Data from: Extensive mitochondrial introgression in North American Great Black-backed Gulls (Larus marinus) from the American Herring Gull (Larus smithsonianus) with little nuclear DNA impact

Recent genetic studies have shown that introgression rates among loci may
greatly vary according to their location in the genome. In particular,
several cases of mito-nuclear discordances have been reported for a wide
range of organisms. In the present study, we examine the causes of
discordance between mitochondrial (mtDNA) and nuclear DNA introgression
detected in North American populations of the Great Black-backed Gull
(Larus marinus), a Holarctic species, from the Nearctic North American
Herring Gull (Larus smithsonianus). Our results show that extensive
unidirectional mtDNA introgression from Larus smithsonianus into Larus
marinus in North America cannot be explained by ancestral polymorphism but
most likely results from ancient hybridization events occurring when Larus
marinus invaded the North America. Conversely, our nuclear DNA results
based on 12 microsatellites detected very little introgression from Larus
smithsonianus into North American Larus marinus. We discuss these results
in the framework of demographic and selective mechanisms that have been
postulated to explain mito-nuclear discrepancies. We were unable to
demonstrate selection as the main cause of mito-nuclear introgression
discordance but cannot dismiss the possible role of selection in the
observed pattern. Among demographic explanations, only drift in small
populations and bias in mate choice in an invasive context may explain our
results. As it is often difficult to demonstrate that selection may be the
main factor driving the introgression of mitochondrial DNA in natural
populations, we advocate that evaluating alternative demographic neutral
hypotheses may help to indirectly support or reject hypotheses invoking
selective processes.