Molecular species discovery in the diatom Sellaphora and its congruence with mating trials

Many diatom and other microbial eukaryote morphospecies consist of a variable number of (pseudo)cryptic species, with obvious consequences for such fields as biogeography and community ecology. Here, we investigated the species limits of morphologically similar small-celled strains of the model diatom Sellaphora from the United Kingdom and Australia, using cox1 mitochondrial and rbcL chloroplast gene sequences. Based on cox1 sequence data, the sequenced strains belonged to six closely related lineages, presumably species, of which one corresponds to the previously described S. auldreekie D.G. Mann & S.M. McDonald. Although rbcL displayed less sequence variation, the same six lineages were also recovered in an rbcL phylogeny of the genus. Molecular species discovery was compared to mating trials involving three of the lineages, showing that they were reproductively isolated. Incomplete evidence from a fourth lineage suggested that it too was reproductively isolated. A posteriori examination of light microscope morphology revealed no simple metrics or presence/absence characters that could consistently separate all species of the auldreekie complex, even though some do differ in pole width or stria density. While it is premature to make conclusions about their biogeography, it is obvious that a number of cryptic Sellaphora species thus far undetected in the UK are easily found at several localities in warm-temperate Australia.