons. These marginal lagoons are important nursery habitats for migratory species recruitment because they provide an ideal habitat for the growth of juveniles with abundant food and relatively high temperatures (Moojen, 1940Moojen, J. 1940. Aspectos ecológicos do alto São Francisco: o pescador. O Campo, 11: 22-24.; Pompeu & Godinho, 2003Pompeu, P. S. & H. P. Godinho. 2003. Ictiofauna de três lagoas marginais do médio São Francisco. Pp. 167-181. In: Godinho, H. P. & A. L. Godinho(Eds.). Águas, peixes e pescadores do São Francisco das Minas Gerais. Belo Horizonte, PUC-Minas.). Unfortunately, important aspects of the fish migrations in the rio São Francisco, such as feeding and reproductive habitats, shoal composition and structure, homing and distances traveled between habitats, remain poorly studied (Sato & Godinho, 2003Sato, Y. & H. P. Godinho. 2003. Migratory fishes of the São Francisco river. Pp. 195-232. In: Carolsfeld, J., B. Harvey, C. Ross & A. Baer (Eds.). Migratory fishes of South America: biology, fisheries and ecological status. Victoria, World Fisheries Trust.).

Prochilodontids occur abundantly in the major drainages of South America and are considered one of the most important components of the commercial and subsistence fisheries in Neotropical freshwater environments (Lowe-McConnell, 1975Lowe-McConell, R. 1975. Fish communities in tropical freshwaters. New York, Longman Publishing.; Goulding, 1981Goulding, M. 1981. Man and fisheries on an Amazon frontier. The Hague, Dr. W. Junk Publishers.; Vari, 1983Vari, R. P. 1983. Phylogenetic relationships of the families Curimatidae, Prochilodontidae, Anostomidae and Chilodontidae (Pisces, Characiformes). Washington, Smithsonian Books.; Castro & Vari, 2004Castro, R. M. C. & R. P. Vari. 2004. Detritivores of the South American Fish Family Prochilodontidae (Teleostei: Ostariophysi: Characiformes): A Phylogenetic and revisionary study. Washington, Smithsonian Books.). The family Prochilodontidae comprises three genera (Ichthyoelephas, Prochilodus, and Semaprochilodus) with highly restructured lips, teeth and jaws; these characteristics distinguish this family externally from all other characiforms (Castro & Vari, 2004Castro, R. M. C. & R. P. Vari. 2004. Detritivores of the South American Fish Family Prochilodontidae (Teleostei: Ostariophysi: Characiformes): A Phylogenetic and revisionary study. Washington, Smithsonian Books.). Prochilodus argenteus, an endemic species from the rio São Francisco basin, nowadays introduced in other Neotropical drainages (Castro & Vari, 2004Castro, R. M. C. & R. P. Vari. 2004. Detritivores of the South American Fish Family Prochilodontidae (Teleostei: Ostariophysi: Characiformes): A Phylogenetic and revisionary study. Washington, Smithsonian Books.), is the largest member of the Prochilodontidae family and is among the most important recreational and commercial fish species in the basin (Camargo & Petrere, 2001Camargo, S. A. F. & M. Petrere. 2001. Social and financial aspects of the artisanal fisheries of Middle São Francisco River, Minas Gerais, Brazil. Fisheries Management and Ecology, 8: 163-171.; Godinho et al., 2003Godinho, A. L., M. F. G. Brito & H. P. Godinho. 2003. Pesca nas corredeiras de Buritizeiro: da ilegalidade à gestão participativa. Pp. 347-360. In: Godinho, H. P. & A. L. Godinho (Eds.). Águas, peixes e pescadores do São Francisco das Minas Gerais. Belo Horizonte, PUC-Minas.). Another endemic species, P. costatus, has an importance in the subsistence fishery, as one of the most captured species in that region (Camargo & Petrere, 2001Camargo, S. A. F. & M. Petrere. 2001. Social and financial aspects of the artisanal fisheries of Middle São Francisco River, Minas Gerais, Brazil. Fisheries Management and Ecology, 8: 163-171.).

Genetic studies using variable markers such as microsatellites and mitochondrial DNA have been conducted to understand the population structure and dynamics of a considerable number of animal species (Zhang & Hewitt, 2003Zhang, D. X. & G. M. Hewitt. 2003. Nuclear DNA analyses in genetic studies of populations: practice, problems and prospects. Molecular Ecology, 12: 563-584.). Microsatellites, or simple sequence repeats (SSRs), are codominant nuclear markers with Mendelian inheritance. Microsatellites are abundantly distributed along genomes and demonstrate high levels of allelic polymorphism (DeWoody & Avise, 2000DeWoody, J. A. & J. C. Avise. 2000. Microsatellite variation in marine, freshwater and anadromous fishes compared with other animals. Journal of Fish Biology, 56: 461-473.). The molecular structure and genetic variability of microsatellites are extensively exploited in evolutionary studies of a wide variety of fish species (see Chistiakov et al., 2006Chistiakov, D. A., B. Hellemans & F. A. M. Volckaert. 2006. Microsatellites and their genomic distribution, evolution, function and applications: A review with special reference to fish genetics. Aquaculture, 255: 1-29.), including some from Neotropical freshwater (Hrbek et al., 2007Hrbek, T., M. Crossa & I. P. Farias. 2007. Conservation strategies for Arapaima gigas (Schinz, 1822) and the Amazonian várzea ecosystem. Brazilian Journal of Biology, 67: 909-917.; Abreu et al., 2009Abreu, M. M., L. H. G. Pereira, V. B. Vila, F. Foresti & C. Oliveira. 2009. Genetic variability of two populations of Pseudoplatystoma reticulatum from the Upper Paraguay River Basin. Genetics and Molecular Biology, 32: 868-873.; Calcagnotto & DeSalle, 2009Calcagnotto, D. & R. DeSalle. 2009. Population genetic structuring in pacu (Piaractus mesopotamicus) across the Paraná-Paraguay basin: evidence from microsatellites. Neotropical Ichthyology, 7: 607-616.; Matsumoto & Hilsdorf, 2009Matsumoto, C. K. & A. W. S. Hilsdorf. 2009. Microsatellite variation and population genetic structure of a neotropical endangered Bryconinae species Brycon insignis Steindachner, 1877: implications for its conservation and sustainable management. Neotropical Ichthyology, 7: 395-402.; Pereira et al., 2009Pereira, L. H. G., F. Foresti & C. Oliveira. 2009. Genetic structure of the migratory catfish Pseudoplatystoma corruscans (Siluriformes: Pimelodidae) suggest homing behavior. Ecology of Freshwater Fish, 18: 215-225.; Sanches et al., 2012Sanches, A., P. M. Galetti, F. Galzerani, J. Derazo, B. Cutilak-Bianchi & T. Hatanaka. 2012. Genetic population structure of two migratory freshwater fish species (Brycon orthotaenia and Prochilodus argenteus) from the São Francisco River in Brazil and its significance for conservation. Latin American Journal of Aquatic Research, 40: 177-186.).

Population genetic studies have been performed with both Prochilodus species from the rio São Francisco basin using Random Amplified Polymorphic DNA (RAPD) technique (Hatanaka & Galetti, 2003Hatanaka, T. & P. M. Galetti. 2003. RAPD markers indicate the occurrence of structured populations in a migratory freshwater fish species. Genetics and Molecular Biology, 26: 19-25.) and microsatellites (Hatanaka et al., 2006Hatanaka, T., F. Henrique-Silva & P. M. Galetti. 2006. Population substructuring in a migratory freshwater fish Prochilodus argenteus (Characiformes, Prochilodontidae) from the São Francisco River. Genetica, 126: 153-159.; Carvalho-Costa et al. 2008Carvalho-Costa, L. F., T. Hatanaka & P. M. Galetti. 2008. Evidence of lack of population substructuring in the Brazilian freshwater fish Prochilodus costatus. Genetics and Molecular Biology, 31: 377-380.; Sanches et al., 2012Sanches, A., P. M. Galetti, F. Galzerani, J. Derazo, B. Cutilak-Bianchi & T. Hatanaka. 2012. Genetic population structure of two migratory freshwater fish species (Brycon orthotaenia and Prochilodus argenteus) from the São Francisco River in Brazil and its significance for conservation. Latin American Journal of Aquatic Research, 40: 177-186.; Barroca et al., 2012Barroca, T. M., F. P. Arantes, B. F. Magalhães, F. F. Siqueira, C. C. R. Horta, I. F. Pena, J. A. Dergam & E. Kalapothakis. 2012. Genetic diversity and population structure of Prochilodus costatus and Prochilodus argenteus preceding dam construction in the Paraopeba River, São Francisco River Basin, Minas Gerais, Brazil. Open Journal of Genetics, 2: 121-130.). However, there is no knowledge about the main nursery areas of the populations of either species or the importance of the marginal lagoons in the maintenance of the genetic variability of these or other migratory species. Considering the threatened marginal regions of the middle rio São Francisco basin and the ecological and economic importance of these species, the goals of this study were to access the main recruitment sites of both species and provide a robust dataset regarding the population genetic structure of these species.

Material and Methods

Sampling sites

Migratory fishes reproduce in the mainstream or tributaries of the rio São Francisco basin, and their eggs and larvae are carried out downstream reaching marginal lagoons (Moojen, 1940Moojen, J. 1940. Aspectos ecológicos do alto São Francisco: o pescador. O Campo, 11: 22-24.) that stay unconnected of the mainstream in the dry season. Thus, samples collected in marginal lagoons were originated from spawning sites located upstream to the lagoons (e.g., samples collected in marginal lagoons from rio Paracatu was derived from eggs spawned only in upstream of the rio Paracatu). Thus, we consider samples from different lagoons of the same tributary as a single sampling group (Fig. 1, Table 1). We divided samples from the mainstream of rio São Francisco in the Três Marias region in three sampling groups to test previous ecological and genetic hypotheses that suggested a population differentiation among them (Godinho & Kynard, 2006Godinho, A. L. & B. Kynard. 2006. Migration and spawning of radio-tagged zulega Prochilodus argenteus in a dammed Brazilian river. Transactions of the American Fisheries Society, 135: 811-824.; Hatanaka et al., 2006Hatanaka, T., F. Henrique-Silva & P. M. Galetti. 2006. Population substructuring in a migratory freshwater fish Prochilodus argenteus (Characiformes, Prochilodontidae) from the São Francisco River. Genetica, 126: 153-159.). Additionally, individuals from Três Marias dam were subdivided in two different units (dry and rainy seasons) trying to check the presence of different migrant shoals inhabiting the same spawning site.

Fig. 1
Map showing the central portion of the rio São Francisco basin and the distribution of the samples of Prochilodus argenteus (yellow) and of P. costatus (black). The circles represent marginal lagoons from tributaries, squares represent marginal lagoons from the rio São Francisco, and triangles represent places in the mainstream rio São Francisco in the Três Marias region. ABA = rio Abaeté; CAR = rio Carinhanha lagoons; JEQ = rio Jequitaí lagoons; PAR = rio Paracatu lagoons; SFR = rio São Francisco lagoons; URU = rio Urucuia lagoons; VEL = rio das Velhas lagoons; TMD = Três Marias Dam.

Table 1
Samples of Prochilodus argenteus and P. costatus from the rio São Francisco basin collected between July/2008 and July/2010. Samples from TMDr and ABAr were collected during the rainy season (November to February), and samples from TMDd were collected during the dry season (March to October). ABAr = rio Abaeté at rainy season; CAR = rio Carinhanha lagoons; JEQ = rio Jequitaí lagoons; PAR = rio Paracatu lagoons; SFR = rio São Francisco lagoons; URU = rio Urucuia lagoons; VEL = rio das Velhas lagoons; TMDd = Três Marias Dam at dry season; TMDr = Três Marias Dam at rainy season.

A total of 273 individuals, that were subdivided into nine sampling groups of Prochilodus argenteus, were collected between July 2008 and February 2010 along the middle rio São Francisco basin. Five sampling groups were from the marginal lagoons of their tributaries, rio Carinhanha (CAR, n = 25), rio Jequitaí (JEQ, n = 33), rio Paracatu (PAR, n = 33), rio Urucuia (URU, n = 33) and rio das Velhas (VEL, n = 28); one sampling group was from the marginal lagoons of the rio São Francisco (SFR, n = 33); two sampling groups were collected downstream of the Três Marias dam, one during rainy season (TMDr, n = 22) and one during dry season (TMDd, n = 33); and one sampling group was collected downstream rio Abaeté during the rainy season (ABAr, n = 33) (Fig. 1, Table 1). In reference to P. costatus, a total of 156 individuals subdivided into five sampling groups were collected along the middle rio São Francisco basin. One sampling group was collected in the marginal lagoons of rio Paracatu (PAR, n = 33); one sampling group was from marginal lagoons of the rio São Francisco basin (SFR, n = 33); two sampling groups were collected downstream of the Três Marias dam, one group was obtained during the rainy season (TMDr, n = 26) and one was obtained during the dry season (TMDd, n = 33); and one sampling group was from downstream rio Abaeté during the rainy season (ABAr, n = 31) (Fig. 1, Table 1). All of the fish used for this study were collected in accordance with Brazilian laws under a permanent scientific collection license. Fin clips or muscles were fixed and preserved in 95% ethanol and deposited in the Laboratório de Biologia e Genética de Peixes, Universidade Estadual Paulista "Júlio de Mesquita Filho", câmpus de Botucatu, São Paulo, Brazil.

Molecular analyses. Genomic DNA was extracted using a saline solution with proteinase K described by Aljanabi & Martinez (1997)Aljanabi, S. M. & I. Martinez. 1997. Universal and rapid salt-extraction of high quality genomic DNA for PCR-based techniques. Nucleic Acids Research, 25: 4692-4693.. Amplifications were carried out using six microsatellite loci developed by Barbosa et al. (2008)Barbosa, A. C. D. R., F. Galzerani, T. C. Corrêa, P. M. Galetti & T. Hatanaka. 2008. Description of novel microsatellite loci in the Neotropical fish Prochilodus argenteus and cross-amplification in P. costatus and P. lineatus. Genetics and Molecular Biology, 31: 357-360. for Prochilodus argenteus: Par66, Par69, Par71, Par76, Par83 and Par85; the same loci were used for P. costatus, exception the locus Par80 was used instead of Par76 (monomorphic for this species). The reactions were performed in a total volume of 12.5 °l with 7.3 °l of ultrapure H2O, 1.25 °l of Taq DNA buffer (10X), 0.45 °l of MgCl2 (50 mM), 0.4 °l of dNTP (2 mM), 0.5 °l of each primer (10 °M), 0.1 °l of Platinum Taq DNA Polymerase enzyme (Invitrogen; www.invitrogen.com) (5 U/°l) and 2.0 °l of genomic DNA (10-50 ng). Polymerase chain reactions (PCR) consisted of an initial denaturation step (5 min at 95ºC) followed by 35 cycles of chain denaturation (30 s at 95ºC), primer hybridization (30 s at 52-54ºC) and nucleotide extension (30 s at 72ºC). After these cycles, a final extension was performed at 72ºC for 10 minutes. Microsatellite amplified fragments were subjected to electrophoresis on 6% polyacrylamide gels for approximately 12 hours at 150 v. The gels were then stained with silver nitrate. The allele lengths were identified by reference to a 10 bp ladder (Invitrogen) using Kodak Digital Science 1D software.

Population genetic analyses

Allele numbers, private allele counts and gene flow (Nm = 0.25(1 - FST )/ FST ) were obtained with Popgene 1.32 (Yeh & Boyle, 1997Yeh, F. C & T. J. B. Boyle. 1997. Population genetics analysis of co-dominant and dominant markers and quantitative traits. Belgian Journal of Botany, 129: 157.). To construct and export the matrices, we used GenAlex 6.1 (Peakall & Smouse, 2006Peakall, R. & P. E. Smouse. 2006. GENALEX 6: genetic analysis in Excel. Population genetic software for teaching and research. Molecular Ecology Notes, 6: 288-295.). The expected and observed heterozygosity (He, Ho), inbreeding coefficient FIS (Wright, 1951Wright, S. 1951. The genetical structure of populations. Annals of Eugenics, 15: 323-354.) were obtained with Arlequin 3.1 (Excoffier et al., 2005Excoffier, L., G. Laval & S. Schneider. 2005. Arlequin (version 3.0): An integrated software package for population genetics data analysis. Evolutionary Bioinformatics Online, 1: 47-50.). The software Microchecker 2.2.1 (van Oosterhout et al., 2004van Oosterhout, C., W. F. Hutchinson, D. P. M. Wills & P. Shipley. 2004. MICRO-CHECKER: software for identifying and correcting genotyping errors in microsatellite data. Molecular Ecology Notes, 4: 535-538.) using the equation 2 of Brookfield (1996)Brookfield, J. F.Y. 1996. A simple new method for estimating null allele frequency from heterozygote deficiency. Molecular Ecology, 5: 453-455. was used to infer the most probable cause of HWE departures produced for null alleles, stuttering and large-allele dropout. The exact test of the Hardy-Weinberg Equilibrium (HWE) (P-value < 0.05) were calculated with 1 million generations of Marchov chain of Monte Carlo (MCMC) with 100,000 'burn-ins' using Arlequin 3.1 (Excoffier et al., 2005Excoffier, L., G. Laval & S. Schneider. 2005. Arlequin (version 3.0): An integrated software package for population genetics data analysis. Evolutionary Bioinformatics Online, 1: 47-50.). Significance levels for the HWE, F- and R-statistics tests were adjusted using Bonferroni corrections (Rice, 1989Rice, W. R. 1989. Analyzing tables of statistical tests. Evolution, 43: 223-225.). The linkage disequilibrium was calculated for all loci using Genepop 4.0.1 online (Raymond & Rousset, 1995Raymond, M. & F. Rousset. 1995. Genepop (version 1.2): population genetics software for exact tests and ecumenicism. Journal of Heredity, 86: 248-249.).

The FST (Wright, 1951Wright, S. 1951. The genetical structure of populations. Annals of Eugenics, 15: 323-354.) assuming the infinite allele model (IAM, Kimura & Crow, 1964Kimura, M. & J. F. Crow. 1964. The number of alleles that can maintained in a finite populations. Genetics, 49: 725-738.), the RST (Slatkin, 1995Slatkin, M. 1995. A measure of population subdivision based on microsatellite allele frequencies. Genetics, 139: 457-462.) assuming the stepwise mutation model (SMM, Kimura & Otha, 1978Kimura, M. & T. Ohta. 1978. Stepwise mutation model and distribution of allelic frequencies in a finite populations. Proceedings of the National Academy of Sciences USA, 75: 2868-2872.) and the analysis of molecular variance (AMOVA) (Excoffier et al., 1992Excoffier, L., P. Smouse & J. Quattro. 1992. Analysis of molecular variance inferred from metric distances among DNA haplotypes: Application to human mitochondrial DNA restriction data. Genetics, 131: 479-491.) calculated for all loci and for all populations with 16,000 permutations (Guo & Thomson, 1992Guo, S. W. & E. A. Thomson. 1992. Performing the Exact Test of Hardy-Weinberg proportion for multiple alleles. Biometrics, 48: 361-372.) were performed using the Arlequin 3.1 program (Excoffier et al., 2005Excoffier, L., G. Laval & S. Schneider. 2005. Arlequin (version 3.0): An integrated software package for population genetics data analysis. Evolutionary Bioinformatics Online, 1: 47-50.). Initially, the samplings of Prochilodus argenteus were considered to be a single group for the AMOVA. Then, the samplings were subdivided into two groups: group 1 (adult fishes collected at mainstream of Três Marias region - ABAr, TMDd, and TMDr) and group 2 (juveniles collected inside unconnected marginal lagoons - SFR, VEL, JEQ, PAR, URU, and CAR). Similarly, the populations of P. costatus were first considered to be a single group and then subdivided into two groups: group 1 (adult fishes collected at mainstream of Três Marias region - ABAr, TMDd, and TMDr) and group 2 (juveniles collected inside unconnected marginal lagoons - LSF and PAR). This criterion was adopted to answer where are the main recruitment areas of the fishes found in the mainstream of Três Marias region (the most fishery activity of the São Francisco basin). In addition, rates of chord genetic distance (Cavalli-Sforza & Edwards, 1967Cavalli-Sforza, L. L. & A. W. F. Edwards. 1967. Phylogenetic analysis: models and estimation procedures. American Journal of Human Genetics, 19: 233-257.) corrected for sample size were calculated with Geneclass2 (Piry et al., 2004Piry, S., A. Alapetite, J. M. Cornuet, D. Paetkau, L. Baudouin & A. Estoup. 2004. Geneclass2: a software for genetic assignment and first-generation migrant detection. Journal of Heredity, 95: 536-539.), and these values were used to construct an unrooted neighbor-joining tree (Saitou & Nei, 1987Saitou, N. & M. Nei. 1987. The neighbor-joining method: a new method for reconstructing phylogenetic trees. Molecular Biology and Evolution, 4: 406-425.) using the software Paup 4.0b10 (Swofford, 2003Swofford, D. L. 2003. Paup*: Phylogenetic analysis using parsimony (*and other methods), version 4. Sunderland, Sinauer Associates.).

Results

Prochilodus argenteus

Genotypes of 273 specimens of nine sampling groups were analyzed. All microsatellite loci were highly polymorphic. A total of 99 alleles were detected for all loci in all populations. The number of alleles per locus ranged from three (Par76) to 24 (Par85), with an overall average of 9.6 alleles (Table 2). Nineteen private alleles were found in low frequency (< 0.05) with the allele 239 (Par85 - URU), presenting a frequency of 0.052. Ho and He ranged from 0.379 (Par69) to 0.937 (Par71) and 0.473 (Par69) to 0.951 (Par85), respectively (Table 2). From the 54 pairwise FIS estimates, 41 showed positive values (heterozygote deficiency), and only 13 showed negative values (an excess of heterozygotes). Positive FIS values indicating heterozygote deficiency was predominant in all loci with a positive overall average (FIS = 0.123, P < 0.008) (Table 2). Only six out of 54 HWE tests performed for P. argenteus (6 microsatellite loci in 9 sampling groups) were significant. In all departure occurrences, the significant values of disequilibrium were associated with the presence of null alleles (Table 2). Null alleles were detected in 14 estimates and stutters were verified only in Par85 in the population TMDr. No evidence of linkage disequilibrium was detected in any loci (P > 0.05).

Table 2
Summary of the six microsatellite loci for each analyzed sampling of Prochilodus argenteus. N, number of individuals; A, number of alleles; P, number of private alleles; Ho, observed heterozygosity; He, expected heterozygosity; FIS, inbreeding coefficient; HWE, probability test for deviation from expected Hardy-Weinberg proportions, *, P-value = 0.05 (adjustment Bonferroni correction P ≤ 0,008; K = 6); r, null alleles frequency per loci. ABAr = rio Abaeté at rainy season; CAR = rio Carinhanha lagoons; JEQ = rio Jequitaí lagoons; PAR = rio Paracatu lagoons; LSF = rio São Francisco lagoons; URU = rio Urucuia lagoons; VEL = rio das Velhas lagoons; TMDd = Três Marias Dam at dry season; TMDr = Três Marias Dam at rainy season.

Low indices of FST obtained by means of pairwise values among local populations of Prochilodus argenteus for all loci were significantly detected (FST = 0.008, P < 0.05), with pairwise FST for all loci ranging from -0.054 (ABA - JEQ) to 0.007 (ABAr - TMDd) (Table 3, above diagonal), clearly indicating an absence of population structure. Additionally, the overall RST also showed significant lower values (RST = 0.059, P < 0.05), ranging from -0.460 (ABAr - PAR) to 0.184 (TMDr - PAR) (Table 3, bellow diagonal). When we considered all specimens to be a single group (see Material and Methods), the AMOVA values (Table 4) revealed that only 0.82% of the total genetic variance was due to differences among populations (FST = 0.008, P < 0.05; RST = 0.069, P < 0.05) and 99.18% of the genetic variation within populations. When we considered the putative existence of two groups (see Material and Methods), the hierarchical AMOVA also revealed that 99.13% of the total variance was found within populations (values are shown in the Table 4).

Table 3
Pairwise FST (above diagonal) and pairwise RST (bellow diagonal) values for Prochilodus argenteus. * P < 0.05, after Bonferroni correction.ABAr = rio Abaeté at rainy season; CAR = rio Carinhanha lagoons; JEQ = rio Jequitaí lagoons; PAR = rio Paracatu lagoons; SFR = rio São Francisco lagoons; URU = rio Urucuia lagoons; VEL = rio das Velhas lagoons; TMDd = Três Marias Dam at dry season; TMDr = Três Marias Dam at rainy season.

Table 4
Analysis of Molecular Variance (AMOVA) among sampling groups of Prochilodus argenteus. Structure tested: All samplings as a single group and divided in two groups: group 1 (Mainstream ABAr, TMDd and TMDr) and group 2 (Marginal lagoons of CAR, JEQ, PAR, SFR, URU and VEL).

The gene flow parameter Nm was calculated from the mean FST value. The mean value obtained was Nm = 9.318, indicating an intense gene exchange occurring among all sampling groups. The gene flow values within each group (group 1: ABAr, TMDd, TMDr; and group 2: CAR, JEQ, PAR, SFR, URU, VEL) were Nm = 12.065 and Nm = 10.331, respectively. The pairwise values of chord distance were estimated and ranged from 24.0% (JEQ - PAR) to 35.3% (ABAr - URU) (Table 5), and the dendrogram of genetic distance showed a low genetic differentiation among sampling groups analyzed (Fig. 2).

Fig. 2
Dendrogram representing the chord genetic distance among sampling groups of Prochilodus argenteus. ABAr = rio Abaeté at rainy season; CAR = rio Carinhanha lagoons; JEQ = rio Jequitaí lagoons; PAR = rio Paracatu lagoons; SFR = rio São Francisco lagoons; URU = rio Urucuia lagoons; VEL = rio das Velhas lagoons; TMDd = Três Marias Dam at dry season; TMDr = Três Marias Dam at rainy season.

Table 5
Chord genetic distance among sampling of Prochilodus argenteus. Values in %. ABAr = rio Abaeté at rainy season; CAR = rio Carinhanha lagoons; JEQ = rio Jequitaí lagoons; PAR = rio Paracatu lagoons; SFR = rio São Francisco lagoons; URU = rio Urucuia lagoons; VEL = rio das Velhas lagoons; TMDd = Três Marias Dam at dry season; TMDr = Três Marias Dam at rainy season.

Prochilodus costatus

Five sampling groups with a total of 156 specimens were analyzed. All loci used in the analysis were highly polymorphic with a total of 112 alleles for all loci in all sampling groups. The number of alleles per locus ranged from four (Par69) to 29 (Par85), with an overall average of 12.9 alleles (Table 6). Nineteen private alleles were found at low frequency (d" 0.04). The observed heterozygosity ranged from 0.231 (Par69) to 0.958 (Par71), and the expected heterozygosity ranged from 0.580 (Par69) to 0.960 (Par85). The FIS indexes showed 24 comparisons with positive values (heterozygote deficiency) and six comparisons with negative values (excess of heterozygotes). The deficiency of heterozygotes was predominant in all loci with a positive overall average (FIS = 0.113, P < 0.008) (Table 6). Only seven out of 30 HWE tests performed for P. costatus (6 microsatellite loci in 5 sampling groups) were significant, and of these, six estimates were associated with the presence of null alleles (Table 6). Presence of stutters was verified in the Par71 loci on PAR. No linkage disequilibrium was detected (P > 0.05), and the allelic variation was treated independently.

Table 6
Summary of the six microsatellite loci for each analyzed sampling of Prochilodus costatus. N, number of individuals; A, number of alleles; P, number of private alleles; Ho, observed heterozygosity; He, expected heterozygosity; FIS, inbreeding coefficient; HWE, probability test for deviation from expected Hardy-Weinberg proportions, *, P-value = 0.05 (adjustment Bonferroni correction P ≤ 0.008; K = 6); r, null alleles frequency per loci. ABAr = rio Abaeté at rainy season; PAR = rio Paracatu lagoons; LSF = rio São Francisco lagoons; TMDd = Três Marias Dam at dry season; TMDr = Três Marias Dam at rainy season.

Our results indicate a significant absence of the population structure based on FST = 0.031, P < 0.05 and RST = 0.044, P < 0.05. In addition, pairwise FST values do not indicated structuring in all estimatives, ranging from -0.005 to 0.049. Median RST values were detected between SFR and TMDd (RST = 0.113, P < 0.05) (Table 7). Considering no predefined hierarchical models, the AMOVA showed that 96.82% of the genetic variance was found within populations (FST = 0.031, P < 0.05; RST = 0.044, P < 0.05). When we considered the putative existence of two groups (see Material and Methods) the estimates remain almost unchanged with the most of the variation within the populations (96.66%) with FST = 0.033, P < 0.05 (values are shown in Table 8).

Table 7
Pairwise FST (above diagonal) and pairwise RST (bellow diagonal) values for Prochilodus costatus. * P < 0.05, after Bonferroni correction. ABAr = rio Abaeté at rainy season; PAR = rio Paracatu lagoons; SFR = rio São Francisco lagoons; TMDd = Três Marias Dam at dry season; TMDr = Três Marias Dam at rainy season.

Table 8
Analysis of Molecular Variance (AMOVA) among sampling groups of Prochilodus costatus. Structure tested: All samplings as a single group and divided in two groups: group 1 (Mainstream ABAr, TMDd and TMDr) and group 2 (Marginal lagoons of PAR and SFR).

The mean value of gene flow obtained for Prochilodus costatus was Nm = 5.911, and the values within each group (group 1: ABAr, TMDd, TMDr; and group 2: PAR, SFR) were Nm = 6.698 and Nm = 13.010, respectively, showing a higher number of migrants per generation. The chord distance ranged from 32.6% (TMDr - PAR) to 39.6% (SFR - ABAr) (Table 9). The SFR sample group has more discrepant indexes, which is shown in the dendrogram (Fig. 3).

Fig. 3
Dendrogram representing the chord genetic distance among sampling groups of Prochilodus costatus. ABAr = rio Abaeté at rainy season; PAR = rio Paracatu lagoons; SFR = rio São Francisco lagoons; TMDd = Três Marias Dam at dry season; TMDr = Três Marias Dam at rainy season.

Table 9
Chord genetic distance among sampling of Prochilodus costatus. Values in %. ABAr = rio Abaeté at rainy season; PAR = rio Paracatu lagoons; SFR = rio São Francisco lagoons; TMDd = Três Marias Dam at dry season; TMDr = Três Marias Dam at rainy season.

Discussion

Genetic diversity and lack of genetic structuring

In the present study, we did not find any genetic evidence of the existence of population structure in the species Prochilodus argenteus or P. costatus from the middle rio São Francisco basin. The population differentiation indexes FST and RST , the chord genetic distance associated to a high gene flow and previous studies (Carvalho-Costa et al., 2008Carvalho-Costa, L. F., T. Hatanaka & P. M. Galetti. 2008. Evidence of lack of population substructuring in the Brazilian freshwater fish Prochilodus costatus. Genetics and Molecular Biology, 31: 377-380.; Sanches et al., 2012Sanches, A., P. M. Galetti, F. Galzerani, J. Derazo, B. Cutilak-Bianchi & T. Hatanaka. 2012. Genetic population structure of two migratory freshwater fish species (Brycon orthotaenia and Prochilodus argenteus) from the São Francisco River in Brazil and its significance for conservation. Latin American Journal of Aquatic Research, 40: 177-186.) corroborate the panmictic unit model for both species. The migration behavior, which is common in prochilodontids (Castro & Vari, 2004Castro, R. M. C. & R. P. Vari. 2004. Detritivores of the South American Fish Family Prochilodontidae (Teleostei: Ostariophysi: Characiformes): A Phylogenetic and revisionary study. Washington, Smithsonian Books.) likely plays an important role in the continuous gene flow in these species, keeping them as a single unit in the studied area.

Variability levels in microsatellites with a mean number of 9.6 alleles per locus in Prochilodus argenteus and 12.9 alleles per locus in P. costatus were similar to those reported in freshwater fish with an average of 9.1 alleles per locus (DeWoody & Avise, 2000DeWoody, J. A. & J. C. Avise. 2000. Microsatellite variation in marine, freshwater and anadromous fishes compared with other animals. Journal of Fish Biology, 56: 461-473.) and were similar to or higher than previous studies with both Prochilodus species (Hatanaka et al., 2006Hatanaka, T., F. Henrique-Silva & P. M. Galetti. 2006. Population substructuring in a migratory freshwater fish Prochilodus argenteus (Characiformes, Prochilodontidae) from the São Francisco River. Genetica, 126: 153-159.; Carvalho-Costa et al., 2008Carvalho-Costa, L. F., T. Hatanaka & P. M. Galetti. 2008. Evidence of lack of population substructuring in the Brazilian freshwater fish Prochilodus costatus. Genetics and Molecular Biology, 31: 377-380.; Sanches et al., 2012Sanches, A., P. M. Galetti, F. Galzerani, J. Derazo, B. Cutilak-Bianchi & T. Hatanaka. 2012. Genetic population structure of two migratory freshwater fish species (Brycon orthotaenia and Prochilodus argenteus) from the São Francisco River in Brazil and its significance for conservation. Latin American Journal of Aquatic Research, 40: 177-186.). High genetic diversity was previously identified in P. lineatus (Sivasundar et al., 2001Sivasundar, A., E. Bermingham & G. Ortí. 2001. Population structure and biogeography of migratory freshwater fishes (Prochilodus: Characiformes) in major South American rivers. Molecular Ecology, 10: 407-417.), which can be explained by the capacity of Prochilodus species to perform great migrations and by their large occurrence along the rivers. In the rio São Francisco basin, shoals of P. argenteus are capable of migrating up to 1,100 km upstream to spawn (Pinheiro, 1981Pinheiro, C. V. L. 1981. Relatório de pesca no lago de Sobradinho para o ano de 1980. Juazeiro, SUDEPE.), while in the rio Mogi-Guaçu (upper rio Paraná basin), P. lineatus migrates a maximum round trip distance of 1,300 km (Godoy, 1975Godoy, M. P. 1975. Peixes do Brasil - Sub ordem Characoidei, Bacia do Rio Mogi Guassu. Piracicaba, Ed. Franciscana.; Toledo et al., 1986Toledo, S. A., M. P. Godoy & E. P. Santos. 1986. Curva de migração do curimbatá, Prochilodus scrofa (Pisces, Prochilodontidae) na bacia superior do rio Paraná, Brasil. Revista Brasileira de Biologia, 46: 447-452.).

Significant deviations from the HWE were found in only six microsatellite loci of Prochilodus argenteus. These deviations are common in microsatellite regions (Alam & Islam, 2005Alam, M. S. & M. S. Islam. 2005. Population genetic structure of Catla catla (Hamilton) revealed by microsatellite DNA markers. Aquaculture, 246: 151-160.; Carreras-Carbonell et al., 2006Carreras-Carbonell, J., E. Macpherson & M. Pascual. 2006. Population structure within and between subspecies of the Mediterranean triplefin fish Tripterygion delaisi revealed by highly polymorphic microsatellite loci. Molecular Ecology, 15: 3527-3539.; Chevolot et al., 2006Chevolot, M., J. R. Ellis, G. Hoarau, A. D. Rijnsdorp, W. T. Stam & J. L. Olsen. 2006. Population structure of the thornback ray (Raja clavata L.) in British waters. Journal of Sea Research, 56: 305-316.). Hatanaka et al. (2006)Hatanaka, T., F. Henrique-Silva & P. M. Galetti. 2006. Population substructuring in a migratory freshwater fish Prochilodus argenteus (Characiformes, Prochilodontidae) from the São Francisco River. Genetica, 126: 153-159. found populations with significant deviations from HWE in two of the four analyzed loci. The occurrence of null alleles and stuttering seems to be the most probable cause of these deviations because all of the deviations were associated with the occurrence of null alleles. Additionally, FIS values in P. costatus were positive in 80% of the estimates, also observed previously in the same species (Carvalho-Costa et al., 2008Carvalho-Costa, L. F., T. Hatanaka & P. M. Galetti. 2008. Evidence of lack of population substructuring in the Brazilian freshwater fish Prochilodus costatus. Genetics and Molecular Biology, 31: 377-380.).

However, we found low levels of population genetic differentiation among local populations of Prochilodus argenteus, as indicated by all population analyses, including FST values that were lower (0.008, P < 0.05) than RST values (0.059, P < 0.05). The RST values under the SMM are expected to be greater than the FST values under the IAM (Slatkin, 1995Slatkin, M. 1995. A measure of population subdivision based on microsatellite allele frequencies. Genetics, 139: 457-462.), although the inverse has been observed in some fish species (e.g., Pereira et al., 2009Pereira, L. H. G., F. Foresti & C. Oliveira. 2009. Genetic structure of the migratory catfish Pseudoplatystoma corruscans (Siluriformes: Pimelodidae) suggest homing behavior. Ecology of Freshwater Fish, 18: 215-225.). Low indexes of FST have been observed in other studies with characiforms, such as in Colossoma macropomum (Santos et al., 2007Santos, M. C. F., M. L. Ruffino & I. P. Farias. 2007. High levels of genetic variability and panmixia of the tambaqui Colossoma macropomum (Cuvier, 1816) in the main channel of the Amazon River. Journal of Fish Biology, 71: 33-44.), Piaractus mesopotamicus (Calcagnotto & DeSalle, 2009Calcagnotto, D. & R. DeSalle. 2009. Population genetic structuring in pacu (Piaractus mesopotamicus) across the Paraná-Paraguay basin: evidence from microsatellites. Neotropical Ichthyology, 7: 607-616.; Iervolino et al., 2010Iervolino, F., E. K. Resende & A. L. S. Hilsdorf. 2010. The lack of genetic differentiation of pacu (Piaractus mesopotamicus) populations in the Upper-Paraguay Basin revealed by the mitochondrial DNA D-loop region: Implications for fishery management. Fisheries Research, 101: 27-31.), Prochilodus argenteus (Hatanaka et al., 2006Hatanaka, T., F. Henrique-Silva & P. M. Galetti. 2006. Population substructuring in a migratory freshwater fish Prochilodus argenteus (Characiformes, Prochilodontidae) from the São Francisco River. Genetica, 126: 153-159.; Sanches et al., 2012Sanches, A., P. M. Galetti, F. Galzerani, J. Derazo, B. Cutilak-Bianchi & T. Hatanaka. 2012. Genetic population structure of two migratory freshwater fish species (Brycon orthotaenia and Prochilodus argenteus) from the São Francisco River in Brazil and its significance for conservation. Latin American Journal of Aquatic Research, 40: 177-186.), Prochilodus costatus (Carvalho-Costa et al., 2008Carvalho-Costa, L. F., T. Hatanaka & P. M. Galetti. 2008. Evidence of lack of population substructuring in the Brazilian freshwater fish Prochilodus costatus. Genetics and Molecular Biology, 31: 377-380.), Prochilodus lineatus (Revaldaves et al., 1997Revaldaves, E., E. Renesto & M. F. P. S. Machado. 1997. Genetic variability of Prochilodus lineatus (Characiformes, Prochilodontidae) in the upper Paraná river. Brazilian Journal of Genetics, 20: 381-388.) and Salminus brasiliensis (Lopes et al., 2007Lopes, C. M., F. S. Almeida, M. L. Orsi, S. G. C. Britto, R. N. Sirol & L. M. K. Sodré. 2007. Fish passage ladders from Canoas Complex - Paranapanema River: evaluation of genetic structure maintenance of Salminus brasiliensis (Teleostei: Characiformes). Neotropical Ichthyology, 5: 131-138.).

The overall FST found in P. argenteus (0.008, P < 0.05) is similar to that found for the same species (FST = 0.008, P = 0.0002, Hatanaka et al., 2006Hatanaka, T., F. Henrique-Silva & P. M. Galetti. 2006. Population substructuring in a migratory freshwater fish Prochilodus argenteus (Characiformes, Prochilodontidae) from the São Francisco River. Genetica, 126: 153-159.). However, these authors suggested a model of structured population based on the occurrence of private alleles and high levels of heterozygosity found in one local population. Our results do not show any genetic structure (pairwise FST ) among sampling groups from the Três Marias region, any differences in the heterozygosity, or the presence of exclusive alleles. Thus, we suggest the existence of only one panmictic unit of P. argenteus in the central portion of the rio São Francisco basin based on FST , RST and chord genetic distance.

Similarly, our results do not show the existence of a genetic structure in Prochilodus costatus. The FST and RST indexes found here were significantly low (FST = 0.031, P < 0.05; RST = 0.044, P < 0.05) with a high levels of gene flow (mean of Nm = 5.911), confirming the null hypothesis of the absence of a population structure. This hypothesis, evidenced by fixation rates is also supported by the homogeneous genetic distance (Table 9). These results corroborate those of Carvalho-Costa et al. (2008)Carvalho-Costa, L. F., T. Hatanaka & P. M. Galetti. 2008. Evidence of lack of population substructuring in the Brazilian freshwater fish Prochilodus costatus. Genetics and Molecular Biology, 31: 377-380., who found low levels of pairwise structuring (FST = -0.009 and 0.006) in populations collected downstream of Três Marias dam, the rio Abaeté and the confluence among the two rivers at the reproductive season.

Migratory routes

Migratory fishes from the rio São Francisco basin spawn only in the reproductive season, which extends from November to February and coincides with the occurrence of rain and flooding, higher temperatures, and long photoperiods (Sato et al., 1996Sato, Y., E. L. Cardoso, A. L. Godinho & H. P. Godinho. 1996. Hypophysation parameters of the fish Prochilodus marggravvi obtained in routine hatchery station conditions. Revista Brasileira de Biologia, 56: 59-64.; Sato & Godinho, 2003Sato, Y. & H. P. Godinho. 2003. Migratory fishes of the São Francisco river. Pp. 195-232. In: Carolsfeld, J., B. Harvey, C. Ross & A. Baer (Eds.). Migratory fishes of South America: biology, fisheries and ecological status. Victoria, World Fisheries Trust.). After total spawning in the mainstream river or in tributaries, eggs and larvae of the migratory fishes are carried downstream by water flow reaching marginal lagoons, which provide essential habitats for juveniles with an abundant food and relatively high temperatures (Moojen, 1940Moojen, J. 1940. Aspectos ecológicos do alto São Francisco: o pescador. O Campo, 11: 22-24.). During the dry season, the marginal lagoons become isolated from the main channel. At the next rainy season, flooding provides water connections, and juveniles are ready to return to the main river (Sato & Godinho, 2003Sato, Y. & H. P. Godinho. 2003. Migratory fishes of the São Francisco river. Pp. 195-232. In: Carolsfeld, J., B. Harvey, C. Ross & A. Baer (Eds.). Migratory fishes of South America: biology, fisheries and ecological status. Victoria, World Fisheries Trust.). Migrations to spawn can explain the existence of a higher number of migrants per generation observed in our analysis (Prochilodus argenteus, Nm = 9.318; P. costatus, Nm = 5.911). Wright (1931)Wright, S. 1931. Evolution in Mendelian populations. Genetics, 16: 97. suggests that values greater than one migrant per generation prevent the substantial local differentiation by genetic drift. In fact, the existence of a single panmictic population for each species associated with the highly migratory behavior support the hypothesis of extensive gene flow among shoals.

The convergence between rio São Francisco and rio Abaeté is considered the most important spawning site of Prochilodus argenteus (Godinho & Kynard, 2006Godinho, A. L. & B. Kynard. 2006. Migration and spawning of radio-tagged zulega Prochilodus argenteus in a dammed Brazilian river. Transactions of the American Fisheries Society, 135: 811-824.), mainly because of the limnological conditions offered by the junction of the two rivers, such as water flow above 600 m³/s, water temperature over 24ºC and concentrations of dissolved oxygen above 5 mg/l (Sato et al., 2005Sato, Y., N. Bazzoli, E. Rizzo, M. B. Boschi & M. O. T. Miranda. 2005. Influence of the Abaeté River on the reproductive success of the neotropical migratory teleost Prochilodus argenteus in the São Francisco River, downstream from the Três Marias Dam, southeastern Brazil. River Research and Applications, 21: 939-950.). As suggested by Hatanaka & Galetti (2003)Hatanaka, T. & P. M. Galetti. 2003. RAPD markers indicate the occurrence of structured populations in a migratory freshwater fish species. Genetics and Molecular Biology, 26: 19-25., one fraction completes its migration toward the dam during the reproductive period, and the majority possibly migrates to locations with environmental conditions more favorable for reproduction. Sato et al. (2005)Sato, Y., N. Bazzoli, E. Rizzo, M. B. Boschi & M. O. T. Miranda. 2005. Influence of the Abaeté River on the reproductive success of the neotropical migratory teleost Prochilodus argenteus in the São Francisco River, downstream from the Três Marias Dam, southeastern Brazil. River Research and Applications, 21: 939-950. found individuals with greater body size and weight and with better reproductive conditions downstream of the rio Abaeté compared with individuals that were collected upstream under the influence of the Três Marias dam. Observations made over the past 25 years as well as reports from local fishermen also relate the downstream confluence of the rio Abaeté and rio São Francisco as a preferential spawning site of other migratory fishes from this basin, such as Brycon orthotaenia, Conorhynchos conirostris, Pseudoplatystoma corruscans, and Salminus franciscanus (Sato et al., 2005Sato, Y., N. Bazzoli, E. Rizzo, M. B. Boschi & M. O. T. Miranda. 2005. Influence of the Abaeté River on the reproductive success of the neotropical migratory teleost Prochilodus argenteus in the São Francisco River, downstream from the Três Marias Dam, southeastern Brazil. River Research and Applications, 21: 939-950.).

Considering two hypothetic scenarios for Prochilodus argenteus, first as a single population and second as divided into two groups, the degree of variation detected by AMOVA among populations was minimal (FST = 0.008, P < 0.05), and variation among the groups was not detected (FCT = 0.001, P > 0.05). These results show that the samples belonging to the Três Marias region (group 1) are not isolated from those recruited in the marginal lagoons (group 2). Based on these data, we can suggest that the populations of P. argenteus, which are recruited in the tributary lagoons, migrate preferentially to the mainstream of the rio São Francisco in the feeding season and then migrate to the Três Marias region or randomly return to the headwaters of tributaries during the reproductive season. In addition, the FST , RST , AMOVA and chord distance could not detect a population genetically distinct from the other populations that possessed similarity with individuals collected downstream rio Abaeté during the rainy season. Thus, this important area (downstream of the rio Abaeté) does not receive individuals of P. argenteus from one exclusive region but instead receives individuals from many regions along the middle rio São Francisco basin, that it must lead to a population homogenization.

Contributions of the marginal lagoons for migratory fishes

The floodplain area from the central rio São Francisco basin was estimated to be about 2,000 km² (Welcomme, 1990). The importance of the marginal lagoons in the recruitment of fishes was previously estimated by Sato et al. (1987)Sato, Y., E. L. Cardoso & J. C. C. Amorim. 1987. Peixes das lagoas marginais do rio São Francisco a montante da represa de Três Marias, Minas Gerais. Brasília, CODEVASF., identifying 37 juveniles including migratory species such as Prochilodus argenteus (< 70 g) and P. costatus (< 80 g). Our results indicate a high gene flow with low genetic differentiation among samples from both marginal lagoons and mainstream rio São Francisco, suggesting a direct connectivity among these shoals.

The continuous and rapid advancement of agriculture and the subsequent sediment deposition in some of the important tributaries from the central basin (Sato & Godinho, 2003Sato, Y. & H. P. Godinho. 2003. Migratory fishes of the São Francisco river. Pp. 195-232. In: Carolsfeld, J., B. Harvey, C. Ross & A. Baer (Eds.). Migratory fishes of South America: biology, fisheries and ecological status. Victoria, World Fisheries Trust.) are factors that directly affect the conservation of the lagoons. Notwithstanding, Pompeu & Godinho (2006)Pompeu, P. S. & H. P. Godinho. 2006. Effects of extended absence of flooding on the fish assemblages of three floodplain lagoons in the middle São Francisco River, Brazil. Neotropical Ichthyology, 4: 427-433. observed a gradual reduction in fish richness and an abundance of lagoons that did not receive annual flooding because of the water flow control by the Três Marias hydroelectric dam. Local extinction with a reduction of almost 70% of the native fish fauna was also observed in a lagoon from the rio São Francisco basin (Pompeu & Alves, 2003Pompeu, P. S. & C. B. M. Alves. 2003. Local fish extinction in a small tropical lake in Brazil. Neotropical Ichthyology, 1: 133-135.). Considering these fish nurseries in the rio São Francisco basin (Sato & Godinho, 2003Sato, Y. & H. P. Godinho. 2003. Migratory fishes of the São Francisco river. Pp. 195-232. In: Carolsfeld, J., B. Harvey, C. Ross & A. Baer (Eds.). Migratory fishes of South America: biology, fisheries and ecological status. Victoria, World Fisheries Trust.) and upper rio Paraná (Agostinho et al., 2000Agostinho, A. A., S. M. Thomaz, C. V. Minte-Vera & K. O .Winemiller. 2000. Biodiversity in the high Paraná River floodplain. Pp. 89-118. In: Gopal, B., W. J. Junk & J. A. Davis (Eds.). Biodiversity in wetlands: assessment, function and conservation. Leiden, Backhuys Publishers.), the Brazilian conservation policy should include these marginal lagoons in the floodplains as a priority for the maintenance of the genetic variability in migratory fishes.

Acknowledgments

This paper benefited from comments and suggestions from Fábio F. Roxo, Fernando F. Mendonça and Luiz H. G. Pereira. We are grateful to CODEVASF-MG and IBAMA-MG for field collection and financial support (YS) and also to Daniel C. Carvalho (PUC-MG) for loan samples. This research was also supported by the Brazilian agencies FAPESP (BFM., FF., and CO), CNPq (FF, CO), and CAPES (B.F.M.). This work was developed as Master's dissertation in Biological Sciences with emphasis in Genetics by the first author.

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Publication Dates

  • Publication in this collection
    Sept 2013

History

  • Received
    14 Dec 2012
  • Accepted
    05 June 2013
Sociedade Brasileira de Ictiologia Universidade Estadual de Maringá, Núcleo de Pesquisas em Limnologia, Ictiologia e Aquicultura/Coleção Ictiologia, Av. Colombo, 5790, 87020-900 Maringá, PR, Brasil, Tel.: (55 44)3011 4632 - Maringá - PR - Brazil
E-mail: neoichth@nupelia.uem.br