Inbreeding Depression Is Purged in the Invasive Insect Harmonia axyridis

SummaryBottlenecks in population size reduce genetic diversity and increase inbreeding, which can lead to inbreeding depression [1]. It is thus puzzling how introduced species, which typically pass through bottlenecks, become such successful invaders [2]. However, under certain theoretical conditions, bottlenecks of intermediate size can actually purge the alleles that cause inbreeding depression [3]. Although this process has been confirmed in model laboratory systems [4], it has yet to be observed in natural invasive populations. We evaluate whether such purging could facilitate biological invasions by using the world-wide invasion of the ladybird (or ladybug) Harmonia axyridis. We first show that invasive populations endured a bottleneck of intermediate intensity. We then demonstrate that replicate introduced populations experience almost none of the inbreeding depression suffered by native populations. Thus, rather than posing a barrier to invasion as often assumed, bottlenecks, by purging deleterious alleles, can enable the evolution of invaders that maintain high fitness even when inbred