Molecular phylogeography of the microturbellarian Monocelis lineata (Platyhelminthes: Proseriata) in the North-East Atlantic

Monocelis lineata is a complex of cryptic species (three in the Mediterranean and one in the Atlantic) widespread in midlittoral habitats. Throughout the range, populations with or without an ocular pigmented shield are found. We investigated the genetic structure of the North-East Atlantic populations with the aim of shedding light on their phylogeography and reconstructing possible patterns of recolonization after the Würmian glaciation. Fourteen samples were investigated using cytochrome c oxidase subunit I (COI) and 13 by inter-simple sequence repeats (ISSRs). COI did not exhibit a clear pattern of decreased genetic diversity along a latitudinal gradient. Populations from Ferrol (Spain), Doolin (Ireland), and Helsingør (Denmark) showed a higher genetic variability, whereas a reduction in the number of haplotypes was found at the northernmost edge of the distribution and in northern Ireland and Scotland. Two genetically differentiated areas (southern Europe and south-western Ireland versus northern Atlantic) were revealed by ISSR data. The results obtained provided evidence of three refugia (Iberian Peninsula, south-western Ireland, and North Sea), and the occurrence of secondary contacts that shaped the genetic variability of some of the populations examined. Two different recolonization pathways of north-western Europe during the post-Würmian glaciations have been detected. Furthermore, ISSR analysis provided evidence of genetic divergence among populations with and without pigmented eyespot, suggesting the action of ecological differentiation.