Origin and adaptive radiation of the exceptional and threatened bembidiine beetle fauna of St Helena (Coleoptera: Carabidae)

Abstract

Abstract The central peaks of the isolated island of St Helena (south Atlantic Ocean) are home to an extraordinary set of ground beetles of the tribe Bembidiini, which belong to three endemic genus-group taxa. These beetles are strikingly different in overall body form from the many bembidiines found elsewhere in the world. At least some of the St Helena species are likely to be extinct, and all are threatened by habitat destruction and invasive species. Through next-generation sequencing of historical museum specimens, we examine the phylogenetic relationships of the St Helena fauna. We find that, in spite of their morphological disparities, the endemic bembidiines of St Helena form a clade of genetically similar species, with their sister group being Bembidion alsium from the Indian Ocean island of La Réunion, and the sister group of this pair being the African subgenus Omotaphus. We propose that the St Helena Peaks Bembidion are an adaptive radiation that arose from a single dispersal event to St Helena from a now-extinct African lineage (sister to Omotaphus) and that this extinct lineage also served as the ancestral source of B. alsium. Given that the St Helena Peaks Bembidion are deeply nested in the genus Bembidion, we move the three taxa back in that genus as subgenera and provide a new name (Bembidion shepherdae) for the now-homonymous Bembidion wollastoni.