Rapid radiation, gradual extinction and parallel evolution challenge generic classification of spathidiid ciliates (Protista, Ciliophora)

Abstract

Reconstruction of evolutionary history of the ciliate order Spathidiida has proven elusive. There are weakly statistically supported deeper nodes in 18S rRNA gene phylogenies on one hand, while several statistically strongly supported clusters containing morphologically dissimilar taxa that usually lack any common apomorphies on the other hand. To clarify whether silent aspects of the spathidiid phylogeny are results of methodological problems of tree‐building algorithms, we examined informativness of the macronuclear rRNA locus using an increased taxon and marker sampling as well as a complex statistical approach. Likelihood mapping revealed that the macronuclear rRNA locus has enough phylogenetic information to infer spathidiid relationships. However, some noise and conflicts hamper inference of deeper branching events, as documented by short parallelograms in the star‐like central part of the split graphs and by low numbers of phylogenetically informative nucleotide homologies supporting deeper nodes of phylogenetic trees. Based on the diversification analyses and the γ‐statistic, we assembled a body of evidence that the spathidiid phylogeny retains the signature of one or several rapid radiations in the Palaeozoic and a subsequent gradual extinction that has started in the Mesozoic. A combination of these two phenomena along with polyphyly of three large spathidiid genera (Spathidium, Epispathidium and Arcuospathidium) are speculated to be the main reasons for fuzzy phylogenetic picture within the order Spathidiida. Because natural classification of spathidiids cannot be provided at the present state of knowledge, we suggest to keep the existing morphology‐based generic classification, but stress that the large spathidiid genera are artificial collective groups.