Phylogeny and ultrastructure of Nephroselmis and Pseudoscourfieldia (Chlorophyta), including the description of Nephroselmis anterostigmatica sp nov and a proposal for the Nephroselmidales ord. nov.

A new species of Nephroselmis, Nephroselmis anterostigmatica, is described from coastal waters of the Palau Islands and the southern part of Japan. The long flagellum was stretched out at angle to the cell and showed a characteristic sigmoid line. A cup-shaped yellowish green chloroplast with two narrow sinuses included a pyrenoid with tubular invaginations of thylakoids. In contrast to other species of Nephroselmis, the eyespot was located at the anterior side of the cell. The transitional region of the flagellum possessed a single transitional plate and a stellate structure. The cell body was covered by two types of square scales (windmill/Maltese cross scales absent), small stellate scales and two types of large spiny scales. The flagellar surface was covered by inner square and outer rod-shaped scales, and ornamented with T- and PI-hairs. Tip hairs are absent. Nephroselmis anterostigmatica is superficially similar to Nephroselmis pyriformis (N. Carter) Ettl, but a detailed comparison of morphological characters, such as chloroplast shape, position of eyespot, composition of hair scales, presence or absence of the two types of small square scales and the large spiny scales, reveals clear differences between the two species. The molecular phylogenetic analysis on the basis of small subunit ribosomal DNA (SSU rDNA) sequences showed that N. anterostigmatica was not closely related to N. pyriformis, but was allied to Nephroselmis astigmatica Inouye & Pienaar. Comparisons of the hair scales and SSU rDNA sequences confirm that the identity of strain CCMP717 is N. pyriformis, and Pseudoscoufieldia marina (Throndsen) Manton has flagella ornamented with a different type of hair scales. The morphological evolution within the genus Nephroselmis is discussed and the order Nephroselmidales is proposed.