Giant Sex Chromosomes in IOmophoita/I Species

Unlike most beetles that have small XY sex chromosomes, the Oedionychina group (Coleoptera, Alticinae) has atypical sex chromosomes, which are called giant sex chromosomes because they are much larger than the autosomes. There is little knowledge about the origin and differentiation of these sex chromosomes. We used Omophoita as a model to investigate the evolution of such giant sex chromosomes, using comparative genomic hybridization (a technique that compares genomes of species by co-hybridizing into chromosomes) and chromosome painting with a probe derived from both X and Y sex chromosomes. It was verified that there is a high degree of similarity between the genomes of male and female O. octoguttata, with a sex-specific region on the Y chromosome. The comparison between the genomes of the species showed that their autosomes are quite different from each other. However, when comparing the sex chromosomes by chromosome painting, it was shown that the sex chromosomes are similar. Based on these results, it can be inferred that the Omophoita sex chromosomes arose and evolved by the same process, and that due to the high degree of similarity between them, there has not yet been enough time for great divergence between the sex chromosomes. The beetles of the subtribe Oedionychina (Chrysomelidae, Alticinae) are the only ones that have the atypical giant and achiasmatic sex chromosomes, which are substantially larger than the autosomes. Previous cytogenetic analyses suggest a large accumulation of repetitive DNA in the sex chromosomes. In this study, we examined the similarity of X and Y chromosomes in four Omophoita species and compared genomic differentiation to better understand the evolutionary process and the giant sex chromosomes origin. Intraspecific genomic comparation using male and female genomes of O. octoguttata and interespecific analyses using genomic DNA of O. octoguttata, O. sexnotata, O. magniguttis, and O. personata were performed. In addition, whole chromosome painting (WCP) experiments were performed with X and Y chromosome probes of O. octogutatta. CGH analysis revealed great genomic similarity between the sexes and a sex-specific region on the Y chromosome, and interspecific analysis revealed a genomic divergence between species. In contrast, WCP results revealed that the sex chromosomes of O. octoguttata have high intra- and interspecific similarity with the studied species. Our data support a common origin under the canonical evolution