Chromosomal Radiation: a model to explain karyotypic diversity in cryptic species

Hypostominae is a subfamily of Loricariidae with great variation in color characters and external morphology. The genus Hypostomus presents the largest number of species ever karyotyped, with Hypostomus ancistroides characterized as a group of cryptic species. In the 15 natural populations of H. ancistroides studied, there are 15 different karyomorphs, with variations in diploid number, sex chromosome systems, and markers, such as C-banding and location of ribosomal cistrons. The objective of this work was to present molecular and chromosomal data of four new populations of Hypostomus ancistroides and to discuss the observed evolutionary trends for this group of a cryptic complex of species. We analyzed specimens from four sampling points in the Tiete, Mogi-Guacu, and Grande river basins, all in the state of Sao Paulo, southeastern Brazil. We performed techniques such as the detection of constitutive heterochromatin and ribosomal sites (5S and 18S), in addition to phylogenetic analyses. All specimens presented 2n=68 chromosomes without supernumerary elements or sex-related heteromorphisms. However, each population has a different karyotype with unique characteristics. The different karyomorphs are a consequence of the presence of Robertsonian rearrangements, such as centric fissions and pericentric inversions, which play an important role in the evolution of Hypostominae. Although variable in relation to the location of constitutive heterochromatin, we observed the presence of banding C in some chromosomes of all karyomorphs, which may indicate the existence of some homology. Another conservative feature is the presence of two pairs of subtelocentric or acrocentric chromosomes carrying 18S rDNA cistrons in the terminal region of the chromosomes. However, we observed the discontinuity of cytogenetic and phylogenetic data, with the formation of different groups (Araras + Indaiatuba and Botucatu + Terra Roxa in cytogenetics, in contrast to Araras + Terra Roxa and Botucatu and Indaiatuba in the phylogeny), suggesting that several derived karyomorphs may be produced from a pluripotent karyomorph as a result of the intrinsic plasticity of the species karyotype. Thus, each new arrangement would be independent in the forms analyzed, as they do not seem to be lineages from the same direct ancestor. Given the above, we believe that the genus Hypostomus continues to be one of the most diverse among the Siluriformes, however, we began to understand a little more about the karyotypic diversity of the group by associating different approaches, such as phylogenetic analyses.