Chromosomal mapping of repetitive DNA and cytochrome C oxidase I sequence analysis reveal differentiation among sympatric samples of Astyanax fasciatus (Characiformes, Characidae).

The genus Astyanax is a specious Neotropical fish group, occurring from the south area of the United States to Argentina. During the past few years, intensive studies on representatives of this group have been performed, broadening the genetic, taxonomic and biogeographical knowledge considerably. However, phylogenetic relationships among many species remain uncertain. Significant karyotypic variations occur in A. bimaculatus, A. scabripinnis and A. fasciatus, suggesting the existence of species complexes. Samples with different diploid numbers may be found in A. fasciatus and, in some cases, morphological differentiation among them is not easily verified due to the absence of clear diagnostic characteristics. In the present study, the organization of the A. fasciatus genome from the Tietê River, Mogi das Cruzes, SP, Brazil, was analyzed, describing the karyotype structure and the chromosomal mapping of 18S and 5S ribosomal DNA (rDNA), H1 and H3 histones genes, telomeric DNA, and Rex3 transposable element. Sequencing of the cytochrome c oxidase I (COI) gene was also performed. Chromosomal analyses highlighted the occurrence of 2 distinct karyomorphs in the referred population, one with 2n = 46 and the other with 2n = 48 chromosomes. Furthermore, physical mapping of the repetitive sequences and nucleotide analysis of the COI gene reinforce the genomic differentiation between those karyomorphs and corroborate the existence of 2 different lineages morphologically similar to A. fasciatus living in sympatry.

Common descent of B chromosomes in two species of the fish genus Prochilodus (Characiformes, Prochilodontidae).

To ascertain the origin of B chromosomes in 2 fish species of the genus Prochilodus, i.e. P. lineatus and P. nigricans, we microdissected them and generated B-specific DNA probes. These probes were used to perform chromosome painting in both species and in 3 further ones belonging to the same genus (P. argenteus, P. brevis and P. costatus). Both probes hybridized with the B chromosomes in P. lineatus and P. nigricans, but with none of the chromosomes in the 5 species. This indicates that the B chromosomes have low similarity with DNAs located in the A chromosomes and suggests the possibility that the B chromosomes in the 2 species have a common origin. The most parsimonious explanation would imply intergeneric hybridization in an ancestor of P. lineatus and P. nigricans yielding the B chromosome as a byproduct, which remained in these 2 species after their phylogenetic origin, but was perhaps lost in other Prochilodus species. This hypothesis predicts that B chromosomes are old genomic elements in this genus, and this could be tested once a species from a relative genus would be found showing homology of its A chromosomes with the B-probes employed here, through a comparison of B chromosome DNA sequences with those in the A chromosomes of this other species.

Chromosomal diversification in populations of Characidium cf. gomesi (Teleostei, Crenuchidae).

Comparative cytogenetic studies carried out in two populations of Characidium cf. gomesi from Botucatu region, SP, Brazil, showed a similar karyotypic structure in a diploid number of 50 chromosomes, 32 metacentric and 18 submetacentric chromosomes for males and 31 metacentric and 19 submetacentric chromosomes for females as well as a ZZ-ZW sex chromosome system. Differences between both populations, however, were found in relation to the occurrence of B chromosomes and the distribution of 18S and 5S ribosomal DNA (rDNA) sites. Characidium cf. gomesi from the Alambari Stream, a component of the Tietê River basin, revealed 18S rDNA on Z and W chromosomes, while this gene was located on autosomes in the sample from the Paranapanema River basin. The 5S rDNA sites were observed in a single chromosomal pair (number 25) in the populations from Paranapanema and in two pairs in the specimens from Tietê (numbers 20 and 25). Besides that, in the sample from Paranapanema, both inter and intra-individual variations were found due to the occurrence of up to four heterochromatic supernumerary chromosomes in the cells. The life mode of this fish, restricted to headwaters and subjected to frequent breakdown into sub-populations, may have contributed to the fixation of such chromosomal differences. The karyotypic similarities found in the analysed populations, however, suggest that all are descended from the same ancestor group whereas their differences indicate that they are already existing in reproductively isolated populations.