Unearthing the historical biogeography of Mediterranean earthworms (Annelida:Hormogastridae)

Abstract

Aim: The genetic diversity and distribution of earthworm species in the Mediterranean Basin has been influenced by their low vagility, as well as by the basin’s complex geological and climatic history. Within this context, our objective was to evaluate the evolutionary history of hormogastrid earthworms by exploring their phylogeny, reconstructing ancestral areas and identifying potential vicariance and dispersal events. Location: Western Mediterranean region, encompassing the geographical range of the family Hormogastridae (four genera, 30 species and subspecies). Methods: We sampled the easternmost area of the hormogastrid range and integrated 606 new sequences with all the molecular data available from previous studies on the westernmost area. Mitochondrial and nuclear markers were amplified and sequenced, and the hormogastrid phylogeny (using Bayesian and likelihood methods) and networks were investigated. Ancestral-area reconstructions were implemented in rasp. Results: Ancestral-area reconstructions provided similar results for alternative phylogenetic hypotheses, placing the origin of Hormogastridae between southern France and the north-eastern Iberian Peninsula, and highlighting north–central Sardinia as a key ancestral area for diversification of eastern Hormogastridae (Hormogaster redii and H. samnitica). Multiple vicariance and dispersal events were detected, indicating a complex evolutionary history. Sardinian populations of H. samnitica and H. redii were not monophyletic but were related to populations from other areas, suggesting that Sardinia is a more complex biogeographical region than anticipated. Our results identified a phylogenetic relationship between the southern Sardinian fauna and that of Sicily, as well as relationships between the fauna of north-eastern Sardinia and those of the Italian Peninsula and the Tuscan Archipelago. Main conclusions: Our results suggest a complex evolutionary history for hormogastrid earthworms, showing the possibility of many dispersal and vicariance events and multiple faunal interchanges between land-masses, which may reflect the biogeographical complexity of the Mediterranean Basin.