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

A new prasinophycean flagellate, Crustomastix didyma Nakayama, Kawachi & In ouye gen. & sp. nov., is described from the northwestern Pacific. This alga is elongate and bean-shaped and has two ventrally inserted, subequal flage lla, with three types of hair scales. The entire cell and flagella are cove red 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. T he 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 Mamiella les. but this alga differs from other members of the Mamiellales in having a MLS and a duct system, which are found in the Pyramimonadales. Crusromast ix 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 Mam iellales and Pyramimonadales. The discovery of this unique prasinophycean a lga provides new insights into the phylogeny and evolution of the green alg ae.