Future directions
Future research on Milnesium should concentrate on the Nearctic
realm to verify whether taxa from this region are mixed with the species
currently known only from the Palaearctic, within clades A and B, or
whether they constitute their own, separate clade(s). Given that the
Nearctic and the Palaearctic have only limited connection through the
Bering strait, but they share the same climate types, mixed clades would
support the EiE hypothesis. If, however, Nearctic and Palaearctic
species form separate clades, then this would further strengthen the
evidence that Milnesium species generally exhibit limited natural
dispersal abilities. Furthermore, in order to uncover the early
evolution of the genus Milnesium , sampling ought to focus in
Antarctica, Australia and New Zealand, as the areas in the Southern
Hemisphere seem to encompass the most basal lineages and are likely to
help us understand the effect of the Gondwana breakdown on the early
diversification of Milnesium .
Crucially, the remaining milnesiid genera need to be included in a
phylogenetic framework to date the diversification events within this
group more reliably. Moreover, as the basal part of the tree is
characterised by shallow branching, which does not allow for resolving
the phylogenetic relationships between the major clades, new molecular
markers or entire genomes should be analysed. Furthermore, the
verification of the mutation rates for other groups of tardigrades could
provide important enhancement to the molecular clock analysis, and help
us better understand the evolution of this phylum. Last but not least, a
greater taxonomic effort should be made to describe the apparently
overwhelming number of putative new species in the genusMilnesium , without neglecting the need to integratively
redescribe the number of classically delineated species.