Macroevolutionary patterns
All clades, except clade E, are restricted mostly to single zoogeographic realms, with the percentage of populations recorded in their respective realms varying from 78% to 100% (89% on average) per clade (Fig. 2). Moreover, the vast majority of analysed species (89%; 57/64) are restricted to a single realm. This is also translated to unequivocal results of the RASP analysis, which indicated a single zoogeographic zone as the place of origin of all clades except clade E (97%–99% and 45% support, respectively). In other words, most species in five of the six major clades recovered in our analysis, i.e. clades A–D and F, seem to have evolved and remained restricted to a single zoogeographic realm during their evolutionary history. In contrast, clade E, comprising widely distributed lineages across tropics and subtropics (Fig. 5), seems to support the EiE hypothesis to some extent.
Clade E (Fig. 5) is the most biogeographically complex group, which is also mirrored in the RASP analysis, with nearly equally supported Oriental (45%) as well as a Neotropical (44%) origin of the ancestor of this clade. In fact, clade E should be regarded as pantropical, considering that it comprises populations from multiple zoogeographic realms distributed along low latitude and often also low altitude areas in: the Orient (7 populations), Neotropic (6 pops), Afrotropic (3 pops), the Panamanian realm (3 pops), and the Oceanic realm (1 pop). Importantly, however, among the nine species preliminarily delineated in clade E, only three (#54, #55, and #58) were found in more than one tropical or subtropical zoogeographic realm (but see two subsections down for more details suggesting that only species #58 can be considered as naturally pantropical). The remaining six species are restricted to single zoogeographic zones, which suggests that dispersal, although it is more frequent than in other clades, is still rare (i.e. one event every several My; see Fig. 5). The pantropical character of clade E may be explained by LDD mediated by trade winds which tend to channel both northern and southern subtropical air masses that could drag tardigrade propagules towards and along the equator and effectively disperse them in a pantropical manner (as was observed in some insects, e.g. Gatehouse, 1997 or plant seeds, e.g. Nie, Deng, Meng, Sun, & Wen, 2013; which may be ecologically similar to dormant stages of tardigrades or their eggs). Given that low altitude tropical regions (which constitute the majority of the tropics) are characterised by similar climate conditions and habitats (mainly tropical and temperate climate types), tardigrade species exhibiting tropical preferences are likely to proliferate and dwell on any continents and islands along the equator provided they had a chance to arrive to there. In contrast, longitudinal dispersal may be much more difficult, as the latitudinally arranged subtropical and tropical areas may constitute a significant barrier, sort of a firewall, for species with temperate climate preferences that could potentially be dispersed by birds migrating between the north and south hemisphere (Mogle, Kimball, Miller, & McKown, 2018; but note that direct evidence for avian dispersal is missing).
Considering that clades C–F comprise exclusively tropical and subtropical species (except for M. bohleberi US.065/species #57 in clade E), it is intriguing why only clade E is pantropical, whereas clades C–D and F are mostly restricted to single zoogeographic realms (Afrotropic and the Australian realm; see Fig. 4 vs Fig. 5). We hypothesise that this could be due to the large geographic distances between South Africa and Australia (ca. 7,800 km) for the southern westerlies to efficiently disperse tardigrades between the continents compared to much smaller distances between land masses within the rings of trade winds (e.g. a continuous land mass between the east coast of northern Africa and west parts of the Orient, or ca. 3,000 km between the west coast of Africa and east coast of South America). In other words, the further south from the equator, the less likely it is to disperse pantropically. Thus, South Africa and Australia may be more efficiently isolated than central Africa and Orient, which is reflected in the geographically conservative compositions of clades C and D. If this hypothesis is true, when the Nearctic is sampled to a greater extent than in the present study, we should expect to observe at least some Holarctic tardigrade species because there are nearly continuous land masses extending from Western Europe via Siberia to North America, and the distance between the western coast of Europe and east coast of North America are separated by ca. 3,000 km. If, however, the latitudinal dispersal is restricted to the tropics and subtropics, then we should observe distinct Palaearctic and Nearctic clades in the northern hemisphere.
Our hypothesis about the limited dispersal capability ofMilnesium species is also supported by the temporal correlation of the diversification of the six main clades recovered in our study with the initial stages of the Gondwana breakdown (but see also the subsection ‘Diversification of the six main lineages’ below). Obviously, both dates are burdened with a considerable estimation error, however, it cannot be ruled out that there is a causal relationship between the Gondwana split and ancient age of the clades that are characterised by geographic clustering. Under this scenario, the barriers created by oceans wedging between the new continents provided effective geographic isolation to drive divergence between the clades. Thus, Milnesiumlineages generally rarely dispersed between the drifting continents, which resulted in the extant pattern in which the major clades are still strongly correlated with their geographic origin. Further studies, including more populations from so far unsampled parts of the world, should be carried out to test whether this correlation is maintained. Ideally, the areas should include former Gondwanan fragments, such as India, the southern parts of South America and the Antarctic, as they could provide crucial data for untangling the origin and early diversification of the genus.