Anucleated cryptophyte endosymbionts in the gonyaulacalean dinoflagellates, Amylax buxus and Amylax triacantha (Dinophyceae)

Cryptophyte endosymbionts, suffering selective digestion of nuclei, were found in gonyaulacalean dinoflagellates, Amylax buxus (Balech) Dodge and Amylax triacantha (Jörgensen) Sournia. They emitted bright yellow-orange fluorescence (= 590 nm emission) under epifluorescent microscopy and possessed U-shape plastids. Under transmission electron microscopy, the plastid was characterized with loose arrangement of two to three thylakoids stacks and with stalked pyrenoid, all coincide to those of cryptophyte genus Teleaulax. Indeed, molecular data based on plastid small subunit rRNA gene demonstrated that the endosymbionts in Amylax are originated from Teleaulax amphioxeia. The stolen plastid (kleptoplastids) in Dinophysis is also acquired from this cryptophyte species. However, in sharp contrast to the case of Dinophysis, the plastid of endosymbiont in Amylax was surrounded by double layer of plastid endoplasmic reticulum, and within the periplastidal area, nucleomorph was retained. The endosymbionts also possessed mitochondria with characteristic plate-like cristae, but lost cell-surface structure. Dinoflagellate's phagocytotic membrane seemed to surround the endosymbionts right after the incorporation, but the membrane itself would probably be digested eventually. Remarkedly, only one cryptophyte cell among 14 endosymbionts in a cell of A. buxus had a nucleus. This is a first finding of kleptoplastidy in gonyaulacalean dinoflagellates, and provides unique strategy of a dinoflagellate in which it selectively eliminate the endosymbiont nucleus