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

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.