Attheya (Bacillariophyta) from the northwestern Sea of Japan: a description of two subgenera based on molecular and morphological data

Strains of the diatoms Attheya ussurensis, A. decora and A. longicornis, isolated from the northwestern Sea of Japan, were studied to infer their phylogeny and determine their systematic position. Morphological studies were conducted using light and transmission electron microscopy, and molecular analyses were based on nuclear-encoded small-subunit (18S) ribosomal DNA (rDNA) and ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit (rbcL) of the plastid gene. The strains studied, as well as sequences of A. longicornis and A. septentrionalis accessed from GenBank, belong to the class Mediophyceae. Molecular data confirmed that Attheya holds a basal position relative to the genera Biddulphia and Chaetoceros (Chaetocerotales). Thus, Attheya is a member of this lineage and not sister to Bacillariophyceae. Strains of Attheya belong to two clearly distinct lineages, differing in morphology and ecology. One lineage included strains of the type species of the genus, A. decora and the species A. ussurensis (psammophytes); the second lineage included the strains A. longicornis and A. septentrionalis (epiphytes or sea-ice diatoms). Morphologically, the two lineages differed in ratio of horn length to valve diameter, shape and number of plastids, presence/absence of girdle band perforation and presence/absence of rimoportula on the valve. Based on morphological and molecular data, we propose Attheya subgenus Attheya, to include A. decora, A. ussurensis, A.arenicola; and Attheya subgenus Dolichoceratia, to include A. longicornis, A. septentrionalis, A. flexuosa, A. gaussii and A. armata.