Concatenated data and dense taxon sampling clarify phylogeny and ecological transitions within Hypotricha

Ciliates are single-cell eukaryotes playing important roles in various ecosystems. Phylogenetic relationships within Hypotricha, one of the most polymorphic and highly derived ciliate groups, remain uncertain. Previous studies suggested that low genetic divergence might be the reason for poorly supported SSU rDNA tree topologies, despite the high morphological diversity of this group. In this study, we substantially increase the number of available hypotrich LSU rDNA gene sequences by the addition of 857 environmental sequences, and we investigate whether a more divergent gene and dense taxon sampling could better resolve the phylogeny of Hypotricha and shed light on the patterns of ecological transitions in the evolutionary history of the group. Pairwise distances of LSU rDNA sequences are generally higher than those for SSU rDNA within each order of Hypotricha, and both concatenated rDNA and LSU rDNA trees provide more resolution for hypotrich phylogenetics. Three traditional (morphology based) hypotrich orders, Stichotrichida, Sporadotrichida and Urostylida, are polyphyletic, but a monophyletic core Urostylida are found in our trees. A brackish/marine environment is inferred as ancestral within Hypotricha, with subsequent ecological diversification into freshwater, soil environments before the origin of major clades and some transitions back to the marine. However, inferred ecological transitions in Hypotricha are influenced by genes, methods and taxa.

Automated summary

By adding 857 environmental sequences to the hypotrich LSU rDNA gene sequences, this study significantly increased the resolution for hypotrich phylogenetics. The traditional hypotrich orders were found to be polyphyletic, with a monophyletic core Urostylida in the trees. The inferred ecological transitions in Hypotricha were found to be influenced by genes, methods, and taxa.