Mito‐nuclear discord in six congeneric lineages of H olarctic ducks (genus A nas )

Abstract

Many species have Holarctic distributions that extend across Europe, Asia and North America. Most genetics research on these species has examined only mitochondrial (mt) DNA , which has revealed wide variance in divergence between Old World ( OW ) and New World ( NW ) populations, ranging from shallow, unstructured genealogies to deeply divergent lineages. In this study, we sequenced 20 nuclear introns to test for concordant patterns of OW – NW differentiation between mt DNA and nuclear (nu) DNA for six lineages of Holarctic ducks (genus Anas ). Genetic differentiation for both marker types varied widely among these lineages (idiosyncratic population histories), but mt DNA and nu DNA divergence within lineages was not significantly correlated. Moreover, compared with the association between mt DNA and nu DNA divergence observed among different species, OW – NW nu DNA differentiation was generally lower than mt DNA divergence, at least for lineages with deeply divergent mt DNA . Furthermore, coalescent estimates indicated significantly higher rates of gene flow for nu DNA than mt DNA for four of the six lineages. Thus, Holarctic ducks show prominent mito‐nuclear discord between OW and NW populations, and we reject differences in sorting rates as the sole cause of the within‐species discord. Male‐mediated intercontinental gene flow is likely a leading contributor to this discord, although selection could also cause increased mt DNA divergence relative to weak nu DNA differentiation. The population genetics of these ducks contribute to growing evidence that mt DNA can be an unreliable indicator of stage of speciation and that more holistic approaches are needed for species delimitation.

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