Scott Baird (Committee Member), Jeffrey Peters (Advisor), John Stireman (Committee Member)
Master of Science (MS)
Sex chromosomes are thought to be an important component of the genome associated with speciation and the buildup of reproductive isolation. Recent advances in sequencing technologies and improvements in population genetics and modeling techniques have made it possible to better assess genomic signatures of selection, genetic drift and gene flow in diverging lineages. Recent studies have shown elevated differentiation on the Z sex-chromosome between the Australian grey teal (Anas gracilis) and chestnut teal (Anas castanea). Here, we used next generation sequencing to scan ~3,400 autosomal loci and ~190 Z loci to examine genomic differentiation and signatures of selection and gene flow between grey and chestnut teals. We also inferred demographic history to assess gene flow and signatures of selection. We found weak differentiation in autosomal loci (mean FST = 0.008), but ~ 28-times higher differentiation on the Z-chromosome (mean FST =0.23). We also found that this higher differentiation was localized on the q-arm of the Z chromosome between 15-million and 40-million base pairs. Whereas we could not reject a strict isolation model on the Z chromosome, models that incorporated gene flow provided a significantly better fit for autosomal loci, which is consistent with reduced local gene flow as expected under a speciation with gene flow model. However, nucleotide diversity within chestnut teal was reduced within the region of elevated differentiation on the Z chromosome, which is more consistent with a selective sweep rather than reduced effective gene flow. Furthermore, linkage disequilibrium within this region was elevated with respects to other regions on the Z chromosome as well as more significantly elevated in chestnut teal than in grey teal. Although we cannot fully reject a model of speciation with gene flow, recent speciation seems a more plausible explanation of the nearly absent divergence of mitochondrial and autosomal DNA. Rather, these data suggest that a selective sweep coupled with strong linkage disequilibrium on the Z chromosome might have played a role in speciation.
Department or Program
Department of Biological Sciences
Year Degree Awarded
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