Comparative approaches to the taxonomy of the genus Galdieria merola (Cyanidiales, Rhodophyta)

Ecophysiological, morphological, ultrastructural, and phylogenetic methods were used to study nine strains of Galdieria isolated from different geographical regions in order to clarify the taxonomy and evolution of the genus and to understand the phylogeny of the Cyanidiales. The studied characters were often conserved in all of the isolates. Confocal laser scanning microscopy (CLSM) observations confirmed the presence of a single plastid and allowed its 3-D reconstruction. Phylogenetic analysis showed that all Galdieria isolates but one (G. maxima) form a strongly supported monophyletic lineage within the Cyanidiales. G. maxima is closely related to Cyanidioschyzon merolae and its taxonomic status needs to be revised. The Cyanidiales is an evolutionarily distinct group within the Rhodophyta. In spite of the uniform morphology among Galdieria strains, analysis of rbcL sequences showed moderate to high levels of sequence divergence within this genus. Apart from G. maxima, which is from Kunashir, Russia, the two other Russian isolates, G. daedala and G. partita grouped together with the remaining Galdieria taxa forming three separate clades. The Galdieria clades reflected the geographic site of origin of the isolates. On the basis of these data, we suggest that Galdieria (excluding G. maxima) represents a species complex. The limited sampling makes it difficult, however, to recommend changes to the existing classification system. We suggest that a broad and systematic study of Galdieria is required to understand fully the evolutionary history and taxonomy of this genus.