Abstract
Until recently, studies of divergence and gene flow among closely-related taxa were generally limited to pairs of sister taxa. However, organisms frequently exchange genes with other non-sister taxa. The “northern oriole” group within genus Icterus exemplifies this problem. This group involves the extensively studied hybrid zone between Baltimore oriole (Icterus galbula) and Bullock's oriole (I. bullockii), an alleged hybrid zone between I. bullockii and black-backed oriole (I. abeillei), and likely mtDNA introgression between I. galbula and I. abeillei. Here, we examine the divergence population genetics of the entire northern oriole group using a multipopulation Isolation-with-Migration (IM) model. In accordance with Haldane's rule, nuclear loci introgress extensively beyond the I. galbula–I. bullockii hybrid zone, while mtDNA does not. We found no evidence of introgression between I. bullockii and I. abeillei or between I. galbula and I. abeillei when all three species were analyzed together in a three-population model. However, traditional pairwise analysis suggested some nuclear introgression from I. abeillei into I. galbula, probably reflecting genetic contributions from I. bullockii unaccounted for in a two-population model. Thus, only by including all members of this group in the analysis was it possible to rigorously estimate the level of gene flow among these three closely related species.
Highlights
The study of phylogeography and population divergence is challenged by the difficulty of distinguishing between shared retained ancestral polymorphism and introgressed alleles (Nielsen and Wakeley 2001)
Slow and stochastic lineage sorting causes a pattern of allele sharing among species that can seem similar to patterns caused by gene flow through introgressive hybridization, even between long-divergent lineages (Hudson and Turelli 2003)
We examined patterns of gene flow among all three members of the northern oriole group using the Bayesian IM model implemented in the program IMa2 (Hey 2010b), which assumes random mating, constant population sizes, selective neutrality, free recombination among loci and no recombination within loci (Hey and Nielsen 2004, 2007; Hey 2010b)
Summary
The study of phylogeography and population divergence is challenged by the difficulty of distinguishing between shared retained ancestral polymorphism and introgressed alleles (Nielsen and Wakeley 2001). Slow and stochastic lineage sorting causes a pattern of allele sharing among species that can seem similar to patterns caused by gene flow through introgressive hybridization, even between long-divergent lineages (Hudson and Turelli 2003). Shared alleles allow multilocus coalescent approaches such as IM to reconstruct the divergence history of closely related species (Nielsen and Wakeley 2001; Hey and Nielsen 2004, 2007). The original IM model was designed for two-population studies between sister taxa that have not exchanged genes with other taxa since they split (Hey and Nielsen 2004, 2007). Organisms frequently exchange genes with other non-sister taxa
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