Individual heterozygosity and fitness in bottlenecked populations during early colonisation

Some populations of alien species, established by a small number of individuals, spread rapidly. This is the 'genetic paradox of invasions' as they must overcome the negative effects of the demographic bottleneck during the establishment phase, which reduces genetic diversity, fitness and evolutionary potential. Using a set of experimentally introduced populations of the Roesel's bush-cricket (Roeseliana roeselii), a nuptial gift-giving insect, we investigated this paradox by examining the relationships between individual heterozygosity (SNP markers), body size (an indicator of insect fitness) and population growth. We found that populations with a lower growth rate (annual increase in the number of stridulating males around the introduction patch) also had lower genetic variation and effective size. Females exhibited significantly higher individual heterozygosity than males. Body size (length of hind femur) increased in females with individual heterozygosity, whereas this was not observed in males. However, population growth was related to heterozygosity in males. Since female body size and male heterozygosity in these insects are related to fecundity and nuptial gift quality, respectively, our results suggest that potential selection on fitness-related phenotypic traits may mitigate effects of inbreeding depression and increase population growth during the establishment phase. The present results cannot fully disentangle complex mechanisms underlying the success of colonisation, but we believe that they will stimulate further experimental research in the field of invasion biology.