versión On-line ISSN 1678-4685
Genet. Mol. Biol. v.22 n.4 São Paulo dic. 1999
GENETIC VARIABILITY IN FIVE SPECIES OF ANOSTOMIDAE (OSTARIOPHYSI - CHARACIFORMES)
Lucimara Chiari and Leda Maria Koelblinger Sodré
Departamento de Biologia Geral, Centro de Ciências Biológicas, Universidade Estadual de Londrina, Campus Universitário, Caixa Postal 6001, 8605l-990 Londrina, PR, Brasil. Send correspondence to L.M.K.S. E-mail: firstname.lastname@example.org
Genetic variability was studied in five fish species (Anostomidae): Schizodon intermedius and S. nasutus and Leporinus friderici, L. elongatus and L. obtusidens, collected at one location on the Tibagi River (Paraná, Brazil). The protein data from seven systems coded collectively for 19 loci in the liver, muscle and heart. Nine of these loci were polymorphic. The estimated proportion of polymorphism loci () varied from 16.7% in S. intermedius to 36.9% in L. friderici; the mean heterozygosity observed (o) was 0.027 ± 0.015 and 0.109 ± 0.042, respectively. The estimated value of the genetic identity among L. friderici and S. intermedius (0.749) and S. nasutus (0.787) suggested that these are "congeneric" species. Morphological characteristics indicate that these species belong to distinct genera, while isoenzymatic data show that they are very similar at the genetic/biochemical level.
Anostomidae is one of the most abundant fish families in the Neotropical region (Vari, 1983). A large number of species from this family were found in a fish fauna survey carried out at five sampling points along the Tibagi River (Paraná, Brazil) by Bennemann et al. (1995). Five species, Leporinus elongatus, L. friderici, L. obtusidens, Schizodon intermedius and S. nasutus, were detected at one of these locations (Sertanópolis).
According to Garavello (1979), the Anostomidae are characterized by a large reduction in the number of teeth. The main character of the genus Leporinus is the presence of wide teeth arranged in steps and a prominent "symphysial" pair. Species of the genus Schizodon do not differ externally from the majority of species of the genus Leporinus. However, the pluricuspidate teeth form a continuous crenulate cutting border, and are quite characteristic of Schizodon (Géry, 1977).
Isozyme electrophoresis studies have been widely employed to analyze ontogenetic development, to quantify the genetic variability in natural fish populations for comparative analysis among populations, and also to trace phylogenetic relationships, e.g. Panepucci et al. (l984 and l987), Basaglia (1989), Rao et al. (1989), Renno et al. (1989), Degani and Veith (1990), Farias and Almeida-Val (l992), Verneau et al. (1994), Zawadzki (1996), Revaldaves et al. (l997) and Almeida and Sodré (l998), among many others.
The present study is a part of an integrated project for the restoration of the Tibagi River Basin - "Aspects of the Fauna and Flora of the Tibagi River Basin". The objective was to determine the electrophoretic profile and the distribution of expression in various tissues of the loci of seven protein systems and to investigate and quantify the genetic variability to estimate the genetic distance data and genetic identity for the five species of Anostomidae found by Bennemann et al. (1995) in Sertanópolis, PR.
MATERIAL AND METHODS
L. elongatus, L. friderici, L. obtusidens, S. intermedius and S. nasutus specimens were collected monthly from the Tibagi River at Sertanópolis region, 2 km extension, PR, Brazil, between June 1994 and May 1996. Fish were collected by gillnets. Samples of liver, muscle, heart, and eye (analyzed for lactate dehydrogenase; LDH) were taken from the captured specimens and kept at -20°C. Individual samples from each tissue were homogenized in Tris-EDTA buffer, pH 7.0 (Degani and Veith, 1990), and centrifuged at 3.000 rpm for 15 min at 4°C.
Seven protein systems, carboxylesterases (EST- E.C. 220.127.116.11.), phosphoglucomutase (PGM- E.C. 18.104.22.168.), glycerol-3-phosphate dehydrogenase (G-3-PDH- E.C. 22.214.171.124), isocitrate dehydrogenase (IDHP- E.C. 126.96.36.199), LDH (E.C. 188.8.131.52), malate dehydrogenase (MDH- E.C. 184.108.40.206) and non-specific proteins (PT), were analyzed using the horizontal electrophoresis technique (Smithies, 1955) in starch gels (Val et al., 1981). Methodologies based on Shaw and Prasad (1970), Harris and Hopkinson (1976) and Lima and Contel (1990) were used for the preparation of the buffer systems and the staining (Table I).
The gene loci nomenclature adopted in this study was proposed by Shaklee et al. (1990). Each locus is designated by the abbreviation of the enzyme name in italics, followed by an Arabic number and an asterisk. The loci and alleles which coded the most anodic isoenzyme were designated by the number 1 or letter A.
BIOSYS-1 application software (Swoford and Selander, 1981) was used for statistical analyses. The genetic variability was estimated by calculating the proportion of polymorphic loci () (99% criterion). The observed intralocus and mean heterozygosities (o) were obtained by direct counting. Expected intralocus and mean heterozygosities (e) were calculated according to Nei (1972). The Nei genetic distance and identity were also calculated by BIOSYS-1, and the genetic distance values were used to construct a dendrogram by the unweighted means method (UPGMA).
Of the seven protein systems, 19 loci were detected for L. friderici and 18 were detected for the other species. All of them exhibited anode migration (Figure 1).
Analysis of each isoenzymatic system of different tissues from one individual sample showed differential expression of each locus (Table II) regarding both number and color intensity of bands.
Of the 19 loci sampled, nine were polymorphic: PT-1*, PGM-1* and PGM-2* for the five species, PGM-3* for the three Leporinus species, PT-2* for S. nasutus, IDHP-1* for L. elongatus and EST-3*, LDH-A* and LDH-B* for L. friderici (Table III).
The loci PT-1* and PGM-1* in S. intermedius; PT-2* in S. nasutus; PGM-2* in S. intermedius, L. elongatus and L. obtusidens and PGM-3* in the three Leporinus species were not at Hardy-Weinberg genetic equilibrium, as indicated by the significantly different genotypic frequencies (P < 0.05).
Leporinus friderici showed the largest proportion of polymorphic loci ( = 36.8%) and L. elongatus showed the largest expected mean heterozygosity (e = 0.142 ± 0.054) (Table IV). The greatst genetic identity value (0.962) was observed between S. intermedius and S. nasutus (TableV). From all three species of Leporinus analyzed, L. friderici was the one which had the smallest genetic distance from S. intermedius and S. nasutus (0.251 and 0.213, respectively) (TableV), which is shown in a dendrogram (Figure 2) constructed with D values by the UPGMA method.
Simultaneous analysis of three different tissues (liver, muscle and heart) of the same individual allowed the detection of a greater number of gene loci compared with other studies, even though the number of protein systems was small. Three loci were detected for PGM. The majority of studies with fish describe only one locus, as found by Renno et al. (1989) for four species of Leporinus. Four loci were found for G-3-PDH in the two Schizodon species and in L. friderici, and three loci in L. obtusidens and L. elongatus; Renno et al. (1989, 1990) observed a single locus in L. friderici muscle.
Almost every bony fish has the LDH-C* locus, which is believed to have originated through a duplication of the LDH-B* locus (Basaglia, 1989; Rao et. al., 1989). The LDH-C* locus in bony fish shows different tissue regulation patterns in different taxons. Primitive orders of bony fish have a generalized tissue expression, while representatives of more advanced orders show a specialized tissue pattern (Whitt, 1975; Kettler and Whitt, 1986; Basaglia, 1989; Rao et al. 1989). LDH-C* locus activity was not detected in the tissues (liver, muscle, heart and eye) analyzed for the lactate dehydrogenase system. The absence of the LDH-C* locus was also observed by Panepucci et al. (1984) in a study on lactate dehydrogenase in several species of the Anostomidae, including those of the present study (except for S. intermedius), and by Renno et al. (1989) in four Leporinus species (including L. friderici).
Divergences from Hardy-Weinberg equilibrium in the expected genotypic frequencies may occur due to mutation, natural selection, preferential crossing, loss or gain of migrants, genetic drift, and/or methodological errors. Methodological errors could explain the deviations shown mainly by the loci PT-2*, PGM-1* and PGM-3*, because of the difficulties in electrophoretic profile interpretation. The deficiency in heterozygotes for these loci may be due to errors in typing. According to Crouau-Rou (1988) (Apud Lima, 1989), many studies have shown significant levels of heterozygote deficiency in natural populations, but these deficiencies are usually observed in some allozymic loci and/or in only some samples of a species, while for other loci the genotypic proportions are in equilibrium.
In the analysis of the genetic variability data, the proportion of polymorphic loci () in the five fish species from the family Anostomidae studied varied from 16.7% in S. intermedius to 36.8% in L. friderici. These data are in agreement with the literature. Renno et al. (1989) observed a value of 33% for L. friderici collected in French Guiana. Nevo (1978) estimated from data in the literature a mean value of 15.2%, for 51 species of Teleostei. Studies carried out in fish species from Brazilian rivers show values with a wide range of variation: Zawadzki (1996) found values from 11.45 to 19.23% in three species of the genus Hypostomus (Iguaçu River); Revaldaves et al. (1997) observed 33.3% for Prochilodus lineatus (Paraná River) and Almeida and Sodré (1998) observed values from 6.67 to 20% in three Pimelodidae species (Tibagi River).
Ward et al. (1992) estimated an He value of 0.051 for about 150 marine and fresh water fish species. The mean heterozygosity observed in the present study varied from 0.027 ± 0.015 in S. intermedius to 0.109 ± 0.042 in L. friderici, and the corresponding expected values ranged from 0.072 ± 0.038 in S. intermedius to 0.142 ± 0.054 in L. elongatus. The e values for Neotropic fish vary from 1.1% in Hypostomus derbyi, a sedentary species (Zawadzki, 1996), to 13.2% in Prochilodus lineatus (Revaldaves et al., 1997). Renno et al. (1989) estimated e at 12% for L. friderici. Thus, the values observed in the five species of Anostomidae do not diverge significantly from those observed in other fish species. The variation in the values, according to Lewontin (1974) and Nei (1978), could be due to the fact that e is greatly affected by the choice and number of loci analyzed.
S. intermedius was the species, among those analyzed, with the lowest and o values (Table IV). The fish fauna survey carried out by Bennemann et al. (1995) at five locations on the Tibagi River shows that S. intermedius specimens were present only in Sertanópolis. L. friderici and L. elongatus, with Ho values 10.9 and 7.5%, respectively, could be found at Sapopema, Londrina and Sertanópolis (Bennemann et al., 1995). According to Zimmerman (1987), high heterozygosity levels and polymorphism may be expected for fish species which are dispersed among several places in one river. The suggestion of a large population would also decrease the probability of inbreeding.
Thorpe (1982), using data available in the literature, established a distribution index for the genetic identity frequencies of Nei (1972) for genetic divergence estimates among conspecific populations (0.95 to 1.0), among species of the same genus (0.35 to 0.85) and among genera of the same family (0.0 to 0.60). According to this index, the values of identity obtained for L. friderici, L. elongatus and L. obtusidens (Table V) show that they may be considered genetically distinct biological species of the same genus. The values obtained between S. intermedius and S. nasutus compared with L. obtusidens and L. elongatus (Table V) are in the identity interval for species of the genera of one family. Although the identity values between S. intermedius and S. nasutus (0.962) and between L. elongatus and L. obtusidens (0.949) are at the limit of the Thorpe's index for conspecific populations, the morphological characteristics make it clear that they are distinct species (Géry, 1977; Shibatta, O.A., personal communication). The estimated identity value between L. friderici and S. intermedius and S. nasutus (Table V) corresponds to the value for congeneric species; however, they are classified as species of distinct genera, and at the genetic-biochemical level, these species share a great number of loci. The dendrogram constructed (Figure 2) from the D values shows that L. friderici is the species within Leporinus that is genetically most similar to the genus Schizodon.
The present study, part of the integrated project "Aspects of Fauna and Flora on the Tibagi River Basin" carried out by Universidade Estadual de Londrina for the restoration of the basin, conflicts with government projects to construct five hydro-electric plants along the Tibagi River. These plants may be barriers to the dispersion of fresh water organisms, specially migratory species. Besides the impact caused by water flow control, they jeopardize survival, successful mating, and gene flow, which surely will alter the genic frequencies in the fish species. Thus, the maintenance of the environmental heterogeneity of the Tibagi River is important for the preservation of aquatic organisms. The construction of hydro-electric plants will lead to a reduction in the genetic variability of migratory species, such as L. friderici, perhaps reaching levels lower than those observed for S. intermedius, a species apparently restricted to a single locality on the Tibagi River.
The authors would like to thank Dr. Oscar Akio Shibatta and the biologist Mário Luís Orsi, from the Departamento de Biologia Animal e Vegetal of Universidade Estadual de Londrina, for the identification and collection of the specimens. We also wish to thank Consórcio Intermunicipal para a Recuperação da Bacia do Rio Tibagi (Intermunicipal Partnership for the Recuperation of the Tibagi River Basin), Klabin Fabricadora de Papel e Celulose (Klabin Paper and Cellulose Factory), CNPq, and Universidade Estadual de Londrina for financial support received for the execution of this study.
A variabilidade genética de 5 espécies da família Anostomidae pertencentes aos gêneros Schizodon (S. intermedius e S. nasutus) e Leporinus (L. friderici, L. elongatus e L. obtusidens) coletadas em uma localidade do rio Tibagi (Paraná, Brasil) foi analisada comparativamente utilizando dados protéicos de 7 sistemas que codificam l9 locos no fígado, músculo e coração. Dos locos identificados, 9 são polimórficos, com valores estimados de proporção de locos polimórficos () que variaram de 16.7% em S. intermedius a 36.85% em L. friderici, e a heterozigosidade média observada (o) foi de 0.027 ± 0.015 e 0.109 ± 0.042, nessas mesmas espécies. O valor estimado de identidade genética (I) entre L. friderici e S. intermedius (0.749) e S. nasutus (0.787) sugere que estas são espécies congenéricas. As características morfológicas determinam que estas espécies pertencem a gêneros distintos, no entanto os dados de identidade e distância genética obtidos demonstram que essas três espécies, no nível genético-bioquímico, têm uma maior similaridade.
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(Received June 15, 1998)