Desmodesmus tropicus (Chlorophyta) in the Danube Delta – reassessing the phylogeny of the series Maximi

Abstract

ABSTRACT Desmodesmus maximus (West & G.S.West) Hegewald, D. perforatus (Lemmermann) E.Hegewald and D. tropicus (W.B.Crow) E.Hegewald form the series Maximi within the genus Desmodesmus. Desmodesmus tropicus is a rare species although it is widely distributed in tropical regions including South-East Asia, South America and the USA. It is less common in temperate fresh waters, known only from Ukraine. In this study, we report for the first time its presence in the Danube Delta (Puiu Lake), Romania. The morphology and molecular phylogeny of two newly isolated strains were studied in relation to other strains of the genus Desmodesmus, series Maximi. Cell wall structures (rosettes, spines, tubes, honeycomb-shaped mesh with hexagonal units) which are important in the taxonomy of these algae were investigated by light-, scanning- and transmission electron microscopy. The phylogenetic position of the strains in the Algae and Cyanobacteria Culture Collection of the Institute of Biological Research was assessed using 18S rDNA sequences, ITS2 primary and secondary structures and by analysis of Compensatory Base Changes. Morphological traits and measurements of cells, coenobia and spines matched the description of D. tropicus, although some differences from the established morphological key (ridge-like structures resembling fine ribs) were also noticed. Identification as D. tropicus var. longiclathratus (Tell) Jeon & Hegewald was based on the presence of inner spines on four-celled and especially on eight-celled coenobia. The ITS2 sequence and secondary structure phylogeny supported the assignment of both AICB strains to D. tropicus, with no divergence from other D. tropicus sequences to support the identification of var. longiclathratus. Nevertheless, the morphological measurements and the ITS2 primary and secondary structure data indicated two distinct subclusters within D. tropicus, and a new possible taxon within the series Maximi.