Reproductive compatibility and rDNA sequence analyses in the Sellaphora pupula species complex (Bacillariophyta)

We tested whether internal transcribed spacer (ITS) rDNA sequence differences are correlated with sexual compatibility in the Sellaphora pupula complex, a model system for investigations of the species concept and speciation in diatoms. The phylogenetic relationships among the demes and the systematic position of the genus within the raphid diatoms were also investigated. The division of clones of S. pupula and S. laevissima into groups, based on sequence similarities and phylogenetic analyses, resembled groupings based on sexual compatibility: A high ITS sequence divergence, making full alignment difficult or impossible, was found among clones whose gametangia do not interact, whereas there was little sequence divergence among interfertile clones. This is clearly consistent with the idea that Z clades exhibit less intraclade than interclade variation in ITS and, as comparisons of secondary structure models for the RECT and PSEUDOCAP clones showed, that there is an equivalence of CBC and Z clades in the rectangular and pseudocapitate demes of S. pupula, as earlier hypothesized for chlorophytes. Intraclonal, presumably intraindividual, variation in ITS was found in S. pupula, though with a degree of variation less than that found within a single Z clade; it was too minor to affect the interclonal relationships in the ITS phylogeny. Sellaphora, which appears monophyletic in 18S phylogenies, with Pinnularia and Naviculapelliculosa as its closest allies, may also include some species currently classified in Eolimna. The S. pupula-S. laevissima group began to diversify in or before the Miocene.