27. Nov 2019
Sex chromosome evolution in Palaeognathae
Datum: 27. November 2019 |
Sprecher: Zongji Wang, University of Vienna
Veranstaltungsort: I22 Lakeside View (I22.01)
Avian species are categorized into two large groups: the Palaeognathae (flightless ratites and volant tinamous) and the Neognathae. Although Palaeognathae only include a small number of species, they are characterized by male parental care, which involves reversed patterns of the sex roles and size dimorphism. Palaegonaths usually have homomorphic sex chromosomes with cytologically indistinguishable Z and W. The reason for this slow differentiation of sex chromosomes is unclear. Moreover, the phylogeny and evolutionary trajectory and of sex chromosomes across the Palaeognathae remains unresolved.
Here we sequenced the female genomes of 9 tinamou and 3 ratite birds, and analyzed a total of 15 Palaeognathae genomes. We identified about 85 Mb Z-linked and different lengths of W-linked scaffolds per species. This indicates a great diversity of sequence differentiation of sex chromosomes across Palaeognathae. We found that almost all ratite birds have long pseudo-autosomal regions (PARs), suggesting these species have a W chromosome that
retains the ancestral status of bird sex chromosomes. In contrast, the PARs and non-recombining regions of tinamou species exhibit a complex pattern, resulting from the suppression of recombination in an independent stepwise manner. We concluded that all Palaeognathae share one stratum with any other birds, and independently formed from one to three strata after their rapid speciation. Contrary to the classic notion, the youngest evolutionary stratum of Elegant crested tinamou formed without chromosomal inversion, and some of its encompassing W-linked genes in Chilean tinamou have unexpectedly upregulated their expression level in ovary, probably due to the female-specific selection. Intriguingly, we found that some CR1 subtypes (i.e. CR1-J1/2_Pass) are only enriched in tinamou Z chromosomes, which indicates the formation of evolutionary strata has also reshaped the TE landscape of the Z chromosomes.
We proposed that the reduced intensities of sexual selection derived from the unique male-only parental care system of Palaeognathae has important contribution to their low rate of sex chromosome divergence evolution. This study uncovers an unexpected complexity of sex chromosomes in Palaeognathae at the genomic level, thus greatly expands our knowledge of vertebrate ZW sex chromosomes evolution.