Contributions to Zoology, 86 (2) – 2017Roberto Guidetti; Sandra J. McInnes; Michele Cesari; Lorena Rebecchi; Omar Rota-Stabelli: Evolutionary scenarios for the origin of an Antarctic tardigrade species based on molecular clock analyses and biogeographic data

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A species’ geographical distribution is the result of a variety of spatio-temporal events, including among others vicariant speciation, diffusion, jump dispersal, extinction, and colonisation. Although we were not able to give a definitive evidence, Mopsechiniscus was shown to be an ancient tardigrade genus with a Gondwanan distribution that has been further articulated at species level by geological events. Our molecular dating, with all its uncertainty, provided an estimate of when two Mopsechiniscus phylogenetic lineages separated. According to these results, the allopatric speciation of M. franciscae and M. granulosus could coincide with two geographical separation/ isolation promoting events: the break-up of Gondwana and the glaciation of Antarctica; our divergence estimates are indeed congruent with other independent studies in dating Gondwanan geological events. From a methodological point of view, our molecular clock results indicated that the molecular dating of tardigrades (using genes coding for ribosomal RNA) is extremely complicated and accompanied by a high degree of uncertainty revealed by: a very wide % HPD, a variable estimate when varying priors, and a relative difficulty in discriminating between competing models. Future studies could increase the number of genes and the taxa considered in the analyses, which should reduce the HPD values.

Overall, our results indicate that Mopsechiniscus can be used for biogeographical and temporal studies, paving the way to further studies to better understand the origin of Antarctic microfauna.