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

Abstract:

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 and thick setae, and several morphological differences were found in the resting spores. In phylogenetic analyses inferred from the LSU sequences, C. subtilis var. abnormis and C. subtilis formed well-separated sister taxa, which supported their morphological similarity and also their distinct differences. Their differences were supported by 42 base pairs, giving a genetic difference of 7.2%. Combined morphological and molecular data revealed that C. subtilis and C. subtilis var. abnormis were distinct species. We therefore reinstate the latter taxon as C. abnormis Proschkina-Lavrenko.

Key Words::

• Chaetoceros subtilis
• Chaetoceros subtilis var. abnormis
• Large-subunit ribosomal gene
• Morphology
• Phylogeny

ACKNOWLEDGEMENTS

This work was supported by grants from the National Natural Science Foundation of China (31570205, 31370235) and Science and Technology Project of Guangzhou (201607010370).

SUPPLEMENTARY DATA

Supplementary data associated with this article can be found online at http://dx.doi.org/10.2216/18-8.1.s1.