Multigene phylogenies of clonal Spumella-like strains, a cryptic heterotrophic nanoflagellate, isolated from different geographical regions

Many nanoflagellate morphospecies comprise an enormous variation of genotypes, probably indicating cryptic species. One of the best-investigated morphospecies with respect to molecular and ecophysiological variation are flagellates of the Spumella morphotype. Here, we have phylogenetically analysed three protein-coding genes (actin, alpha-tubulin, beta-tubulin), internal transcribed spacers (ITS1, ITS2) and the 5.8S rDNA of 17 Spumella-like strains isolated from soil, freshwater and marine samples in order to (i) test the validity of the current Spumella-like phylogenetic classification system based exclusively on small subunit (SSU) rDNA, (ii) elucidate the phylogenetic associations of SSU rDNA-unresolved strains and (iii) evaluate the validity of the assignment of ecophysiological adaptations to previously identified SSU rDNA sequence clades. All single-gene analyses show different patterns of support, are incongruent and identify a number of conflicting nodes. Likewise, a concatenation of all protein genes fails to recover specific SSU rDNA clades. However, a combined analysis of all genes confidently resolved the conflicts of the single genes and the protein-gene concatenations and resulted in a tree topology that is identical to the SSU rDNA analysis, but with enhanced phylogenetic resolution and decisively greater support. We conclude that, depending on the genes concatenated, a 'supergene' analysis minimizes artefactual effects of single genes and may be superior in its performance in phylogenetically analysing cryptic species. We confirm the validity of the SSU rDNA Spumella-like phyloclades and support the suggestion that these clades indeed seem to reflect certain ecophysiological adaptations.