Genomic Monitoring of a Reintroduced Butterfly Uncovers Contrasting Founder Lineage Survival

Genetic factors can have a major influence on both short- and long-term success of reintroductions. Genomic monitoring can give a range of insights into the early life of a reintroduced population and ultimately can help to avoid wasting limited conservation resources. In this study, we characterise the genetic diversity of a reintroduced Carterocephalus palaemon (Chequered Skipper butterfly) population in England with respect to the spatial genetic structure and diversity of the source populations in south Belgium. We aim to evaluate the success of the reintroduction, including the effectiveness of the donor sampling strategy, and assess genetic vulnerabilities that may affect the population's future. We also use an isolation-by-distance approach to make quantitative inferences about dispersal, and we explore covariance between host mitochondrial and Wolbachia genomes. We find that, four generations following the initial release, the reintroduced population, founded by 66 wild-caught adults, has an effective size of c. 33, yet has retained similar levels of genomic heterozygosity to those in the source subpopulations in Belgium and shows low levels of inbreeding. However, the restricted number of founders and variance in reproductive success among the surviving families have resulted in a higher level of kinship, likely to result in somewhat higher rates of inbreeding in the future. Furthermore, there is a distinct split between two source landscapes in Belgium, and all genomic evidence suggests that the reintroduced population is descended from only one of these landscapes (called Fagne). We discuss potential causes behind these results, including whether Wolbachia strains are causing genetic incompatibility between clades. We conclude that a conservative strategy for any further translocations would prefer Fagne sites as sources because of the strong evidence of their ability to survive. However, our results warrant further investigation into the reasons for the divergence found in Belgium.