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Speciation in Passerina buntings: introgression patterns of sex-linked loci identify a candidate gene region for reproductive isolation



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      Present address: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, 14853, USA; Cornell Laboratory of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY, 14850, USA


    1. Museum of Natural Science, 119 Foster Hall, Louisiana State University, Baton Rouge, LA, 70803, USA; Department of Biological Sciences, Louisiana State University, Baton Rouge, LA, 70803, USA
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Matthew D Carling, Fax: 607–254–2486; E-mail:


Sex-chromosomes are thought to play an important role in speciation, but few studies of non-model organisms have investigated the relative influence of multiple sex-linked markers on reproductive isolation. We collected 222 individuals along a geographical transect spanning the hybrid zone between Passerina amoena and P. cyanea (Aves: Cardinalidae). Using maximum-likelihood cline fitting methods, we estimated locus-specific introgression rates for 10 z-linked markers. Although the cline width estimates ranged from 2.8 to 584 km, eight of 10 loci had cline widths between 224 and 271 km. We also used coalescent-based estimates of locus-specific divergence times between P. amoena and P. cyanea to test a recently proposed hypothesis of an inverse relationship between divergence time and cline width but did not find a significant association. The narrow width (2.8 km) of the cline estimated from the VLDLR9 locus indicates strong selection retarding introgression of alleles at this locus across the hybrid zone. Interestingly, a mutation in the very low density lipoprotein receptor (VLDLR) gene, in which VLDLR9 is an intron, is known to reduce the egg-laying ability of some chickens, suggesting a possible link between this gene region and reproductive isolation between P. amoena and P. cyanea. These results underscore the importance of sampling multiple loci to investigate introgression patterns across a chromosome or genome and support previous findings of the importance of sex-linked genes in speciation.