Random mating in a hybrid zone between two putative climate-adapted bird
lineages with predicted mitonuclear incompatibilities
Abstract
Mitonuclear interactions have been proposed as evolutionary drivers of
sexual reproduction, sexual selection, adaptation, and speciation. We
investigated the role of pre-mating isolation in maintaining functional
mitonuclear interactions in a wild population with divergent sets of
proposed co-adapted mitonuclear genotypes. Two lineages have been
identified in the eastern yellow robin Eopsaltria australis - putatively
climate-adapted to ‘inland’ and ‘coastal’ climates. The lineages differ
by ~7% of mitochondrial DNA positions, whereas nuclear
genome differences are concentrated into a sex-linked region enriched
with mitochondrial genes. This pattern can be explained by female-linked
selection accompanied by male-mediated gene flow across the narrow
hybrid zone in which the two lineages coexist. It remains unknown
whether lineage divergence is driven by intrinsic incompatibilities
(particularly in females, under Haldane’s rule), extrinsic
incompatibilities, or both. We tested whether non-random mating with
respect to partners’ mitolineages or Z-linked variation could facilitate
lineage divergence. We used field data, Z-linked and mitolineage genetic
markers from two locations where the lineages hybridize, to test whether
females choose to mate with (1) males of their own mitolineage and/or
bearing similar Z-linked variation, as might be expected if hybrids
experience intrinsic incompatibilities, or (2) putatively
locally-adapted males, as expected under environmental selection.
Comparisons of field observations and simulations present no evidence of
non-random mating: the observed reduced female gene flow likely operates
via post-mating isolation. Future studies testing for female-biased
mortality at different life stages and female habitat selection should
clarify the mechanisms of selection.