The last days ofAporia crataegi(L.) in Britain: evaluating genomic erosion in an extirpated butterfly

Abstract

Current rates of climate change and habitat degradation are causing biodiversity declines globally, with global extinction rates far beyond estimated background extinction rates. Studies on vertebrates highlight how conservation genomics can be an effective tool to identify and manage threatened populations. However, it is unclear how metrics of genomic erosion, commonly applied in conservation genomics, translate to invertebrate species which can have markedly different population sizes and life history strategies. The Black-veined White butterfly (Aporia crataegi)was extirpated from Britain in the 1920s. Here, we sequenced historical DNA from 17 museum specimens collected between 1854 and 1924 to determine levels of genomic erosion in the species as it approached extirpation. We compare these results to those from the Common Blue butterfly (Polyommatus icarus) - a species with stable demographic trends in Great Britain. We provide evidence for a bottleneck of British butterflies around the period of post-glacial colonisation of the British Isles. Despite a 4X difference in effective population sizes around the time of post-glacial colonisation, reduction in genome-wide heterozygosity (our metric for genetic diversity) inA. crataegi(17.9%) was comparable to that inP. icarus(14.5%). More symptomatic ofA. crataegi’s disappearance were marked increases in size and proportion of runs-of-homozygosity (RoH) and accumulation of putatively mildly and weakly deleterious variants. Our results support the notion that metrics with more direct links to host fitness and function, such as inbreeding and genetic load, may be more informative of population decline and thus more valuable in identifying vulnerable insect populations.

Assisted colonisation prospects for the black-veined white butterfly in England

“…Extinction from the UK has primarily been attributed to a deterioration in climatic suitability, including a run of wet Septembers; however, specific weather events such as these would be unlikely to cause extinction unless the background climatic conditions were already marginal (see climatic analysis, below). While there are signs from museum specimens of genetic erosion in A. crataegi prior to its extinction in Britain (Whitla et al 2023), this seems likely to be a consequence of the butterfly's decline to low numbers, rather than the primary cause of decline. A number of small-scale reintroductions have been attempted but none was successful.…”