Endosymbionts as an overlooked threat for insect reintroductions

Species reintroduction - or augmentation - into restored habitat will be increasingly used to combat biodiversity loss in a rapidly changing world, and used for a wider range of taxa, including invertebrates. Key tenets of this biodiversity restoration approach are to use source populations that are ecologically compatible with the target site, and to maximise genetic diversity.

However, it is unusual to consider potential incompatibilities between different lineages arising from the effects of endosymbionts, which are common and may significantly impact conservation objectives.

In 2018, Butterfly Conservation began a reintroduction programme of the Chequered Skipper butterfly (Carterocephalus palaemon), which went extinct from England in 1976, using source populations from Belgium. Recent genetic studies have shown an unexpected degree of divergence between the source populations, and that only one of the reintroduced genetic lineages has persisted in England.

There is strong circumstantial evidence that the elimination of one founder lineage was caused by the presence of incompatible Wolbachia strains. The bacterial endosymbiont Wolbachia is common across insects, and uses cytoplasmic incompatibility and male killing to promote its own transmission. In our study population it is ubiquitous, and shows two diverged strains that correspond with the mitochondrial phylogeny.

Better understanding of the endosymbiont interaction is critical to the establishment of a functioning metapopulation of Chequered Skipper across the reintroduction landscape, and more generally to developing best practice protocols for reintroduction biology.

In this PhD project, we aim to establish the operation of cytoplasmic incompatibility in the Chequered Skipper, and to predict its consequences. Our main objectives are to: 1. Establish direct evidence for cytoplasmic incompatibility in C. palaemon using crossing experiments.

2. Explore the mechanism of cytoplasmic incompatibility with respect to candidate genes in Wolbachia and coevolutionary selection in C. palaemon.

3. Reconstruct the evolutionary origins and spread of the incompatible Wolbachia strains, using a phylogeographic approach. 4. Track the survival of C. palaemon/Wolbachia lineages in reintroduced English populations.