There and Back Again: Parallel Evolution of Cell Coverings in Centrohelid Heliozoans.

Centrohelid heliozoans are non-ciliary axopodial protists of uncertain affinities. Cell coverings (organic spicules, siliceous plate-scales and spine-scales) are the main character for their taxonomy. Phylogenetic reconstructions of this group are mostly based on the sequences of 18S rDNA. Here molecular and microscopical data on Raphidiophrys heterophryoidea, R. drakena sp. nov., Spiculophrys agregata gen. et sp. nov. and four strains here referred to as Heterophrys-like organisms (HLOs) are provided. The family Raphidiophryidae is shown to be polyphyletic. The position of Spiculophrys is uncertain, but it probably represents the most basal branch in the order Pterocystida. Three strains of HLOs branch deeply inside the genus Acanthocystis and one strain inside Polyplacocystis, even though they differ considerably in morphology of the cell coverings from other members of these two genera. The possible reasons for this phenomenon are discussed. On the basis of newly obtained sequences and EDX-analysis an interpretation of the evolution of cell coverings is proposed. It appears that cell coverings were ancestrally organic, then silicified independently on two occasions and after that multiple cases of desilicification or even complete loss of coverings took place. So, in the two centrohelid orders the evolution includes similar sequences of parallel events.