Evolution and conservation of Characidium sex chromosomes.

Fish species exhibit substantial variation in the degree of genetic differentiation between sex chromosome pairs, and therefore offer the opportunity to study the full range of sex chromosome evolution. We used restriction-site associated DNA sequencing (RAD-seq) to study the sex chromosomes of Characidium gomesi, a species with conspicuous heteromorphic ZW/ZZ sex chromosomes. We screened 9863 single-nucleotide polymorphisms (SNPs), corresponding to ~1 marker/100 kb distributed across the genome for sex-linked variation. With this data set, we identified 26 female-specific RAD loci, putatively located on the W chromosome, as well as 148 sex-associated SNPs showing significant differentiation (average FST=0.144) between males and females, and therefore in regions of more recent divergence between the Z and W chromosomes. In addition, we detected 25 RAD loci showing extreme heterozygote deficiency in females but which were in Hardy-Weinberg equilibrium in males, consistent with degeneration of the W chromosome and therefore female hemizygosity. We validated seven female-specific and two sex-associated markers in a larger sample of C. gomesi, of which three localised to the W chromosome, thereby providing useful markers for sexing wild samples. Validated markers were evaluated in other populations and species of the genus Characidium, this exploration suggesting a rapid turnover of W-specific repetitive elements. Together, our analyses point to a complex origin for the sex chromosome of C. gomesi and highlight the utility of RAD-seq for studying the composition and evolution of sex chromosomes systems in wild populations.

Genetic differentiation among distinct karyomorphs of the wolf fish Hoplias malabaricus species complex (Characiformes, Erythrinidae) and report of unusual hybridization with natural triploidy.

In this study, genetic differentiation between karyomorphs A (2n = 42) and D (2n = 39/40) of the wolf fish Hoplias malabaricus, which is comprised of several cryptic species that present a wide variety of diploid chromosome numbers and sex chromosome systems, resulting in the identification of seven distinct karyomorphs (A-G), was investigated using a combination of molecular and cytogenetic tools. Deep sequence divergences for both karyomorphs were observed and indicate a long period of reproductive isolation between karyomorphs A and D. Additionally, one individual with 61 chromosomes was identified, which, as far as is known, is the first case of natural triploidy resulting from the hybridization between these highly differentiated karyomorphs of H. malabaricus. Molecular and cytogenetic analyses revealed that this allotriploid specimen carries two sets of maternal chromosomes from karyomorph D (2n = 40) and one set of chromosomes from karyomorph A (n = 21). Moreover, ribosomal sites and active nucleolus organizer regions from both parental contributors were found in the triploid hybrid. Considering the significant genetic distances between karyomorphs A and D, one of the primary reasons for the lack of recurrent reports of hybridization in the H. malabaricus species complex may be due to post-zygotic barriers, such as hybrid sterility or unviability.

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.

Molecular characterization and physical mapping of two classes of 5S rDNA in the genomes of Gymnotus sylvius and G. inaequilabiatus (Gymnotiformes, Gymnotidae).

The nucleotide sequences of the 5S rRNA multigene family and their distribution across the karyotypes in 2 species of Gymnotiformes, genus Gymnotus (G. sylvius and G. inaequilabiatus) were investigated by means of fluorescence in situ hybridization (FISH). The results showed the existence of 2 distinct classes of 5S rDNA sequences in both species: class I and class II. A high conservative pattern of the codifying region of the 5S rRNA gene was identified, contrasting with significant alterations detected in the nontranscribed spacer (NTS). The presence of TATA-like sequences along the NTS of both species was an expected occurrence, since such sequences have been associated with the regulation of the gene expression. FISH using 5S rDNA class I and class II probes revealed that both gene classes were collocated in the same chromosome pair in the genome of G. sylvius, while in that of G. inaequilabiatus, class II appeared more disperse than class I.