Evolutionary history and hybridization in Passerina buntings
Understanding the genetic basis of speciation is of fundamental importance to evolutionary biology and hybrid zones offer unique natural laboratories in which to investigate the ecological and evolutionary processes important in creating and maintaining biological diversity. By comparing introgression patterns of different loci, researchers can begin to identify genetic regions that contribute to reproductive isolation between hybridizing taxa. In taxa, like birds, with heterogametic females, Haldane’s rule predicts that mtDNA and z-linked loci will introgress less than autosomal loci. I tested this prediction using the hybrid zone between Passerina cyanea (Indigo Bunting) and Passerina amoena (Lazuli Bunting), two species that hybridize where their breeding ranges overlap in the Great Plains of North America. Although a recent mtDNA-based phylogenetic hypothesis of the genus Passerina suggested these two species are not sister taxa, I found, using DNA sequence data from ten nuclear loci, that they are more closely related to each other than either is to P. caerulea (the mtDNA sister to P. ameona). Both cline-based and coalescent-based analyses of mtDNA (two genes), z-linked (two loci), and autosomal (four) loci indicated a reduction in introgression of both mtDNA and z-linked loci, relative to autosomal loci. These patterns, consistent with the predictions of Haldane’s rule, suggested the sex-chromosomes may play a large role in reproductive isolation between P. cyanea and P. amoena. Using DNA sequence data from an additional eight z-linked loci, I explored patterns of differential introgression of ten z-linked loci. Introgression of one z-linked locus, VLDLR9, was significantly less than introgression of the other nine loci, pointing to a candidate region for reproductive isolation between P. cyanea and P. amoena. Interestingly, VLDLR9 is an intron of the very-low density lipoprotein receptor, which plays an active role in egg laying. Additionally, in a particular strain of chickens, a point mutation in the VLDLR gene produces females that do not lay eggs. While my data are insufficient to adequately address the role VLDLR may play in maintaining reproductive isolation between P. cyanea and P. amoena, the hypothesis that female buntings may have trouble laying eggs warrants further investigation.
introgression, speciation, Indigo Bunting, Lazuli Bunting
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