On the phylogenetic position of the genus Raphidocystis (Haptista: Centroplasthelida) with notes on the dimorphism in centrohelid life cycle

Abstract

Centrohelid heliozoan Raphidocystis glabra was reisolated for the first time after initial description and put into a clonal culture. Its correct identification was confirmed by scanning and transmission electron microscopy of scales. The first light microscopy data from the living cells were obtained. Phylogenetic analysis of its position using 18S rDNA sequences was also performed. This species branches inside of the Polyplacocystis clade, being closely related to the strain HLO4. The latter was isolated from the same sample with R. glabra and has spicules, not siliceous scales, unlike all the other members of this clade. Using another strain isolated from this sample the co-specificity of HLO4 and R. glabra was demonstrated, and the presence of spicule-bearing stages in the life cycle of Raphidocystis was shown. Polyplacocystis revealed to be paraphyletic with Raphidocystis embedded in it. Moreover, representatives of both genera share the similar monolayered plate scales with hollow inflected margin. Thus, a family-level name Raphidocystidae for the whole clade was proposed and all the Polyplacocystis species were transferred to Raphidocystis which name has a priority. The evolution of centrohelid coverings in the light of new findings was discussed, as well as the possible wide distribution of dimorphism in centrohelid life cycles.

Keywords: 18S rDNA phylogeny; Cell coverings; Centrohelids; Dimorphism; Life cycle; Ultrastructure.