Introduction
The distribution of animals across the World has been studied since the
nineteenth century, and one of the first results of global biogeographic
analyses was the division of the Earth into zoogeographic realms
(Wallace, 1876). However, such studies were conducted primarily on
large, easily observable organisms, mainly vertebrates (Sclater, 1858).
In contrast, for microscopic forms of life, the ‘Everything is
Everywhere but environment selects’ hypothesis (EiE) was postulated
(Baas-Becking, 1934; Fenchel & Finlay, 2004; Foissner, 2006): because
of their small size, <1 mm, such organisms were believed to
have unlimited long distance dispersal (LDD) abilities, and therefore
they should not show any biogeographical patterns, dwelling wherever the
environment is suitable. This hypothesis was argued to be especially
applicable to groups with dormant stages, which can be easily dispersed
over long distances (Fontaneto, Barraclough, Chen, Ricci, & Herniou,
2008; Incagnone, Marrone, Barone, Robba, & Naselli-Flores, 2015).
Earlier studies, which were based on morphological characters, seemed to
confirm the EiE hypothesis across different taxonomic groups (e.g.
Fenchel, Esteban, & Finlay, 1997; Martiny et al., 2006; Heino et al.,
2010). However, with the development of molecular tools, many species of
microscopic animals that were previously thought to be cosmopolitan,
were demonstrated to actually be species complexes (Fontaneto, Kaya,
Herniou, & Barraclough, 2009; Cesari, Bertolani, Rebecchi, & Guidetti,
2009), with respective species that often exhibit limited geographic
ranges. Therefore, recently, evidence against the EiE hypothesis in
microscopic animals started to accumulate (e.g. Fontaneto et al. 2008;
Baltanás & Danielopol, 2013; Garraffoni & Balsamo, 2017; Worsaae,
Kerbl, Vang, & Gonzalez, 2019; Gąsiorek, Vončina, Zając, & Michalczyk,
in review), undermining the hypothesis or at least its universal
application to all micrometazoan species.
One of the groups of microscopic animals (up to ca. 1 mm) that have the
ability to withstand hash environmental conditions are limnoterrestrial
tardigrades (e.g. Hengherr & Schill, 2018). These ubiquitous
invertebrates inhabit a wide variety of environments across the planet
(e.g. Nelson, Bartels, & Guil, 2018). Many tardigrade species have been
considered cosmopolitan, but the knowledge on the distribution of
particular species is usually extremely fragmentary (Gąsiorek et al.,
2019b). In recent years, the discussion on whether tardigrades do
support or undermine the EiE hypothesis has been debated (e.g. Pilato &
Binda, 2001; Guil, 2011), but only a limited number of geographically
and/or taxonomically restricted studies on this topic have been
conducted so far (e.g. Jørgensen, Møbjerg, & Kristensen, 2007; Guil
Sanchez-Moreno, & Machordom, 2009; Cesari, McInnes, Bertolani,
Rebecchi, & Guidetti, 2016; Morek, Stec, Gąsiorek, Surmacz, &
Michalczyk, 2019a; Gąsiorek et al. 2019b; Gąsiorek, Vončina, Degma, &
Michalczyk, 2020; Morek & Michalczyk 2020). Nevertheless, these initial
studies already indicate that many species may not be as widely
distributed as previously assumed. Importantly, in recent years, many
species complexes and pseudocryptic species have been detected (e.g.
Bertolani, Rebecchi, Giovannini, & Cesari, 2011; Stec, Morek, Gąsiorek,
& Michalczyk, 2018; Morek et al., 2019a), further questioning the
putative cosmopolitan distribution of tardigrade species. Moreover, the
impact of geological events on the distribution of these animals has
been hypothesised to be important (McInnes & Pugh, 1998; Guidetti,
McInnes, Cesari, Rebecchi, & Rota-Stabelli, 2017), which could not be
the case if tardigrade species were generally cosmopolitan. In parallel,
anthropogenic dispersal has to be taken into consideration, as it is
likely to obscure the natural distribution of tardigrade species
(Gąsiorek Vončina, & Michalczyk, 2019a; Morek et al. 2019b) most likely
by artificially broadening geographic ranges of at least some species,
which may provide false evidence in favour of the EiE hypothesis.
One of the tardigrade genera with a long history of biogeographic
records is Milnesium Doyère, 1840. Importantly, the genus was
erroneously considered monotypic for many decades and, as a consequence,
the type species, Milnesium tardigradum Doyère, 1840, was
reported from numerous localities throughout the globe. Therefore,M. tardigradum was regarded cosmopolitan for over 170 years, but
this view has been challenged by its integrative redescription by
Michalczyk, Wełnicz, Frohme, & Kaczmarek (2012ab) and further
questioned by Morek et al. (2019b). Moreover, the most recent
phylogenetic analysis of the genus Milnesium (Morek &
Michalczyk, 2020), despite the limited number of analysed populations
(34) and species (25), suggested that the dispersal capability ofMilnesium species is limited and that the geographic origin of
species is a better predictor of a phylogenetic position of any givenMilnesium species than the morphological characters traditionally
used in the apochelan taxonomy.
Importantly, however, with small sample size, the detection of the same
species in multiple localities is not likely, especially in species-rich
genera in which species are not common/abundant. Thus, even if species
are widespread and their geographic distributions conform to the EiE
hypothesis, analyses based on small sample size may produce artefactual
patterns. In other words, the correlation of geography with phylogeny
reported by Morek & Michalczyk (2020) could be an artefact, resulting
from undersampling, falsely suggesting low dispersal abilities ofMilnesium species and prematurely rejecting the EiE hypothesis to
explain the geographic distribution of species within this genus.
Therefore, to verify the biogeographic conclusions stemming from Morek
& Michalczyk (2020), we considerably enlarged the phylogeneticMilnesium dataset (from 34 in Morek & Michalczyk, 2020 to 127
populations herein), originating from nine zoogeographic realms
(compared to six in Morek & Michalczyk, 2020). Moreover, in order to
understand what geological or historical events may have shaped the
diversification and distribution of Milnesium , we dated our
phylogeny using molecular clock analysis for the first time in
Eutardigrada Richters, 1926.