Centromere repositioning explains fundamental number variability in the New World monkey genus Saimiri.

Cytogenetics has historically played a key role in research on squirrel monkey (genus Saimiri) evolutionary biology. Squirrel monkeys have a diploid number of 2n = 44, but vary in fundamental number (FN). Apparently, differences in FN have phylogenetic implications and are correlated with geographic regions. A number of hypothetical mechanisms were proposed to explain difference in FN: translocations, heterochromatin, or, most commonly, pericentric inversions. Recently, an additional mechanism, centromere repositioning, was discovered, which can alter chromosome morphology and FN. Here, we used chromosome banding, chromosome painting, and BAC-FISH to test these hypotheses. We demonstrate that centromere repositioning on chromosomes 5 and 15 is the mechanism that accounts for differences in FN. Current phylogenomic trees of platyrrhines provide a temporal framework for evolutionary new centromeres (ENC) in Saimiri. The X-chromosome ENC could be up to 15 million years (my) old that on chromosome 5 as recent as 0.3 my. The chromosome 15 ENC is intermediate, as young as 2.24 my. All ENC have abundant satellite DNAs indicating that the maturation process was fairly rapid. Callithrix jacchus was used as an outgroup for the BAC-FISH data analysis. Comparison with scaffolds from the S. boliviensis genome revealed an error in the last marmoset genome release. Future research including at the sequence level will provide better understanding of chromosome evolution in Saimiri and other platyrrhines. Probably other cases of differences in chromosome morphology and FN, both within and between taxa, will be shown to be due to centromere repositioning and not pericentric inversions.

Evolutionary plasticity of acipenseriform genomes.

Acipenseriformes is an order of ray-finned fishes, comprising 27 extant species of sturgeons and paddlefishes inhabiting waters of the Northern Hemisphere. The order has a basal position within Actinopteri (ray-finned fish minus polypterids) and is characterized by many specific morphological and genomic features, including high diploid chromosome numbers, various levels of ploidy between species, unclear sex determination, and propensity to interspecific hybridization. Recent advances in molecular genetics, genomics, and comparative cytogenetics produced novel data on different aspects of acipenseriform biology, including improved phylogenetic reconstructions and better understanding of genome structure. Here, we discuss the cytogenetic and genomic traits of acipenseriforms and their connection with polyploidization and tolerance to interspecific hybridization.