Poster Presentation Society for Molecular Biology and Evolution Conference 2016

Evolutionary dynamics of old, homomorphic sex chromosomes in Paleognathous birds. (#533)

Tim Sackton 1 , Luohao Xu 2 , Scott Edwards 1
  1. Harvard University, Cambridge, MA, United States
  2. Dept of Evolutionary Biology, Uppsala University, Uppsala

In species with stable systems of chromosomal sex determination, sex chromosomes typically evolve recombination suppression, which is followed by degeneration of the heteromorphic chromosome. In most birds, which have a ZW sex determination system, the endpoint of this process is a highly degenerate female-specific W chromosome with a typically very small pseudo-autosomal region (PAR), leading to heterogametic (ZW) females and homogametic (ZZ) males. However, many paleognathous birds (including the flightless ratites and volant tinamous), have old, homomorphic sex chromosomes, with large PARs and largely non-degenerate W chromosomes. The mechanisms by which old, homomorphic sex chromosomes can be maintained remain mysterious. To better understand the evolutionary dynamics of Paleognath sex chromosomes, we have identified Z-linked sequence from 10 newly sequenced Palaeognaths, including 7 flightless ratites and 3 tinamous. We annotated the PAR in a subset of these species, revealing considerable variation in the extent of the PAR and sex chromosome degeneration across this group. Transcriptome sequencing in Chilean tinamou and emu reveals surprising, albeit highly preliminary, evidence for complete dosage compensation in adult emus via down-regulation of male genes in the degenerated region, in contrast to Chilean tinamous and other previously studied birds which lack complete dosage compensation. Consistent with many other taxa, we find evidence for faster evolutionary rates of proteins on the Z chromosome compared to autosomes; unexpectedly, we find that in Palaeognaths genes in both the PAR and the degenerated region show elevated rates of protein evolution compared to autosomes. This pattern is consistent with reduced recombination rates in the PAR (reducing the efficacy of selection), and suggests that palaeognath sex chromosomes may experience reduced recombination rates without subsequent degeneration. Taken together, our work suggests that the evolution of old, homomorphic sex chromosomes in palaeognaths may be an exception to standard models of sex chromosome evolution.