Phylogeography of a riparian earthworm shows environmental factors influence genetic structure

Abstract

Aim: The study of cosmopolitan earthworms could be even more interesting than that of endemic species in revealing evolutionary processes. Previous research on the cosmopolitan worm Eiseniella tetraedra has indicated some phylogeographic structure among populations, but the factors responsible remain unresolved. We hypothesized that environmental factors and dispersal have shaped the distribution of the species' lineages. Location: Spain and Portugal; Iberian Peninsula. Taxon: Eiseniella tetraedra (Lumbricidae, Oligochaeta, Annelida). Methods: We collected 739 specimens of Eiseniella tetraedra from 65 localities around the Iberian Peninsula between 2012 and 2016. We performed phylogenetic analysis (Bayesian Inference and maximum likelihood) using two mitochondrial (COI and 16S) and one nuclear marker (28S). Furthermore, we studied their genetic diversity and historic demography based on the COI gene. Correlations between genetic diversity and 22 environmental factors were tested. Results: Eiseniella tetraedra showed high diversity in the Iberian Peninsula, with eight different lineages nested in two clades. We found lineages mostly restricted to the northern region, while others were distributed throughout the Iberian Peninsula. Habitat stability, that is, constant availability or lack of water, also correlated with genetic diversity. Thus, although no clear phylogeographic pattern was found, environmental factors (such as precipitation, temperature, and soil pH) and habitat stability influenced the distribution of genetic variability. Main Conclusions: Eiseniella tetraedra is an earthworm with great genetic variability. We show that the ranges of species with high relative dispersal ability and ambiguous phylogeographic patterns may be better explained by influence of environmental conditions rather than specific geographic features. Adaptation to unstable conditions has been shown to confer more success on one of the two major genetic clades recovered, pointing to ecological plasticity as a key for evolutionary success.