Morphological and genetic variability of assemblages of <i>Cyclotella ocellata</i> Pantocsek/<i>C. comensis</i> Grunow complex (Bacillariophyta, Thalassiosirales)

Centric diatom taxa in the <i>Cyclotella ocellata</i> and <i>C. comensis</i> complexes show high morphological variability and often apparently continuous morphological transitions. In this study, we investigated natural assemblages of the <i>C. ocellata/C. comensis</i> complex from Hungarian and Croatian lakes and from Turkish streams using morphological and molecular methods. The studied assemblages contained cells with morphologies resembling <i>C. ocellata</i> as well as other, closely related, species: <i>C. comensis</i>, <i>C. pseudocomensis</i>, <i>C. costei</i>, and <i>C. trichnoidea</i>. The goal of our paper was to assess whether the observed morphological differences were due to intraspecific variability or suggest the existence of several, putatively distinct species.Ten morphometric characters were measured, which, either individually, or in pairs, did not differentiate the nominal taxa in our assemblages. However, multivariate discriminant analysis has revealed a group including <i>C. ocellata</i> and <i>C. trichonidea</i> morphologies could be separated from another containing <i>C. comensis</i>, <i>C. pseudocomensis</i> and <i>C. costei</i>.A nuclear (18S rDNA) and a chloroplast (<i>rbc</i>L) gene were amplified and partially sequenced from environmental DNA or from isolated cells. The sequences showed little variability among the assemblages and nominal species. Although general congruence of molecular and morphometric separation supports the species level separation of <i>C. ocellata/trichonidea</i> from the probably conspecific <i>C. comensis</i>/<i>pseudocomensis</i>/<i>costei</i>, sequence divergences between the groups are in the same range as within them, so that a conspecificity of all four taxa cannot be unequivocally excluded.