Adrift across tectonic plates: molecular phylogenetics supports the ancient Laurasian origin of old limnic crangonyctid amphipods

摘要

Crangonyctidae is a speciose and almost exclusively freshwater Holarctic family of amphipod crustaceans. Its members inhabit groundwater as well as epigean biotopes with groundwater connections, and often exhibit endemic, relict distributions. Therefore, it has been proposed that this poorly dispersing, yet intercontinentally distributed family must have ancient Mesozoic origins. Here, we test the hypothesis that Crangonyctidae originated before the final break-up of Laurasia at the end of the Cretaceous. We used molecular phylogenetic analyses based on mitochondrial and nuclear markers and incorporated six out of the seven recognized genera. We calculated divergence times using a novel calibration scheme based exclusively on fossils and, for comparison, also applied substitution rates previously inferred for other arthropods. Our results indicate that crangonyctids originated during the Early Cretaceous in a northerly temperate area comprising nowadays North America and Europe, supporting the Laurasian origin hypothesis. Moreover, high latitude lineages were found to be generally older than the ones at lower latitudes, further supporting the boreal origin of the group and its relict biogeography. The estimated substitution rate of 1.773% Ma(-1) for the COI marker agrees well with other arthropod rates, making it appropriate for dating divergences at various phylogenetic levels within the Amphipoda. Furthermore, our extensive phylogeny reinforces the polyphyly of the intercontinental genera Crangonyx, Stygobromus, and Synurella, supports the monophyly of Bactrurus, and elucidates the position of Lyurella. We conclude that crangonyctids are an ideal model for testing continental-level vicariance hypotheses and should be in the focus of future phylogenomic studies.