Simple SummaryUnlike 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 usedOmophoitaas 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 femaleO. 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 theOmophoitasex 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.AbstractThe 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 fourOmophoitaspecies and compared genomic differentiation to better understand the evolutionary process and the giant sex chromosomes origin. Intraspecific genomic comparation using male and female genomes ofO. octoguttataand interespecific analyses using genomic DNA ofO. octoguttata,O. sexnotata,O. magniguttis, andO. personatawere performed. In addition, whole chromosome painting (WCP) experiments were performed with X and Y chromosome probes ofO. 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 ofO. octoguttatahave high intra- and interspecific similarity with the studied species. Our data support a common origin under the canonical evolution of the sex chromosomes in this group, as they have high genomic similarity between them.