Opening PandoraU+2019s box: species diversity, evolutionary history and biogeography of the Pinnularia borealis species complex

Because of their immense diversity, the systematics of protists, and diatoms in particular, is complex and challenging. Since species taxonomy has far-reaching consequences for our understanding of diatom evolutionary history, ecology and biogeography, detailed studies into species limits and diversity are needed if we are to advance our understanding of the range dynamics and diversification of diatoms. However, the biogeography and ecology of (pseudo)cryptic diatom species is highly understudied and there are virtually no data available on the timing of lineage splitting within species complexes. Recent advances in next generation sequencing currently make it possible to undertake such large-scale molecular based studies. In the present study, we used the semiterrestrial diatom complex Pinnularia borealis Ehrenberg as a case study for speciation, evolutionary history and biogeography of diatoms and more generally microscopic eukaryotes. During field campaigns on various localities, including Arctic and Antarctic regions, more than 500 samples from (semi)terrestrial habitats were collected. About 25 % of these contained living P. borealis cells. In total, more than 700 monoclonal cultures were established. A five-marker molecular phylogeny of P. borealis was reconstructed and time calibrated using five Pinnularia fossils. Species accumulation curves were calculated to assess the sampling coverage. Although morphological differentiation between several lineages is incomplete, automatic species delimitation based on molecular methods revealed the presence of at least 42 species-level lineages worldwide. The age of the complex is estimated at 30-47 million years old and the diversification of P. borealis started between 15 and 27 million years ago. Whereas some lineages are cosmopolitan, most seem to have a very restricted geographic distribution, including most Maritime Antarctic lineages. Whenever lineages were recovered from several localities, various haplotypes were found, suggesting phylogeographic patterns. Altogether, the results indicate a high degree of hidden species diversity in P. borealis worldwide, and hint at a high regional species diversification in Maritime Antarctica and the presence of regional endemics in the (sub)Antarctic. The topology of the phylogenetic tree and the age of the lineage splitting suggest multiple independent colonization events in or out the Antarctic and Arctic regions during the past 20-30 million years.

Jaar van publicatie:2016