Sex and neo-sex chromosome evolution in beetles.

Beetles are the most species-rich group of animals and harbor diverse karyotypes. Most species have XY sex chromosomes, but X0 sex determination mechanisms are also common in some groups. We generated a whole-chromosome assembly of Tribolium confusum, which has a neo-sex chromosome, and utilize eleven additional beetle genomes to reconstruct karyotype evolution across Coleoptera. We identify ancestral linkage groups, termed Stevens elements, that share a conserved set of genes across beetles. While the ancestral X chromosome is maintained across beetles, we find independent additions of autosomes to the ancestral sex chromosomes. These neo-sex chromosomes evolve the stereotypical properties of sex chromosomes, including the evolution of dosage compensation and a non-random distribution of genes with sex-biased expression. Beetles thus provide a novel model to gain a better understanding of the diverse forces driving sex chromosome evolution. Author summary: The evolution of differentiated sex chromosomes (e.g., X and Y chromosomes in male heterogametic species) from ordinary autosomes has occurred many times in animals and plants. Investigation of sex chromosomes in key model species (such as mammals, birds, flies and worms) has revealed that sex chromosome evolution is driven by both general and lineage-specific forces, but a detailed investigation and understanding of sex chromosomes from many clades is lacking. Importantly, cytogenetic studies in beetles–the most species-rich group of animals—have shown that chromosomal sex determination is conserved across Coleoptera. However, the identity and conservation of sex chromosomes in beetles at the molecular level and the evolutionary processes shaping them have not been studied in a systematic way. We use genomic approaches to understand karyotype and sex chromosome evolution across Coleoptera to establish general principles underlying the evolution of sex chromosomes in the most speciose group of animals on earth. [ABSTRACT FROM AUTHOR]