Relations between the Yellowhammer (Emberiza citrinella) and the Pine Bunting (Emberiza leucocephalos) in the Forested Steppe of the Trans-Urals

Extreme Heterogeneity in Genomic Differentiation between Phenotypically Divergent Songbirds: A Test of Mitonuclear Co-introgression

“…Yellowhammers and pine buntings show negligible mtDNA differentiation (Irwin et al 2009) but are well differentiated phenotypically (Panov et al 2003, Rubtsov & Tarasov, 2017). Prior to this study, a possible explanation for this pattern was simply rapid phenotypic evolution between two genetically similar sister taxa.…”

“…Mitonuclear incompatibilities are thought to represent an important post-zygotic reproductive barrier between taxa (Gershoni et al 2009; Burton & Barreto, 2012; Hill, 2019), meaning mitonuclear co-introgression has the potential to weaken species boundaries. Support for such breakdown may be seen in the extensive and dynamic hybrid zone between yellowhammers and pine buntings (Panov et al 2003; 2007; Rubtsov, 2007; Rubtsov & Tarasov, 2017). Further, careful examination of genetic differentiation and reproductive barriers within the yellowhammer and pine bunting hybrid zone would shed light on the possibility of their merging in the future.…”

“…Phenotypic scores of 0 are consistent with a phenotypically pure yellowhammer and scores of 7 are consistent with a phenotypically pure pine bunting. Phenotypic classes included: pure citrinella (PC), almost citrinella (SC), citrinella hybrid (CH), yellow hybrid (YH), white hybrid (WH), leucocephalos hybrid (LH), almost leucocephalos (SL) and pure leucocephalos (PL) (Rubtsov & Tarasov, 2017). Unless stated otherwise, any SC and SL individuals that appeared in the allopatric zones were grouped together with PC and PL individuals respectively and treated as phenotypic yellowhammers and phenotypic pine buntings in subsequent analyses (Figure 1B).…”

“…An example of one such system includes the yellowhammer (Passeriformes: Emberizidae: Emberiza citrinella ) and the pine bunting ( Emberiza leucocephalos )—an avian species pair thought to have diverged during the Pleistocene glaciations when they were separated on either side of Eurasia by an area of unsuitable habitat (Irwin et al 2009). These taxa are highly divergent in plumage and moderately divergent in song and ecology (Panov et al 2003; Rubtsov & Tarasov, 2017). Despite their differences, yellowhammers and pine buntings hybridize extensively in a large contact zone in central and western Siberia (Panov et al 2003; 2007; Rubtsov, 2007; Irwin et al 2009; Rubtsov & Tarasov, 2017).…”

“…These taxa are highly divergent in plumage and moderately divergent in song and ecology (Panov et al 2003; Rubtsov & Tarasov, 2017). Despite their differences, yellowhammers and pine buntings hybridize extensively in a large contact zone in central and western Siberia (Panov et al 2003; 2007; Rubtsov, 2007; Irwin et al 2009; Rubtsov & Tarasov, 2017). Genomic work has identified mitonuclear discordance between allopatric yellowhammers and pine buntings (Irwin et al 2009) as they possess almost no mtDNA divergence but show moderate differentiation in nuclear AFLP (Amplified Fragment Length Polymorphism) markers.…”