On the description of Tisochrysis lutea gen. nov sp nov and Isochrysis nuda sp nov in the Isochrysidales, and the transfer of Dicrateria to the Prymnesiales (Haptophyta)

The Isochrysidaceae is a family of non-calcifying organisms within the haptophyte order Isochrysidales. Isochrysis galbana, a species widely used as a food source in aquaculture, is the best-known representative of this family that contains three genera but only six described species. We sequenced partial nuclear small subunit (SSU) and large subunit rDNA and mitochondrial cytochrome oxidase 1 genes of 34 isochrysidacean culture strains (including authentic strains when available) and compared molecular phylogenetic inferences with cytological and ultrastructural observations. The isochrysidaceaen culture strain Isochrysis affinis galbana (Tahiti isolate), widely used in aquaculture and commonly known as T-Iso, is clearly genetically distinct from Isochrysis galbana, despite seemingly being morphologically identical. A strain with a similar ultrastructure to that of Isochrysis galbana except for the lack of body scales had sequences that were more similar to but still distinct from those of Isochrysis galbana. Dicrateria inornata, a species that lacks body scales, is classified within the Isochrysidaceae, but the SSU rDNA sequence of the authentic strain of this species matches that of Imantonia rotunda within another haptophye order, the Prymnesiales. D. inornata and Imantonia rotunda have similar ultrastructure except for the respective absence/presence of scales. These results lead us to propose the erection of one new genus (Tisochrysis gen. nov.) and two new species (Tisochrysis lutea sp. nov. and Isochrysis nuda sp. nov.). D. inornata is reclassified within the Prymnesiales, and Imantonia rotunda is transferred to this genus (Dicrateria rotunda comb. nov.).