Abstract
For many years, Paramacrobiotus richtersi was reported to consist of populations with different chromosome numbers and reproductive modes. To clarify the relationships among different populations, the type locality of the species (Clare Island, Ireland) and several Italian localities were sampled. Populations were investigated with an integrated approach, using morphological (LM, CLSM, SEM), morphometric, karyological, and molecular (18S rRNA, cox1 genes) data. Paramacrobiotus richtersi was redescribed and a neotype designed from the Irish bisexual population. Animals of all populations had very similar qualitative and quantitative characters, apart from the absence of males and the presence of triploidy in some of them, whereas some differences were recorded in the egg shell. All populations examined had the same 18S haplotype, while 21 haplotypes were found in the cox1 gene. In four cases, those qualitative characters were correlated with clear molecular (cox1) differences (genetic distance 14.6–21.8%). The integrative approach, which considered the morphological differences in the eggs, the reproductive biology and the wide genetic distances among putative species, led to the description of four new species (Paramacrobiotus arduus sp. n., Paramacrobiotus celsus sp. n., Paramacrobiotus depressus sp. n., Paramacrobiotus spatialis sp. n.) and two Unconfirmed Candidate Species (UCS) within the P. richtersi complex. Paramacrobiotus fairbanksi, the only ascertained parthenogenetic, triploid species, was redescribed and showed a wide distribution (Italy, Spain, Poland, Alaska), while the amphimictic species showed limited distributions. The difference in distribution between apomictic and amphimictic populations can be explained by the difference in the dispersal potentials associated with these two types of reproduction.
Highlights
Molecular approaches are becoming increasingly essential in organismal systematics, phylogeny, biogeography and phylogeography
Principal component analysis of the pt indexes of the sclerified structures does not show any defined separate cluster (Fig. 4a, b), confirming the data presented above. These results allow the conclusion that there is a P. richtersi complex characterized by species with extreme similarity in the morphologies of the sclerified structures of the animals
Our results suggest that the P. richtersi complex is formed by pseudo-cryptic species, as they do not show morphological differences among the animals, but are genetically distinct with differences in reproductive biology and/or a limited degree of differences in egg shell morphology
Summary
Molecular approaches are becoming increasingly essential in organismal systematics, phylogeny, biogeography and phylogeography This is true for the taxonomy of meiofaunal metazoans, which often consist of taxa that are neglected and difficult to identify due to their small sizes and low numbers of morphological characters. Before the use of molecular approaches, several cases of different “cytotypes” of the same morphospecies were identified in several genera of eutardigrades (for a review, see [20]). These were often characterized by different chromosome numbers (namely diploid and polyploid) and/or a different reproductive mode (amphimixis vs parthenogenesis)
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