Taxonomic revision of the genus Chorda (Chordaceae, Laminariales) on the basis of sporophyte anatomy and molecular phylogeny

摘要

Chorda kikonaiensis Sasaki & Kawai sp. nov. is newly described from Hokkaido, Japan, on the basis of morphological and molecular data. Furthermore, based on the latter, the common Chorda species that has been identified as Chorda filum in Japan is shown to be independent from the European (true) Chorda filum at the species level. Therefore, Chorda asiatica Sasaki & Kawai sp. nov. is described for the cryptic Japanese taxon. Chorda kikonaiensis resembles C. filum and C. asiatica, but is distinguished by the shorter (0.4-1.3 m) and softer sporophyte, and the thinner cortex composed of fewer (2-4) cells. The independence of this species is further supported by molecular phylogenetic analyses using rbcL gene and internal transcribed spacer (ITS) rDNA sequences. Chorda asiatica is more variable in morphology (length of erect thallus and number of cell layers composing the cortex) than C. kikoniensis and C. rigida, and is difficult to distinguish from C. filum on the basis of morphology, but is clearly separated from the other species (C. filum, C. kikoniensis and C. rigida) on the basis of ITS rDNA data. Chorda filum is distributed in the Atlantic, whereas C. asiatica, C. kikonaiensis and C. rigida are distributed in the Pacific Ocean. Furthermore, the presence of one or two additional cryptic species is suggested in the northern Pacific using molecular data. Therefore, it is shown that the genus Chorda has considerably higher taxonomic and genetic diversity in the Pacific than in the Atlantic. Although no molecular data are available for other eastern Asian (southeastern Russian coast, Korea and China) Chorda species, on the basis of morphology and geographical distributions, they are likely referable to C. asiatica.