Repetitive DNA chromosomal organization in the cricket Cycloptiloides americanus: a case of the unusual X1X20 sex chromosome system in Orthoptera

A common placement for most sex chromosomes that is involved in their evolutionary histories is the accumulation of distinct classes of repetitive DNAs. Here, with the aim of understanding the poorly studied repetitive DNA organization in crickets and its possible role in sex chromosome differentiation, we characterized the chromosomes of the cricket species Cycloptiloides americanus, a species with the remarkable presence of the unusual sex chromosome system X(1)X(2)0a (TM),/X(1)X(1)X(2)X(2)a (TM) Euro. For these proposes, we used C-banding and mapping through the fluorescence in situ hybridization of some repetitive DNAs. The C-banding and distribution of highly and moderately repetitive DNAs (C (0)t-1 DNA) varied depending of the chromosome. The greater accumulation of repetitive DNAs in the X-2 chromosome was evidenced. The microsatellites were spread along entire chromosomes, but (AG)(10) and (TAA)(10) were less enriched, mainly in the centromeric areas. Among the multigene families, the 18S rDNA was spread throughout almost all of the chromosomes, except for pair 5 and X-2, while the U2 snDNA was placed exclusively in the largest chromosome. Finally, the 5S rDNA was exclusively located in the short arms of the sex chromosomes. The obtained data reinforce the importance of chromosomal dissociation and inversion as a primary evolutionary mechanism to generate neo-sex chromosomes in the species studied, followed by the repetitive DNAs accumulation. Moreover the exclusive placement of 5S rDNA in the sex chromosomes suggests the involvement of this sequence in sex chromosome recognition throughout meiosis and, consequently, their maintenance, in addition to their avoiding degeneration.