Taxonomy and the phylogenetic position of a new prasinophycean alga, Crustomastix didyma gen. & sp. nov. (Chlorophyta)

Abstract

A new prasinophycean flagellate, Crustomastix didyma Nakayama, Kawachi & Inouye gen. & sp. nov., is described from the northwestern Pacific. This alga is elongate and bean-shaped and has two ventrally inserted, subequal flagella, with three types of hair scales. The entire cell and flagella are covered by a thin coat, which is probably derived from the Golgi apparatus and released through the duct system. The yellowish chloroplast is divided into two lobes connected by a narrow bridge. There is no pyrenoid or eyespot. The flagellar apparatus comprises the distal and proximal fibres, the Id and Is microtubular roots, and a duct fibre. A multilayered structure (MLS) is associated with the Id root. Some features, such as the swimming behaviour, the transitional region, and the absence of microtubular roots associated with the number 2 basal body, suggest a close relationship to the Mamiellales, but this alga differs from other members of the Mamiellales in having a MLS and a duct system, which are found in the Pyramimonadales. Crustomastix didyma appears to be the most primitive member of the Mamiellales found so far and retains some characters of a putative common ancestor of the Mamiellales and Pyramimonadales. The discovery of this unique prasinophycean alga provides new insights into the phylogeny and evolution of the green algae.