C 1997 The Japan Mendel Society Cytologia 62: 241-247, 1997

Cytogenetic Analysis of Four Curimatids from the Paraguay Basin, Brazil (Pisces: : )

M. C. Navarrete1 and H. F. Julio Jr.2

1 Departamento de Biologica/CCBS, Universidade Federal de Mato Grosso do Sul, CP 649, Campo Grande, MS, Brazil-79070-900 2 Departamento de Biologia Celular/CCB, Universidade Estadual de Maringa, Av. Colombo, 3690, Maringa, PR, Brazil-87020-900

Accepted May 26, 1997

The Curimatidae, with eight genera and about 100 (Vari 1989), are distributed

over most of the Neotropical region. East of the Andes they are found in all major basins from the Rio da Prata northwards and in transandean waters they occur from northern Peru to the

drainage of the Caribbean and southern Costa Rica (Vari 1988). Curimatids are micro-

phagous, feeding on microorganisms and detritus ingested with mud, playing an important role in nutrient cycling (Vari 1989). In the great river systems of South America ilyophagous

(, mainly, and Curimatidae) frequently dominate the biomass (Bowen 1983). According to Vari (1983), the Curimatidae are phylogenetically related to the Prochilod- ontidae, and Chilodontidae, these four families forming a monophyletic unit. The

cytogenetic data available for these families support the hypothesis of the monophyly since the karyotype of majority of the species comprises 54 metacentric and submetacentric chromo-

somes (Scheel 1973, Galetti et al. 1981, 1991, Pauls and Bertollo 1990, Cestari et al. 1990, Feldberg et al. 1992). The Parodontidae (Moreira-Filho et al. 1985) and Hemiodontidae

(Porto 1992) also show the same karyotypic macrostructure and were considered to be closely related to that monophyly by Galetti et al. (1994), who proposed that 54 biarmed chromosomes

could be a synapomorphy shared by those six families. Despite the great karyotypic similarity observed among the species belonging to those

families, some species of the Curimatidae have different diploid numbers, in most cases derived from fission events (Scheel 1973, Venere 1991, Feldberg et al. 1987, 1992, 1993, Navarrete

1996). In this study we present cytogenetic data on four species of Curimatidae, on the one hand reinforcing that karyotypic similarity but also showing that in the

occurs the lowest number of chromosomes in the family, possibly derived by centric fusion.

Material and methods

Sixteen specimens of dorsalis (6 males/10 females) were analyzed, twenty-one

of curviventris (7/14), seventeen of brevipinna (6/11) and two of Curimatopsis myersi (0/2). The collecting sites are situated between 56°58'32"-57°04'38"W

and 19 •‹21'31"-19•‹34'13"S (Corumba, MS, Brazil), consisting of lateral lakes of the Miranda river (connected with each other at least during the rainy season) for C. dorsalis and P.

curviventris, one lake near MS-184 road for C. myersi and both localities for S. brevipinna. Somatic chromosome preparations were obtained from cell suspensions of the cephalic

portion of the kidney by the air-drying technique of Bertollo et al. (1978). Nucleolus organizer regions were identified by silver nitrate staining following the method of Howell and Black

(1980) and constitutive heterochromatin was detected by C-banding technique of Sumner

(1972). 242 M. C. Navarrete and H. F. Julio Jr. Cytologia 62

A

B

C

D

Fig . 1 . Karyotypes (Giemsa) of (A), Psectrogaster curviventris (B), Steindachnerina brevipinna (C) and Curimatopsis myersi (D). Ag-NOR-bearing chromosomes in the inset. 1997 Cytogenetic Analysis of Four Curimatids 243

A

B

C

Fig . 2 . C-banding in Curimatella dorsalis (A), Psectrogaster curviventris (B) and Steindach- nerina brevipinna (C).

Chromosomes were measured on photographs with calipers and classified as metacentric or submetacentric following Levan et al. (1964) and then put in order by size.

Results Curimatella dorsalis has a complement of 46 meta- and 8 submetacentric chromosomes, with NORs located terminally on the short arms of 13th pair (Fig. 1A). With respect to pattern of distribution of C-positive constitutive heterochromatin, this species revealed distinct peri- centromeric blocks in some chromosomes (Fig. 2A). The NOR-bearing pair shows strong 244 M. C. Navarrete and H. F. Julio Jr. Cytologia 62 staining adjacent to the NORs. In Psectrogaster curviventris 42 meta- and 12 submetacentric chromosomes were observed, with terminal NORs on the short arms of the 20th metacentric pair (Fig. 1B). The analysis of heterochromatin showed a pattern similar to the one described above for C. dorsalis (Fig. 2B). The telomeric staining seen on 20th pair is coincident with staining by AgNO3, revealing C-positive NORs. Steindachnerina brevipinna presents 48 metacentric and 6 submetacentric chromosomes, the NORs appearing terminally on the 17th metacentric pair (Fig. 1C). This species has less heterochromatin than the first two, with centromeric and telomeric blocks (Fig. 2C). Pair 5, besides showing strong telomeric and centromeric staining, stands out due to a big hetero- chromatic block present on the short arm of one of the homologues. The pair of nucleolar organizer chromosomes stain strongly on the telomeric region of the short arm, a region coincident with the NOR sites. In Curimatopsis myersi the complement is composed of 42 metacentric and 4 sub- metacentric chromosomes (Fig. 1D). As to NORs, only one weakly AgNO3-stained metaphase figure was obtained, which was not possible to photograph. As two stained chromosomes were observed in it and interphase nuclei presented always two nucleoles, it seems that this species also has one pair of NOR-bearing chromosomes. There is no data regarding the constitutive heterochromatin of this species. In four species described above no sex chromosome heteromorphism was observed at the level of the analyses made.

Discussion As shown in Table 1, of thirty-one curimatid species for which there are cytogenetic data, twenty-five show 2n = 54 meta- and submetacentric chromosomes, testimony of great karyo- typic stability in the family. Other characteristic that reinforces the idea of stability is the presence of simple NORs in all curimatids.

Table 1. Karyotypic characteristics in the genera Curimatella, Psectrogaster, Steindachnerina and Curimatopsis

Source: 1—Scheel 1973, 2—Venere and Galetti Jr. 1989, 3—Venere 1991, 4—Feldberg et al. 1992 and 5-this study. 1997 Cytogenetic Analysis of Four Curimatids 245

Venere and Galetti (1989) suggested that curimatids show similar karyotypic macro- structures but does not a conserved macrostructure. Of the four species analysed in this study, P. curviventris, C. dorsalis and S. brevipinna show the general karyotype of the family (Table 1, Figs. 1A-C) but can be distinguished by differences in chromosome formula, NOR location, pattern of distribution of constitutive heterochromatin, and further evidence of the occurrence of non-Robertsonian rearrangements during the karyotypic evolution of the group (Venere and Galetti Jr. 1989, Venere 1991, Feldberg et al. 1992). Two different chromosome formulae have been described for Curimatella : 46 M and 8 SM in C. dorsalis, C. albuma and C. meyeri, and 50 M and 4 SM in C. lepidura (Table 1). In the first two species the NORs are located in a medium-sized chromosome pair, possibly the same one, but in different arms. In the other two species the NOR-bearing pair is possibly different. Vari (1992) considered that C. albuma and C. meyeri could be sister species on the basis of one sinapomorphy. The data on chromosome formulae available for the genus Curimatella suggest that 46 M and 8 SM could be a character state placing those two species and C. dorsalis in the same intrageneric clade. P. curviventris and P. rutiloides showed the same chromosome formula but different NOR-bearing pairs (Table 1). The first species revealed some individuals with two small NORs and others with the NOR in one of the homologous chromosomes being twice as large as the other. This phenomenon has been observed frequently in species of several groups of fishes, for example, in Gymnotiformes (Foresti et al. 1981), Characiformes (Moreira Filho et al. 1984, Galetti et al. 1984, 1995), Cyprinodontiformes (Howell and Black 1979), Cypriniformes (Amemiya and Gold 1988) and Salmoniformes (Sanchez et al. 1990). For the Curimatidae, NOR heteromorphism was observed in some species by Venere (1991) and in squamoralevis (Navarrete 1996). The genus Steindachnerina (Table 1) seems to show greater variation in chromosome formulae, three different ones being shown by six species. As regards NOR location, at least two situations have been reported. In S. conspersa the NORs are interstitial on the second pair of the complement. In three species (S. elegans, S. insculpta and S. sp.) the NOR-bearing pair is the 25th, a small metacentric. In S. brevipinna and S. leucisca 17th and 15th pairs, respectively, were identified as the NOR-bearing chromosomes. Venere and Galetti Jr. (1989) and Venere (1991) arranged the chromosomes only by size, but in Feldberg et al. (1992) and the present study the chromosomes were put in order first by type. Moreover the precise identification of the NOR-bearing pair is hindered by homogeneity in chromosome size (Venere and Galetti Jr. 1989). Therefore, the NORs in this five species could be in the same pair. In relation to the distribution of C-positive constitutive heterochromatin, centromeric- and telomeric C-bands were observed in this study. C. dorsalis and P. curviventris showed mainly centromeric staining (Figs. 2A, B), a pattern similar to that observed in four species of (Venere 1991) and in Potamorhina pristigaster (Feldberg et al. 1993). S. brevipinna revealed a smaller amount of heterochromatin and staining in centromeric and telomeric regions (Fig. 2C), similar to the pattern described for S. insculpta by Venere (1991). The occurrence of very similar patterns in species of the first four genera suggests that it could be a plesiomorphic state for the family, the condition described for the two species of Steindachnerina being apomorphic. Venere (1991) reported intensely stained C-bands on the second pair of three species of Cyphocharax and S. insculpta, suggesting that this could be a basal element in the Curimatidae. In the present study similar C-bands were observed in P. curviventris and C. dorsalis, but not in S. brevipinna. A big metacentric with strong staining on the centromeric region was observed in 4th pair of C. dorsalis and P. curviventris and 5th pair of S. brevipinna, a condition similar to what Venere (opus cit.) described for 5th pair of Cyphocharax sp. and 6th pair of S. insculpta, 246 M. C. Navarrete and H. F. Julio Jr. Cytologia 62 suggesting that this could be another homeologous chromosome. In all species studied by Venere (1991), NORs are adjacent to or coincident with heterochromatic blocks as was the case for P. curviventris, C. dorsalis and S. brevipinna. Despite the macrostructural stability of the karyotype in the family, some species do show variation in diploid number, a condition considered to be derived by Galetti et al. (1994). Cyphocharax sp. (2n = 58, Venere 1991), ocellata (2n = 56, Feldberg et al. 1992), Potamorhina altamazonica (2n =102, Feldberg et aL 1993), P. latior (2n = 56, Feldberg et al. 1993) and P. squamoralevis (2n = 102, Navarrete 1996) reveal increase of chromosome number due to centric fission. Curimatopsis, however, shows reduction in diploid number (Table 1), suggesting that rearrangements involving fusion or deletion were also very important in the karyotypic evolution of this genus. Vari (1989) presented a cladogram for the genus Curimatopsis where C. myersi and C. macrolepis belong to lineages that diverged during the first vicariant event in the genus. This could suggest that the diploid number of 46 chromosomes is basal in Curimatopsis. The haploid number of 26 in one lineage of C. macrolepis could be a derived condition or an artifact of the method. According to Kirpichnikov (1981), reductions in chromosome numbers (by centric fusion or deletion) are, in many cases, related with specialization, but can also result of adaptations to specific environmental conditions or necessity of adjustment of metabolic level. Most species of Curimatopsis show smaller sizethan what is found in the other genera of the family, a condition that Vari (1989) regarded as derived.

Summary Four species of Curimatidae, collected in lakes situated in the floodplain of the Miranda river (Pantanal-MS), were analyzed cytogenetically. Karyotypes of Curimatella dorsalis, Psectrogaster curviventris and Steindachnerina brevipinna were characterized by 2n = 54 meta- centric and submetacentric chromosomes, one pair of chromosomes bearing NORs in terminal position (46 M/8 SM and 13th, 42 M/12 SM and 20th, 48 M/6 SM and 17th, respectively). The first two species showed constitutive heterochromatin on the centromeric region of some chromosomes and in the other species heterochromatic blocks were observed on the centrom- eric and telomeric regions of a few chromosomes. The karyotype of Curimatopsis myersi showed 2n = 46 (42 M/4 SM), one pair bearing terminal NORs, representing the lowest diploid number for the family.

Acknowledgements The authors are grateful to H. A. Britski, for confirming the identities of the species studied, to an anonymous reviewer for sussestions on the manuscript, and to M. Uetanabaro, J. C. Louzan and O. Froehlich for assistance with specimen collection. This work was supported by CAPES, CECITEC and the Federal University of Mato Grosso do Sul.

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