Inbreeding Depression Is Purged in the Invasive Insect Harmonia axyridis

Bottlenecks 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. ► Introduced populations of Harmonia axyridis have passed through a bottleneck. ► Native populations exhibit severe inbreeding depression ► Invasive populations experience little to no inbreeding depression ► Invaders have evolved higher mean fitness overall than their native counterparts.