Quadricoccopsis gen. nov., a new genus of Quadricoccus-like algae in Oocystaceae from China (Trebouxiophyceae, Chlorophyta)

Abstract

Members of Quadricoccus-like algae are characterized by oval to ellipsoid cells adherent to the bowl-shaped or stretched, empty mother cell walls and are common in phytoplankton of inland waters. To date, the morphologically similar genera Quadricoccus and Lobocystis are accepted for this group and the former had been phylogenetic positioned in Oocystaceae. In this study, seven strains of Quadricoccus-like algae were identified and successfully cultured in the laboratory. Light and electron microscope observations and phylogenetic analysis revealed that the strains represent three different species within a new genus, described here as Quadricoccopsis gen. nov. It differed from genus Quadricoccus by characteristically cell adherent mode that two pairs of daughter cells connected to the mother cell remnant respectively by the pole and median portion and distinguished from Lobocystis by variable 2-4-8 autospores, characteristically cell adherent mode and only found in limnetic water. The three new species, described here as Q. simplex, Q. parva and Q. glomerata, differed in cell size, colony morphology and autospore number. Phylogenetic analysis revealed the genus Quadricoccopsis in Oocystaceae and a close relationship with Oocystidium, which is far away from the phylogenetic position of Quadricoccus in one of the granulated clades. The Quadricoccus-like algae were, therefore, proved to be a paraphyly. Furthermore, apart from Planctonema-like algae, the Oocystaceae characteristic cell wall ultrastructure, which is multi-layered with cellulose fibrils in each layer perpendicular to those of the adjoining layer, is not shown again in genus Quadricoccopsis. Different cell wall substructures may be related to the colony formation mechanism. Whether the ultrastructure criterion is applied to all the Oocystaceae needs to be re-evaluated and, further, the definition of this family should be discussed.