Evolutionary relationships between the varieties of green algae Pediastrum boryanum and P. duplex s.l. (Chlorophyceae, Hydrodictyaceae)

The infraspecific relationships of the two most variable species, Pediastrum boryanum and P. duplex s.l. identified on the basis of morphological criteria, have been poorly studied so far. The present study focused on 12 original strains isolated from Polish water bodies and 29 strains from the GenBank database, which represent P. boryanum, P. duplex s.l. varieties and other morphologically similar taxa, against the background of 14 other strains of the family Hydrodictyaceae. In order to estimate the level of congruence between the phylogeny and the classical taxonomic system based on morphological criteria, the strains of P. boryanum and P. duplex s.l., and other morphologically similar taxa were subjected to parallel phylogenetic analyses applying Bayesian and Maximum Likelihood methods to combined molecular data (26S rDNA and rbcL cpDNA) and morphological analyses based on light and scanning electron microscopy, and iconographic documentation of almost all strains published elsewhere. The gene topologies revealed many discrepancies in the morphological features used to delimit the analysed taxa. The polyphyly of both P. boryanum and P. duplex s.l. was confirmed. Pseudopediastrum boryanum var. cornutum (formerly P. boryanum var. cornutum) formed a well supported monophyletic clade, whereas other varieties, including var. boryanum, brevicorne and longicorne, proved to be complex taxa. Previous description of Lacunastrum gracillimum (formerly P. duplex var. gracillimum) was well supported. A clade composed mostly of European strains identified as P. duplex var. rugulosum should be regarded as this taxon, whereas clades containing strains exhibiting P. duplex var. rugulosum-like morphology represent new taxa. Taxonomical changes within both P. boryanum and P. duplex s.l. are required on the basis of a higher number of strains and detailed morphometric data.