Revisiting Chaetoceros subtilis and C. subtilis var. abnormis (Bacillariophyceae), reinstating the latter as C. abnormis

Chaetoceros is one of the most species-rich diatom genera, but about one-third to one-half of the taxa are considered synonyms, and a modern revision is needed on the basis of a combination of morphological and molecular data. Chaetoceros subtilis Cleve and C. subtilis var. abnormis (Proschkina-Lavrenko) Proschkina-Lavrenko were revisited in the present study. Monoclonal strains were established from the southeast Chinese coast. Morphology of vegetative cells and resting spores was observed using light microscopy, as well as scanning and transmission electron microscopy. The hypervariable D1–D3 region of the nuclear large-subunit ribosomal RNA-encoding gene (LSU ribosomal DNA) was sequenced for information on phylogenetic relationships. Species delineations were evaluated for the two taxa. Morphologically, C. subtilis var. abnormis was quite similar to C. subtilis, but differences were found in the number of thick terminal posterior setae, detailed structure and morphometric data of intercalary a...