PhD defence: Unraveling hidden biodiversity: An example of integrative taxonomy applied to the hybridizing Brachionus calyciflor

Brachionus calyciflorus is a widespread freshwater zooplankton. It was first suggested to comprise a species complex more than a decade ago but a formal description based on phylogeny and morphometry is only available recently. So far, a comprehensive study that intended to explore the species differentiation of the four known species (i.e., B. dorcas, B. elevatus, B. calyciflorus s.s. and B. fernandoi) is still lacking. Therefore, in this thesis, we applied an integrative taxonomic approach to disentangle the cryptic species complex from the aspects of life history traits, reproductive isolation and phylogeographic structure.

In Chapter 2, we found B. fernandoi differed strongly from the other species and was characterized by significantly lower investment in sexual reproduction and longer egg development times. But in population growth, demographic costs associated with long egg development times seemed compensated by reduced costs associated with the sexual investment. In Chapter 3 and 4, we found the two phylogenetically most closely related species, B. calyciflorus s.s. and B. elevatus, are strongly isolated by both pre- and postzygotic barriers although in some cases hybridization is still possible and offspring maybe fertile.

In Chapter 5, we found deep intraspecific phylogeographic subdivisions between and within subcontinents and large-scale patterns of mitochondrial displacement in three of the four species which indicate on one hand, despite the high potential for long-distance dispersal, the effective gene flow at the continental level is very limited; on the other hand, postglacial expansions are the most likely process that shaping contemporary phylogeographic structure.