,960 Carlos, SP, B r a d Artoni, R. F. and Bertollo, L. A. C. 2001. Trends in the karyotype evolution of Loricariidae fish (Siluriformes).-Hered-itas 134: 201-210. Lund, Sweden. ISSN 0018-0661. Six species of Loricariidae belonging to the subfamilies Hypostominae (Hypostomus emarginatus, Rhinelepsis aspera, Pogonopoma wertheimeri), Ancistrinae (Panaque cf. nigrolineatus, Hemiancistrus sp.) and Loricariinae (Sturisoma cf. nigrirostrum) were studied cytogenetically. The results show that 2n = 54 represents the basal diploid number for this fish family. Different trends in the karyotypic evolution can be seen among the subfamilies: Hypostominae and Loricariinae species present diversified karyotypic macrostructures, while the Ancistrinae appear to show more conserved karyotypes. Among the Hypostominae, the genus Hypostomus had a wide karyotypic variation (2n = 52 to SO), where centric fissions seem to play an important role in this chromosomal divergence. The nucleolar organizing regions were diversified, and occurrence of multiple NORs was frequent. Heteromorphic chromosomes belonging to distinct sex chromosome systems can also occur infrequently among the Loricariidae.

BackgroundThe fish, Erythrinus erythrinus, shows an interpopulation diversity, with four karyomorphs differing by chromosomal number, chromosomal morphology and heteromorphic sex chromosomes. Karyomorph A has a diploid number of 2n = 54 and does not have differentiated sex chromosomes. Karyomorph D has 2n = 52 chromosomes in females and 2n = 51 in males, and it is most likely derived from karyomorph A by the differentiation of a multiple X1X2Y sex chromosome system. In this study, we analyzed karyomorphs A and D by means of cytogenetic approaches to evaluate their evolutionary relationship.ResultsConspicuous differences in the distribution of the 5S rDNA and Rex3 non-LTR retrotransposon were found between the two karyomorphs, while no changes in the heterochromatin and 18S rDNA patterns were found between them. Rex3 was interstitially dispersed in most chromosomes. It had a compartmentalized distribution in the centromeric regions of only two acrocentric chromosomes in karyomorph A. In comparison, in karyomorph D, Rex3 was found in 22 acrocentric chromosomes in females and 21 in males. All 5S rDNA sites co-localized with Rex3, suggesting that these are associated in the genome. In addition, the origin of the large metacentric Y chromosome in karyomorph D by centric fusion was highlighted by the presence of internal telomeric sites and 5S rDNA/Rex3 sites on this chromosome.ConclusionWe demonstrated that some repetitive DNAs (5S rDNA, Rex3 retroelement and (TTAGGG)n telomeric repeats) were crucial for the evolutionary divergence inside E. erythrinus. These elements were strongly associated with the karyomorphic evolution of this species. Our results indicate that chromosomal rearrangements and genomic modifications were significant events during the course of evolution of this fish. We detected centric fusions that were associated with the differentiation of the multiple sex chromosomes in karyomorph D, as well as a surprising increase of associated 5S rDNA/Rex3 loci, in contrast to karyomorph A. In this sense, E. erythrinus emerges as an excellent model system for better understanding the evolutionary mechanisms underlying the huge genome diversity in fish. This organism can also contribute to understanding vertebrate genome evolution as a whole.

Karyotype and other chromosomal markers of Characidium cf. gomesi were analyzed using conventional (Giemsa-staining, Ag-NOR and C-banding) and molecular (Fluorescent in situ hybridization (FISH) with 18S and 5S rDNA biotinylated probes) techniques. Both sexes had invariably diploid chromosome number 2n = 50 while karyotypes of males and females differed. That of male consisted of 32 metacentric + 18 submetacentric chromosomes and that of female consisted 31 metacentric + 18 submetacentric + 1 subtelocentric chromosomes. The Z chromosome was medium-sized metacentric, while W was highly heterochromatinized subtelocentric element. NORs as revealed by Ag-staining were situated at 2-7 telomeric regions while FISH with 18S probes showed consistently 10 signals at telomeric regions. FISH with 5S rDNA probe showed constantly signals at one metacentric pair. Distribution of centromeric heterochromatin was mostly in all chromosome pairs, besides some telomeric sites. The common origin of the sex chromosome system of ZZ/ZW type in the karyotypes of other representatives of the genus analyzed so far might be hypothesized based on biogeography and partial phylogeny of the group.

Astyanax fasciatus may be characterized as a chromosomally diversified ‘species’ presenting distinct cytotypes, each with its specific variants. The sympatric and syntopic occurrence of different cytotypes reinforces the hypothesis in which A. fasciatus may represent a group of species currently placed under a single common designation. Specimens from three collection points spread along the Mogi-Guaçu River in southeast Brazil were examined in the present work: (1) near its headwaters (Ouro Fino – MG), (2) in the middle region of the river (Cachoeira de Emas, Pirassunun ga – SP) and (3) close to its confluence with the Pardo River (Barrinha – SP). The 2n = 48 chromosomes cytotype was found in all sampling points, while cytotype 2n = 46 was only encountered in Barrinha and Cachoeira de Emas. In the latter locality, cytotype 2n = 46 predominated; nevertheless, other karyotype forms with 2n = 45 and 47 chromosomes also occurred, besides a structural variant of cytotype 2n = 46. One specimen with 2n = 47 chromosomes was also found in Ouro Fino. The Ag-NOR analysis, as well as the location of the 18S and 5S ribosomal genes, were conserved in all cytotypes. The data indicate that the variant karyotypes are a consequence of interbreeding between the standard cytotypes (2n = 46 and 48) and/or its descendants. This suggests a karyotype plasticity for this species, where at least a few variant karyotypes would not have deleterious effects on their bearers.

We present cytogenetic analyses of four fish species, belonging to four Loricariidae subfamilies: Neoplecostomus microps (Neoplecostominae) with 2n ¼ 54 chromosomes, Harttia loricariformis (Loricariinae) with 2n ¼ 56 chromosomes, Hypostomus affinis (Hypostominae) with 2n ¼ 66 chromosomes and Upsilodus sp. (Upsilodinae), with 2n ¼ 96 chromosomes. In addition to karyotypes, data on the location of 18s rDNA sites are presented, derived from indirect (silver nitrate impregnation) and direct (FISH) methods. There is only one pair of nucleolar organizing regions (NORs) per species, except in H. affinis. Diversity and NOR macrokaryotypic evolution in the species analyzed are discussed in relation to the evolution of the Loricariidae as a whole. In addition, a revision of the cytogenetic data available for this family is presented.

A chromosome analysis was carried out in two sympatric fish species of the genus Parodon, Parodon sp. and P. tortuosus, from the Paraná basin, São Paulo State, Brazil. Although both species showed the same diploid number (2n=54), an interspecific diversity was detected concerning their karyotypic formulas and banding patterns, besides a ZZ/ZW sex chromosome system detected in Parodon sp., which was caracterized as a new species for this genus. No heteromorphic sex chromosomes were found in P. tortuosus. These data are discussed concerning the characterization of the regional ictiofauna and its evolutionary aspects.

SUMMARY -Some species of Hypostomus from the Upper Parana Basin (SP) were studied cytogenetically. The diploid chromosome number ranged from 2n = 68 to 2n = 80, with a considerably variable karyotypic structure between species, but no chromosomal differences were observed between males and females. The nucleolar organizer regions (NORs) were also quite variable both intra-and interspecifically in terms of number and size. In general, constitutive heterochromatin was not very abundant and usually located in small blocks. The data suggest that some chromosomal rearrangements, as the robertsonian ones and pericentric inversions, were important for the karyotypic evolution of Hypostomus. These fishes also appear to be a good material for cytotaxonomic studies.

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