Diversification of a ZZ/ZW sex chromosome system in Characidium fish (Crenuchidae, Characiformes)

Abstract

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.

Interspecific chromosomal divergences in the genus Characidium (Teleostei: Characiformes: Crenuchidae)

“…Thus, the heterochromatinization mechanism appears to play an important role in the differentiation of W chromosome from the protosex chromosome among populations/species in an independent pathway. In the present study, the identification and analysis of sexual chromosomes in C. lanei, C. pterostictum, C. lauroi, C. oiticicai, C. schubarti, and Characidium sp., which characterize Z and W heteromorphic chromosome systems in distinct stages of differentiation, supports the propositions postulated by Centofante et al (2001Centofante et al ( , 2003 and Vicari et al (2008) that heterochromatinization events could have occurred in the ancestors of these species and that sex chromosomes differentiated independently during the speciation process. This could explain the differences found among the level of heterochromatinization in some species analyzed, possibly reinforcing the hypothesis of a common origin for heteromorphic sex chromosomes followed by an independent process of differentiation in the Characidium species.…”

“…The species studied presented a conserved karyotypic macrostructure, mainly in relation to their diploid number and the presence of metacentric and submetacentric chromosomes. Nevertheless, the presence of B chromosomes in C. pterostictum and C. oiticicai, the occurrence of sex chromosomes in most of the analyzed species, the occurrence of NORs associated with sex chromosomes, and the occurrence of a pair of acrocentric chromosomes in C. pterostictum (Table 2) reinforce the evidence already postulated for some species of Characidium, which suggested the existence of a great chromosome variability among the representatives of this genus (Maistro et al, 1998(Maistro et al, , 2004Centofante et al, 2001Centofante et al, , 2003Vicari et al, 2008;Noleto et al, 2009).…”

“…The analysis of Characidium cf. gomesi from the Quebra Perna Stream, Tibagi River basin, conducted by Vicari et al (2008) registered another occurrence of ZZ/ZW chromosomes, where Z was represented by a metacentric chromosome with pericentromeric heterochromatin and W by a subtelocentric chromosome totally heterochromatic.…”

“…The heterochromatinization process, when observed in one of the chromosomes of the previously homomorphic pair in a region adjacent to the sexual determining sequence, enables the suppression of recombination events and favors the independent evolution of this chromosome and/or chromosome region (John, 1988;Rice, 1996;Steinemann & Steinemann, 1992). Vicari et al (2008) consider the existence of a protosex chromosome pair for some Characidium species and, both the Z and W chromosomes would have undergone shape and size modifications, probably due to duplications, deletions and/ or inversions, in relation to the putative ancestor condition. Thus, the heterochromatinization mechanism appears to play an important role in the differentiation of W chromosome from the protosex chromosome among populations/species in an independent pathway.…”

“…A cladistic analysis suggested that Characidiinae and Crenuchinae belong to the monophyletic family Crenuchidae (Buckup, 1998). Cytogenetic studies in this group are scarce, recent, and limited to some species of the genus Characidium (Miyazawa & Galetti Jr., 1994;Maistro et al, 1998Maistro et al, , 2004Centofante et al, 2001Centofante et al, , 2003Silva & Maistro, 2006;Vicari et al, 2008;Noleto et al, 2009). The analyzed species present a conserved karyotypic macrostructure, showing a constant diploid number of 2n=50 chromosomes and a predominance of metacentric and submetacentric chromosomes.…”