‘Everything is not everywhere’: time-calibrated phylogeography of the
genus Milnesium (Tardigrada)
Abstract
There is ample evidence that macroscopic animals form geographic
clusters termed as zoogeographic realms (zones), whereas distributions
of species of microscopic animals are still poorly understood. The
common view has been that micrometazoans, thanks to their putatively
excellent dispersal abilities, are subject to the ‘Everything is
Everywhere but environment selects’ hypothesis (EiE). One of such
groups, <1 mm in length, are limnoterrestrial water bears
(Tardigrada), which can additionally enter cryptobiosis that should
further enhance their potential for long distance dispersion (e.g. by
wind). However, an increasing number of studies, including the most
recent phylogeny of a eutardigrade genus Milnesium, seem to question the
general applicability of the EiE hypothesis to tardigrade species.
Nevertheless, all the Milnesium phylogenies published to date were based
on a limited number of populations, which are likely to falsely suggest
limited geographic ranges. Thus, in order to comprehensively test the
EiE hypothesis, here, we considerably enlarged the Milnesium dataset
both taxonomically and geographically, and we analysed it in tandem with
climate type and reproductive mode. Additionally, we time-calibrated our
phylogeny to align it with major geological events. Our results show
that, although cases of long distance dispersal are present, they seem
to be rare and mostly ancient. Overall, Milnesium species are restricted
to single zoogeographic realms, which suggests that these tardigrades
have limited dispersal abilities. Finally, our results also suggest that
the breakdown of Gondwana may influenced the evolutionary history of
Milnesium. In conclusion, phylogenetic relationships within the genus
seem to be determined mainly by paleogeography.