Karyotype and sex chromosomes evolution in neotropical fishes

Abstract

Fish species are a model for the analysis and understanding of various evolutionary processes among vertebrates, considering the basal position they occupy in this group. In the Erythrinidae family, Hoplias malabaricus likely represents a species complex, with 02 large evolutionary groups (I and II) concerning their karyotype composition. At least 04 karyomorphs (A-D) comprise the group I and 03 karyomorphs (E-G) comprise the group II. A variety of sex chromosome systems can be found in this "nominal species", including sex chromosomes in early stages of differentiation, highly differentiated sex chromosomes, as well as the absence of heteromorphic sex chromosomes. The evolutionary relationship of sex chromosomes in karyomorphs of the Group I was analyzed in our previous studies (FAPESP Process. 2012 / 01778-2). On the contrary, Hoplias lacerdae group appears to show an unexpected pattern of karyotype evolution among the Erythrinidae family, pointing to both numerical and structural conservation and absence of heteromorphic sex chromosomes. This fact is particularly noteworthy considering that H. malabaricus and H. lacerdae are congeneric groups and, therefore, closely related. This research aims at continuing the evolutionary investigations in this Neotropical family, now focusing in karyomorphs of the Group II of H. malabaricus and in species of H. lacerdae group. It will give emphasis to the likely emergence of an XX/XY sex system in the karyomorph F of H. malabaricus, which may be closely related to XX/XY1Y2 sex system found in the karyomorph G. In turn, the species of the group H, lacerdae will be investigated regarding their karyotype evolution in the light of different chromosomal markers and based on the recent taxonomic revision that identified 05 different species composing this group. Additionally, further investigation on the sex chromosome´s differentiation process among fishes will be focused on species of the genus Triportheus (Characidae), which has been studied by our research group for several years. The species of this genus bear a remarkable ZZ / ZW sex chromosome system, probably basal to the genus, with the conservation of the Z chromosome and the accumulation of different classes of repetitive DNA on the W chromosome. In addition to expanding the knowledge of the karyotype evolution of species not yet studied, it will be deepened the differentiation process of the sex-specific chromosome by analyzing its homology among different species. In summary, excellent evolutionary scenarios are found in the models presented by H. malabaricus and H. lacerdae groups and the genus Triportheus, which will be addressed by procedures of the modern molecular cytogenetic, including chromosome microdissection, chromosome painting by multicolor FISH (m-FISH) and chromosomal mapping of repetitive sequences. The expected results will enable to improve the understanding of the karyotype evolution, especially regarding the differentiation of the sex chromosomes in lower vertebrates, the characterization of species complexes and the biodiversity of the Neotropical ichthyofauna. (AU)