Diversification of the six main lineages
The phyletic relationships obtained in this contribution are overall in
line with the first comprehensive phylogeny of the genusMilnesium (Morek & Michalczyk, 2020; see also Fig. 2 herein).
Three out of the five main clades obtained in the previous paper were
also recovered in the analysis presented herein (A, B and D), and only
the two smallest clades (E and C in Morek & Michalczyk, 2020,
consisting therein of one and two species, respectively) changed their
composition and slightly changed their position in the currently
presented tree. These changes are not surprising given the inclusion of
a large portion of new data in the current analysis (Fig. 2) and the
shallow divergence of the deep nodes.
The short branches and weakly supported nodes in the basal part of the
tree may suggest ancient and rapid diversification within the genus. On
the other hand, such a pattern could be an artefact resulting from
fragmentary sampling. Although both taxonomic and geographic sampling in
the present study are widest to date, the considerable number of species
that were sequenced for the first time (see also below for details)
suggests that new phyletic lineages could be out there, but have not
been detected yet. Apart from new species, DNA sequences for the
remaining three apochelan genera are likely to shed some strong light on
the evolution of the genus and entire family. Moreover, the relatively
long branch from the Apochela-Parachela split to the first
diversification observed in the ingroup (187–270 My long; see Fig. 2)
suggests that there probably were ancient lineages that did not persist
to the present time. If this is indeed the case, it is possible that the
genus diversified also earlier or later than the diversification
recovered in our analysis. However, assuming that our phylogenetic tree
accurately reflects the evolutionary history of the genus, the question
what may have caused the rapid divergence calls for a hypothesis.
There are two mutually non-exclusive possible explanations for such a
scenario: the breakdown of Gondwana and the switch to carnivory of the
common ancestor of extant Milnesium species. The first hypothesis
stems from the coincidence of the divergence and Gondwana breakdown (see
Fig. 2). Under this scenario, the force behind the diversification was
geographic isolation. The apparent low dispersal abilities of
apochelans, as demonstrated in the present study, seem to support this
hypothesis. On the other hand, the sister order Parachela started to
diversify 83 My earlier and, if continental drift was supposed to be
responsible for the diversification of limnoterrestrial tardigrades in
general, we should expect to see roughly the same temporal pattern in
both apochelans and parachelans. Moreover, it needs to be stressed that
the Gondwana breakdown was a long process that did not result in an
immediate geographic isolation and our molecular clock analysis is
burdened with considerable error, meaning that the time correlation
between geology and phylogeny observed in the present study may be
accidental.
Whereas the Gondwana split hypothesis concerns an external, abiotic
factor, the carnivory hypothesis concerns a physioecological property ofMilnesium itself. All known species in the genus are strictly
carnivorous and are not able to reproduce on a herbivorous diet
(Bryndová, Stec, Schill, Michalczyk, & Devetter, 2020). At the same
time, all limnoterrestrial heterotardigrades and many eutardigrades are
bacteri-, herbi- and/or fungivorous, only with macrobiotids being mostly
omnivorous. Apart from tardigrades, there are no predators within this
body size range dwelling in habitats occupied by apochelans that would
hunt for the same prey (i.e. mainly nematodes, rotifers and
non-apochelan tardigrades; Bryndová et al. 2020). In other words, the
ecological niche of a microscopic predator in moss and lichen habitats
is occupied exclusively by apochelans. Moreover, taking into
consideration that global biodiversity grows over geological time
(Crame, 2001), it is even more likely than the ecological niche of a
microscopic predator was unoccupied at the time of the diversification
of the genus Milnesium uncovered in our analysis (ca. 160 My ago
or even earlier). Assuming that Milnesium was carnivorous at the
time of the observed lineage diversification, which is the most
parsimonious scenario, our analysis also suggests that the genus is
older than other tardigrades that feed on micrometazoans, such as the
omnivorous Paramacrobiotus Guidetti, Schill, Bertolani, Dandekar,
& Wolf, 2009 (162 My vs 138 My, respectively), further lending
credence to this hypothesis. On the other hand, this hypothesis requires
a bold assumption that carnivory evolved within a relatively short
period of time and relatively late in the natural history of the
apochelan lineage (if it appeared earlier, then the divergence should
have occurred earlier too). Moreover, the other three apochelan genera
(which are probably also carnivorous) must have diverged at a similar
time as the ‘basal’ lineages of Milnesium . Given that no
molecular data for Bergtrollus Dastych, 2011, LimmeniusHorning, Schuster & Grigarick, 1978 and Milnesioides Claxton,
1999 are available, this hypothesis cannot be currently tested.
Similarly, the Gondwana breakdown hypothesis remains a speculation and
may require fossil evidence to be verified, meaning that it is likely to
remain untested for a long time.