Evidence for meiotic drive as an explanation for karyotype changes in fishes.

The process of preferential chromosome segregation during meiosis has been suggested to be responsible for the predominance of certain chromosome types in the karyotypes of mammals, birds and insects. We developed an extensive analysis of the fixation of mono- or bibrachial chromosomes in the karyotypes of the large Actinopterygii fish group, a key link in the evolution of terrestrial vertebrates, in order to investigate the generality of meiotic drive in determining karyotypic macrotrends. Unlike mammals, fishes have markedly undergone several types of preferential chromosomal rearrangements throughout evolution. Data from the analyzed orders indicate a prevalence of karyotypes with few (<33%) or many (>66%) acrocentric chromosomes and a low number of karyotypes with balanced numbers of mono- and bi-brachial elements. Parallel trends towards a higher number of karyotypes with prevalence of monobrachial chromosomes occurred in phylogenetically close orders (e.g. Perciformes and Tetraodontiformes, and in the order Mugiliformes) and in clades with prevalence of bibrachial elements (e.g. Characiformes, Gymnotiformes, Siluriformes, and Cypriniformes). Some orders where fewer species were available for study, such as Atheriniformes and Anguilliformes, showed karyotype assemblages where both trends were present. Our results strongly suggest a primary role of meiotic drive in karyotypic evolution as indicated by the accumulation of monobrachial chromosomes in Perciformes and Cypriniformes, or bibrachial chromosomes in Siluriformes and Characiformes. Further examinations of the interaction between life history traits, environmental characteristics, and the fixation of chromosomal rearrangements would be exceedingly valuable.

Mapping of ribosomal genes and chromosomal markers in three species of the genus Serrasalmus (Characidae, Serrasalminae) from the Amazon basin

Karyotypic characteristics of three species of the genus serrasalmus (S. altispinnis, S. gouldingi and S. Serrulatus) from the middle and lower Negro River, Amazon Basin, were investigated using different staining techniques and Fluorescent in situ hybridization with 5S and 18S rDNA probes. The diploid number was invariably 2n = 60 and the fundamental number was FN = 110. Nevertheless, the karyotypes differed from each other in composition: 24m, 20sm, 6st, 10a in S. altispinnis; 22m, 22sm, 6st, 10a in S. gouldingi and 20m, 22sm, 8st, 10a in S. serrulatus. The karyotype of S. altispinnis differed from the one previously described in a population from the Pitinga River. C-positive constitutive heterochromatin was mainly pericentromeric in the karyotypes of all species. Nucleolar organizer regions were multiple and preferentially located terminally on the short arms of the subtelocentric/acrocentric chromosomes, as evidenced by both silver nitrate staining and fluorescent in situ hybridization with the 18S rDNA probe. The maximum number of NORs varied among species, as did the NOR-bearing chromosomes. FISH with the 5S rDNA probe produced an interstitial signal on the long arms of the pair 7 in all species, coincident with a C-positive heterochromatic band. While some chromosome features were shared by the three species, some were species-specific and thus useful for cytotaxonomy.