Cytogenetics and biogeography: considerations about the natural origin of Hoplias malabaricus (Characiformes, Erythrinidae) on the Iguaçu river

BIOLOGICAL AND APPLIED SCIENCES

Cytogenetics and biogeography: considerations about the natural origin of Hoplias malabaricus (Characiformes, Erythrinidae) on the Iguaçu river

Marcelo Ricardo VicariI, ^*; Rubens Pazza^I; Roberto Ferreira Artoni^II; Vladimir Pavan Margarido^III; Luiz Antonio Carlos Bertollo^I

^IUniversidade Federal de São Carlos; Departamento de Genética e Evolução; Via Washington Luís, km 235; C. P. 676; 13565-905; [e-mail]; São Carlos - SP - Brasil

^IIUniversidade Estadual de Ponta Grossa; Departamento de Biologia Estrutural; Molecular e Genética; 84030-900; Ponta Grossa - PR - Brasil

^IIIUniversidade Estadual do Oeste do Paraná; Centro de Ciências Biológicas e da Saúde; 85814-110; Cascavel - PR - Brasil

ABSTRACT

Hopliasmalabaricus (trahyra) is a widespread fish species over the Neotropical region with diversified inter-populational karyotypes (cytotypes), which may correspond to a species complex. Despite the wide distribution in the South American basins, some authors have questioned its natural origin in the Iguaçu river, an important Brazilian river basin which is characterized by several endemic fish species. We have analyzed the karyotype of H. malabaricus from different collection sites of this river, by conventional and banding methods. Our results, in addition to our previous data concerning geographic distribution of the cytotypes, contribute to better understand the origin of H. malabaricus on the Iguaçu river, reinforcing the proposition that it is a natural fish species in this river basin.

Key words: Chromosome evolution, karyotypes distribution, endemism

RESUMO

Hoplias malabaricus (traíra) é uma espécie de peixe com ampla distribuição pela região Neotropical e alta diversidade cariotípica interpopulacional (citótipos), que podem corresponder a um complexo de espécies. Apesar da sua ampla distribuição pelas bacias hidrográficas da América do Sul, alguns autores têm questionado sua origem natural no rio Iguaçu, uma importante bacia hidrográfica brasileira que é caracterizada pela presença de várias espécies de peixes endêmicas. Assim, nós temos analisado o cariótipo de H. malabaricus de diferentes locais de coleta neste rio por métodos convencionais e bandamentos cromossômicos. Nossos resultados, em adição a dados prévios a respeito da distribuição geográfica dos citótipos contribuem para um melhor entendimento da origem de H. malabaricus no rio Iguaçu, reforçando a proposta que esta é uma espécie de peixe natural desta bacia hidrográfica.

INTRODUCTION

The fish fauna of the Iguaçu river is well known for its high degree of endemism and for the absence of several fish families common in other Brazilian basins (Júlio Jr. et al., 1997). The Iguaçu falls have acted as an efficient geographical barrier between the ichthyofauna from the first and the second plateaus along most of the river length (Maack, 1968). Garavello et al. (1997) described 36 endemic species, 11 introduced species and 3 exotic species at Iguaçu river. The occurrence of Hoplias malabaricus in this basin, popularly known as traíra, is quite controversial being referred as an introduced species (Garavello et al. op. cit.). Dergam et al. (1998) stated that the introduction of H. malabaricus in the Iguaçu river could have been done from the Tibagi headwaters, based on the similar genetic patterns of the populations, as well as in the proximity of both rivers.

Cytogenetical data clearly indicate that H. malabaricus is a complex species, probably related to its wide distribution over Neotropical region, requiring a careful taxonomic revision (Bertollo et al., 1983; Dergam and Bertollo, 1990; Scavone et al., 1994; Bertollo et al., 2000). Bertollo et al. (2000) differentiated seven distinct cytotypes in this fish group, based on their chromosome numbers and morphology and the absence or occurrence of distinct sex chromosome systems.

The present paper aimed to contribute for a better understanding of the biogeography of H. malabaricus and its probable natural origin on the Iguaçu river basin.

MATERIAL AND METHODS

Forty-six specimens of Hoplias malabaricus were sampled in the main stream and in marginal lakes belonging to the Iguaçu river. Twelve specimens were collected from a population in the western region of the Paraná State (Brazil), in the township of Nova Prata do Iguaçu. Thirty-four specimens were captured from a population in the eastern region of the same state, in the township of Palmeira (Fig. 1).

Metaphases were obtained according to Bertollo et al. (1978), using a cell suspension from the anterior kidney, after in vivo treatment with colchicine (1ml/100g body weight). The detection of the nucleolar organizing regions (NORs) and the constitutive heterochromatin followed the methods described by Howell and Black (1980) and Sumner (1972), respectively. Chromomycin A₃ staining was performed according to Schmid (1980). Chromosomes were arranged by size and shape, according to Levan et al. (1964) and the cytotypes were classified as stated by Bertollo et al. (2000).

RESULTS

The two populations analyzed, from now named eastern and western populations, showed the same karyotypic macrostructure, with a diploid number of 2n=42, with 24 metacentric (M) plus 18 submetacentric (SM) chromosomes. The karyotypes were identical between the sexes, without any evidence of heteromorphic sex chromosomes (Figs. 2a, c).

Ag-NORs (silver stained nucleolar organizing regions) were variable in number in both populations. Two to seven Ag-NORs were evidenced in the eastern population and three to eight in the western population, generally located on the telomeric region of the long arm of meta/submetacentric chromosomes. Specifically in the eastern population, it was found Ag-NOR in the telomeric region of the long arm of the smaller submetacentric chromosome pair and an interstitial Ag-NOR in the long arm of the only one homologue of the 16^th pair (Fig. 3b). Bi-telomeric Ag-NORs (NORs located on the two telomeric regions of a same chromosome) were also found in both populations (Fig. 3b).

Constitutive heterochromatin was evidenced in the centromeric/pericentromeric regions of all chromosomes and, eventually, in a telomeric position (Figs. 2b, d). In the eastern population, the 16^th chromosome pair showed a polymorphic GC-rich heterochromatic region in agreement with the NOR location (Fig. 3a). In the western population, GC-rich heterochromatic regions were detected in the centromeric region of the 6^th, 13^th and 15^th chromosome pairs, without association with NORs (Fig. 3c).

DISCUSSION

Both eastern and western populations showed a karyotypic structure that corresponds to cytotype A in the Bertollo's classification (Bertollo et al., 2000). However, some chromosomal differentiation was found between the populations, particularly in respect of the Ag-NORs and GC-rich regions locations. Indeed, the Ag-NORs located on the telomeric region of the smaller submetacentric chromosomes and in the interstitial region of the 16^th chromosome (Fig. 3b) were only seen in the eastern population. In fact, interstitial NOR is an uncommon feature for H. malabaricus. On the other hand bitelomeric NORs, as those observed in both eastern and western populations, have been commonly found in H. malabaricus cytotypes (Bertollo, 1996; Born and Bertollo, 2001), representing a probable sinapomorphic condition for this fish group. Concerning the distinct numbers of Ag-NORs found in the eastern (2-7) and in the western (3-8) populations, although this variation may be related to a differential expression of some rDNA cistrons, the results may also represent real karyotypic differences, as verified by Born and Bertollo (2001) and Vicari et al. (2003) for distinct H. malabaricus populations belonging to cytotype A.

Two classes of heterochromatin were observed in both populations. The first one possesses a GC-rich DNA, which is present only in few chromosomes. This heterochromatin was not found to be related to NORs in the western population, which might represent an exception feature amongst fishes (Souza et al., 1996; Artoni et al., 1999; Margarido and Galetti Jr., 2000). Indeed, GC-rich heterochromatin associated with NORs is a common character in fish species (Mayr et al., 1985; Schmid and Guttenbach, 1988; Phillips and Hartley, 1988; Sola et al., 1992). The location of this heterochromatin in distinct chromosome pairs, besides its relation with NOR only in the eastern population, are good evidences that a karyotypic diversification have already occurred at some degree between the populations. A second heterochromatin class, present on the centromeric region of most chromosomes, does not present any fluorescent signal after chomomycin A₃ staining.

Lemos et al. (2002) found sympatric populations of H. malabaricus belonging to cytotype A (2n=42 chromosomes without a sex chromosome system) and cytotype B (2n=42 chromosomes with a XX/XY sex chromosome system) in the Iguaçu river, the latter being also found in coastal Brazilian basins (Bertollo et al., 2000). The occurrence of the cytotype B in the Iguaçu basin could be explained by dispersion from the coastal basins, as a consequence of a preterit contact between the basins, a headwater capture in small streams or even an occasional communication during the wet seasons (Lemos et al., 2002). Whereas cytotype A show a wide distribution along the Iguaçu basin (Bertollo et al., 2000; Lemos et al., 2002; present study), cytotype B was found thus far only in a restricted area of the first plateau of that basin (Fig. 1d). Thus, it is likely that the dispersal event of cytotype B have a recent origin, in addition to the low vagility of H. malabaricus and the possible geological barrier formed by the Ponta Grossa arc, that is, the Devonian scarp that delimits the first and the second plateaus of the Paraná State (Fig. 1b).

The Iguaçu river basin is not recent in origin. As other tributaries of the Paraná river, as well as the Paraná river itself, its origin dates from the basaltic settlement during the formation of Serra Geral, in the Cretaceous (Potter, 1997). Its fish fauna finds itself isolated from that of the Paraná river due to the formation of the Iguaçu falls, approximately 22 millions years ago (Oligo-Miocene period). This fact may have favored the speciation and the considerable degree of endemism of that basin (Severi and Cordeiro 1994). H. malabaricus has been considered a non-native species in the Iguaçu basin. However, cytogenetics data show that cytotype A finds itself broadly distributed in Brazilian southeast and southern regions (where the Iguaçu basin is inserted), reaching Uruguay and Argentina (Bertollo et al., 2000). A priori, it seems very unlikely that H. malabaricus was originally absent from the Iguaçu basin, although present in many other neighboring basins. Thus, the endemism of the Iguaçu river appears to be valid for many fish species, but does not seem to apply for others, as H. malabaricus (Lemos et al., 2002). Our hypothesis is that H. malabaricus cytotype A is a native form in the Iguaçu river, and that the karyotypic diversification of the populations here analyzed is a throughout time consequence of vicariant events.

ACKNOWLEDGEMENTS

This study was supported by FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), Fundação Araucária (Fundação de Apoio ao Desenvolvimento Científico e Tecnológico do Estado do Paraná), Universidade Estadual do Oeste do Paraná and Universidade Estadual de Ponta Grossa. The authors also thank Dra. Onildes Maria Taschetto and Dr. Horácio Ferreira Julio Jr. for their contributions, and Miguel Airton Carvalho for field and laboratory assistance.

Received: August 30, 2004;

Revised: December 28, 2004;

Accepted: July 28, 2005.

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*

Author for correspondence

Publication Dates

• Publication in this collection
  24 Apr 2006
• Date of issue
  Mar 2006

History

• Accepted
  28 July 2005
• Received
  30 Aug 2004
• Reviewed
  28 Dec 2004