Chromosome Characterization of a Neotropical Fish Poptella paraguayensis from Paraguay River Basin

Transcription

1 1998 The Japan Mendel Society Cytologia 63: 73-77,1998 Chromosome Characterization of a Neotropical Fish Poptella paraguayensis from Paraguay River Basin (Stethaprioninae, Characidae) P. D. Freitas1, M. C. Navarrete2, C. S. Miyazawa3 and P. M. Galetti Jr.1 1 Departamento de Genetica e Evolucao, Universidade Federal de Sdo Carlos, Sao Carlos, SP, Brazil (Correspondence to P.M.G.J.) 2 Departamento de Biologia, Universidade Federal do Mato Grosso do Sul, Campo Grande, MS, Brazil 3 Departamento de Biologia e Zoologia, Universidade Federal do Mato Grosso, Cuiaba, MT, Brazil Accepted December 1, 1997 Summary The subfamily Stethaprioninae, a monophyletic group among characids, is almost unknown from the cytogenetic viewpoint. This paper shows the first chromosome characterization of Poptella paraguayensis, an endemic species of the Paraguay river basin. Analysis in Giemsa-stained preparations showed 2n = 50: 10 M + 26SM + 8ST +6A. The nucleolar organizing regions (NORs) detected by silver nitrate and GC-specific fluorochrome mithramycin (MM) staining showed two NORbearing chromosomes in the diploid complement. DA/DAPI staining showed no differential fluorescence along the chromosomes. C-banding evidenced small blocks of heterochromatin in the centromeric and telomeric regions. Phylogenetic relationships between the Stethaprioninae and other characids are discussed on the base of chromosome data. Stethaprioninae, a subfamily of the family Characidae, is distributed along the main hydrographic systems of South America, including the Amazonas, Parana-Paraguay, Sdo Francisco, and the coastal systems from northeastern to the Orinoco river (Reis 1989). Fishes of this group show compressed and deep body shape, developed pectoral fins, premaxilla with two teeth series and dentary with a series of multicuspid teeth (Britski et al. 1988). Osteological studies have demonstrated a great uniformity among genera of this subfamily. A well developed predorsal spine and anal-fin hooks are synapomorphies of this group, which is represented by four genera, Brachychalcinus, Orthospinus, Poptella and Stethaprion (Reis 1989). Despite its significant importance to the neotropical fish fauna, chromosome data thus far reported in the Stethaprioninae are restricted to two species, Poptella orbicularis (P. compressa according to Reis 1989), with n=25 (Scheel 1973), and Orthospinus franciscensis with 2n=50: 10M+32SM+8A (Pfister and Moreira-Filho 1996). Among the stethaprionin genera, Poptella shows wider distribution and inhabits all large cisandian hydrographic systems of South America, except the Sao Francisco, Upper Parana and the eastern coastal rivers. Four species are currently recognized in this genus: Poptella brevispina, P compressa, P. longipinnis and P. paraguayensis (Reis 1989). In the present study, the chromosome structure of Poptella paraguayensis, an endemic species of the Paraguay river system is characterized by means of Giemsa, C-banding, Ag-NOR, Mithramycin and DAPI staining, and phylogenetic relationships between the Stethaprioninae and other characids are discussed.

2 74 P. D. Freitas, M. C. Navarrete, C. S. Miyazawa and P. M. Galetti Jr. Cytologia 63 Materials and methods Eight individuals of Poptella paraguayensis (2 females, 4 males and 2 of unidentified sex) from oxbow lakes of the Miranda river, near the township of Passo do Lontra (MS), and ten individuals (5 females, 5 males) from the Bento Gomes river, municipality of Pocone (MT), both rivers in the Pantanal region, Paraguay basin, were cytogenetically studied. Chromosome preparations were obtained from suspension cells of the kidney by a conventional air-drying method (Bertollo et al. 1978). Silver nitrate staining was used to identify nucleolar organizing regions (Howell and Black 1980). C-banding pattern was studied using barium hydroxide (Sumner 1972). Fluorescent bands were produced by DNA-binding base-specific fluorochromes, mithramycin A (MM) and 4'6-diamidino- 2 -phenylindole (DAPI), both counterstained with dis - tamycin A (DA) (Schweizer 1976, Schmid 1980). Results Giemsa - stained metaphases of Poptella paraguayensis from both localities showed a diploid number of 50 chromosomes which comprises 10 metacentrics (M), 26 submetacentrics (SM), 8 subtelocentrics (ST) and 6 acrocentrics (A) (Fig. 1). Analysis of nucleolar organizer regions (NORs) by silver nitrate staining evidenced two NOR-bearing chromosomes in the diploid complement. Ag-NOR sites were detected at the end of the short arm of a submetacentric pair, probably number 8 of the karyotype (Fig. 2). Size differences between homologous Ag - NORs were also observed. C-banded heterochromatin appears restricted to small blocks on the centromeres and telom - Fig. 1. Giemsa-stained karyotype of Poptella paraguayensis. Fig. 2. Ag-stained karyotype of Poptella paraguayensis. Ag-NOR sites shown at the end of the short arm of the chromosome 8.

3 1998 Chromosomes of Stethaprionin Fish, Poptella paraguayensis 75 Fig. 3. C-banded karyotype of Poptella paraguayensis. A B C Fig. 4. A and B: metaphases of Poptella paraguayensis after MM staining. C: DA/DAPI staining. NOR-bearing chromosomes are indicated by large arrows. A third chromosome showing a MM+ band is indicated by a small arrow.

4 76 P. D. Freitas, M. C. Navarrete, C. S. Miyazawa and P. M. Galetti Jr. Cytologia 63 eres (Fig. 3). DA/MM staining showed no differential fluorescence along the chromosomes of P. paraguayensis, except for the two homologous NOR sites. In some cells, another chromosome showed a smaller and less bright MM+ band at the end of the short arm (Fig. 4A, B). DA/DAPI staining showed no differential fluorescence along the chromosomes of this species (Fig. 4C). Discussion The karyotype of P.paraguayensis studied here is 2n=50 comprising 10M + 26SM+ 8ST +6A, and although it differs from that of O. franciscensis (2n=50: 10M+32SM+8A) (Pfister and Moreira - Filho 1996), suggests that 2n=50 could be the general diploid chromosome number of this subfamily. Pericentric inversions might have a major role in the chromosome evolution of these fishes. Two Ag-NOR bearing chromosomes detected in P. paraguayensis were also reported in O. franciscensis (Pfister and Moreira - Filho 1996). In P. paraguayensis, Ag - NORs were detected at the end of the short arm of a submetacentric pair, comparable to pair 8 in the complement, and were often heteromorphic in their size. Fluorescent MM+ bands were congruent with Ag - NORs in P paraguayensis, a common feature among fishes and some other vertebrates (e.g., Schmid and Gut - tenbach 1988). In O. franciscensis, MM positive bands were observed in some chromosome regions other than the Ag-NOR sites (Pfister and Moreira - Filho 1996) and may indicate minor chromosome differenti - ations involving heterochromatin in this species. In P paraguayensis, another chromosome showing a dot-like less bright MM+ band detected in some cells could be inactive secondary NOR (e.g., Galetti et al. 1995) or related to heterochromatin stainability. There have been several hypotheses about systematic position of the subfamily Stethaprioninae. Fowler (1950) considered the Stethaprioninae as a subfamily of the Serrasalmidae. However, this hypothesis cannot be supported by the chromosome data. Serrasalmids so far studied show their karyotype structure varying from 2n= 54 to 64. Unlike the Stethaprioninae, the serrasalmids show the largest diploid numbers in the order Characiformes, and may represent a cytogenetical unit in which increase of the chromosome number is a derived condition (Arefjev 1990, Cestari and Galetti 1992). The Stethaprioninae was also reported as a tribe of Tetragonopterinae (Gery 1964). Karyotypes of the subfamily Tetragonopterinae comprising Moenkhausia, Tetragonopterus, Astyanax and other genera, are similar to those of stethaprionins. But stethaprionins (2n=50, NF =92-94) differ from Moenkhausia (2n=50, NF =98-100) in having a smaller arm number, and from Tetragonopterus (2n=52, NF =68-74) because of the diploid chromosome number (Portela et al. 1988, Foresti et al. 1989, Alberdi and Fenocchio 1997). In contrast, several species of Astyanax (2n=50, NF =64-94) show karyotypes quite similar to those of stethaprionins (Morelli et al. 1983, Moreira - Filho and Bertollo 1991). Slight differences related to the number of biarmed and uniarmed chromosomes between Astyanax species and P. paraguayensis could have diverged from a common ancestral karyotype. Acknowledgments The authors thank O. Froehlich and M. Uatanabaro for help and material support during fish collection. This work was in part supported by the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico/Brazil (CNPq). References Alberdi, A. J. and Fenocchio, A. S Karyotypes of five Tetragonopterinae species (Pisces, Characidae) from Argentina. Cytologia 62:

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