Hidden diversity: multiple Arctic and Antarctic lineages in the cosmopolitan diatom Pinnularia borealis

Pinnularia borealis Ehrenb. is a common diatom (Bacillariophyta) found on all continents. Although it has been observed in Antarctic waters with low nutrient and mineral content, P. borealis is mainly confined to (semi)terrestrial habitats such as seepages, moss vegetation and moist to dry soils. In the past, a large number of morphological forms have been described as subspecies, varieties or formas, but many of these show considerable morphological overlap resulting in an uncertain taxonomic status. Recently, detailed morphological analysis of several forms occuring in Amsterdam Island (sub-Antarctic) and Livingston Island (Maritime Antarctic) resulted in the description of inter alia P. sylviae Van de Vijver and P. quesadae Van de Vijver & Zidarova as species new to science. In parallel, a molecular phylogenetic approach was taken to tackle the species diversity, evolutionary history and biogeography of this enigmatic diatom. During field campaigns on various localities, including alpine, Arctic and Antarctic regions, samples were taken from soils, moss, seepages and the littoral zones of freshwater ponds and lakes. When P. borealis was present, cells were isolated, brought into culture and harvested for DNA and morphology analysis. In a first effort, samples were collected from Schirmacher Oasis (Continental Antarctic), Canada, Chile, Mongolia, Czech Republic, France and Belgium. Molecular phylogenies based on the plastid gene rbcL and the nuclear LSU rDNA revealed the presence of 8 lineages in P. borealis, including a distinct continental Antarctic lineage (Souffreau et al. 2013). A molecular clock estimates the origin of P. borealis at 30-47 million years ago and the age of the continental Antarctic lineage at 8 million years. Since then, the addition of 31 strains from Marion Island (sub-Antarctic), Vega Island (Maritime Antarctic), Spitsbergen (High Arctic) and Belgium resulted in the discovery of no less than 8 additional lineages and the co-existence of multiple lineages within a single region. Within individual lineages, different haplotypes were detected from different locations, suggesting the presence of an intra-lineage phylogeographic structure. Morphological differentiation between the different lineages seems incomplete, as several taxa cannot be reliably separated using light microscopy or scanning elecktron microscopy, while others do correspond to newly described varieties or species in the complex. Very recently, new strains were obtained from the Pyrenees (Spain), Norway, James Ross Island and the South Shetland Islands (Maritime Antarctic), and morphological and genetic data will be assessed in the coming months. All together, the results already indicate a high degree of hidden species diversity in P. borealis, and hint at a high regional species diversity in the Maritime Antarctic and the presence of regional endemics in the (sub)Antarctic. Future in-depth analyses of this complex in polar regions based on additional isolates and environmental sequencing approaches, will allow addressing key questions concerning its species diversity, evolutionary history, biogeography and ecological differentiation in Arctic and Antarctic regions