Species delimitation
The bPTP analyses identified 76 entities in both the Maximum Likelihood
and BI approaches, with supports ranging from 0.65 to 1.00 (0.97 on
average). However, after considering the lack of morphological
differences (data not shown) and low genetic variance among some of the
delimited entities, and considering that bPTP and other tree-based
species delimitation methods usually oversplit species into multiple
entities (Dellicour & Flot, 2018), we concluded that the following
populations represent single species: the eleven M. tardigradumpopulations (indicated as two entities by the bPTP; following Morek et
al., 2019a), the six populations of M. eurystomum Maucci, 1991
(recovered as three entities by the bPTP; following Morek, Blagden,
Kristensen, Michalczyk, 2020a), the three populations of M.
quadrifidum Nederström, 1919 (indicated as two entities by the bPTP)
both populations of M. variefidum Morek et al., 2016 (recovered
as two entities by the bPTP), Milnesium sp. TZ.075, ZA.204 and
ZA.180+ZA.218 (indicated as three entities by the bPTP);Milnesium sp. ID.432+ID.940, ID.711+ID.950, PH.014 and VN.045
(recovered as four entities by the bPTP); and Milnesium sp.
AR.437+AR.470 and GF.089+GF.093+GF.193 (indicated as two entities by the
bPTP). Thus, we concluded that our dataset comprises 64 species
recognised as distinct taxa when both molecular and morphological data
were in agreement. Slightly more than a half of the species (around
56%) were represented by a single population (singletons), whereas 44%
were formed by multiple populations.
The most abundant species were: M. tardigradum (represented by
eleven populations), M. eurystomum (six populations); M.
inceptum Morek et al., 2019b (five populations), M. reductumTumanov, 2006 (five populations), and two undescribed species, one from
the Oriental realm, Milnesium sp. #54 (six populations), and the
second one from the South America, Milnesium sp. #55 (five
populations). Whether the Oriental and South American populations
represent single species or groups of species, should be a subject of
further research. Among the 64 delimited species, as little as 15
include previously described taxa. The other 49 species are most likely
new to science, which would sharply increase the number of recognised
species in the genus Milnesium , currently amounting to 43 (as of
23.09.2020; Degma, Bertolani, & Guidetti, 2009–2020; Morek, Ciosek, &
Michalczyk, 2020b).