Abstract
Karyotypes of seven fish species of the genus Characidium, three of them studied for the first time, were characterized using conventional cytogenetic techniques (Giemsa staining, Ag-NOR, and C-banding). All species presented a diploid number of 2n=50, with only metacentric and submetacentric chromosomes, as observed in all Characidium species studied. In two species cells with one to three B chromosomes were observed. All species analyzed have a single NOR-bearing chromosome pair with morphological differences among them. Characidium cf. zebra shows heterochromatic blocks restricted to the pericentromeric regions of all chromosomes denoting the absence of a sex chromosome system. On the other hand, the species Characidium lanei, C. pterostictum, C. lauroi, C. oiticicai, C. schubarti, and Characidium sp., besides presenting pericentromeric heterochromatic blocks, exhibited large interstitial and/or terminal heterochromatic blocks, and a ZZ/ZW sex chromosome system. The constitutive heterochromatin seems to play a relevant role in the chromosome differentiation process of the studied species, mainly in relation to the sex chromosomes. The geographical isolation of the rivers in which the species were sampled, associated with their way of life restricted to headwaters environments, may have favored the process of fixation of different karyotypes found in each of the analyzed species.
Highlights
Fish represent the sister group to modern vertebrates and most species are gonochoristic and do not present differentiated sex chromosomes (Devlin & Nagahama, 2002) or are hermaphrodite
Characidium cf. zebra presented a small amount of constitutive heterochromatin restricted to the pericentromeric areas of all chromosomes (Fig. 2b)
The karyotypes of C. oiticicai, C. schubarti, and Characidium sp. were studied for the first time, increasing the number of species karyotyped in the genus to ten
Summary
Fish represent the sister group to modern vertebrates and most species are gonochoristic and do not present differentiated sex chromosomes (Devlin & Nagahama, 2002) or are hermaphrodite. Fish species that present morphologically differentiated sex chromosomes, mainly those found in the Neotropical region (Oliveira et al, 2007), reveal a great variability of systems. The differentiated sex chromosomes may have developed independently in several lineages of Neotropical fishes, because their occurrence is sporadic and probably recent in certain groups. In some families, such as Erythrinidae and Parodontidae, different heteromorphic sex chromosome systems were identified among their representatives (Moreira-Filho et al, 1993; Born & Bertollo, 2000).
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