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

The central peaks of the isolated island of St Helena (in the south Atlantic Ocean) are home to an extraordinary set of ground beetles of the tribe Bembidiini, and 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 extinct, and all are threatened by habitat destruction and invasive species. Through next-generation sequencing of old museum specimens, we examine the phylogenetic relationships of the St Helena fauna. We find that, in spite of their morphological disparities, the endemic St Helena bembidiines form a clade of genetically very similar species, their sister group is Bembidion alsium from the island of La Réunion (east of Madagascar), and the sister group of this pair is 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 Bembidion alsium. As the St Helena Peaks Bembidion are deeply nested within the genus Bembidion, we move the three taxa back within that genus as subgenera, and provide a new name (Bembidion shepherdae) for the now-homonymous Bembidion wollastoni.位于南大西洋的圣赫勒拿孤岛中央峰，栖息着一类隶属于Bembidiini族（Bembidiini）的特殊地甲类群，该类群包含三个特有属级分类单元。这类地甲的整体躯体形态，与全球其他区域分布的各类Bembidiini族甲虫存在显著差异。圣赫勒拿岛的该类地甲物种中至少有部分已濒临灭绝，所有物种均受到栖息地破坏与入侵物种的威胁。本研究通过对馆藏老旧博物馆标本开展下一代测序，解析了圣赫勒拿岛该类动物区系的系统发育关系。研究结果显示，尽管圣赫勒拿岛的特有Bembidiini族甲虫存在形态分化，它们却构成了一个遗传相似度极高的物种演化支；该演化支的姊妹群为分布于马达加斯加东侧留尼旺岛的Bembidion alsium（Bembidion alsium），而这一组姊妹群的姊妹群则是非洲地区的Omotaphus亚属（Omotaphus）。本研究提出，圣赫勒拿峰区的Bembidion属（Bembidion）甲虫属于一次适应辐射事件的产物：其祖先类群源自某一现已灭绝的非洲演化支系（为Omotaphus亚属的姊妹群），仅通过单次扩散事件抵达圣赫勒拿岛；且该灭绝演化支系同时也是Bembidion alsium的祖先来源。鉴于圣赫勒拿峰区的Bembidion属甲虫完全嵌套于Bembidion属内部，本研究将这三个分类单元重新归入该属并作为其亚属；同时针对当前存在同名问题的Bembidion wollastoni（Bembidion wollastoni），我们为其拟定了新学名Bembidion shepherdae（Bembidion shepherdae）。