Absolute orientation of the flagellar apparatus of Hibberdia magna comb. nov. (Chrysophyceae)

Abstract

The first flagellum of Hibberdia magna comb. nov. bears mastigonemes that have both short and long lateral filaments attached to the tubular shaft. The second flagellum is very short (ca. 850 nm) and is directed posteriorly approximately 160° from the first flagellum. Three microtubular flagellar roots (R1, R2 and R4) and a rhizoplast (= striated root) are present. The R1 root consists of four microtubules that arise near the right surface of the first flagellum basal body; the R1 root extends to the dorsal side of the cell and then curves back along the left side of the cell. Cytoskeletal microtubules are nucleated from the R1 root including one loose cluster of cytoskeletal microtubules that extends down the left side of the cell adjacent to the contractile vacuole. The R2 root is a single microtubule that arises along the left surface of the first flagellum basal body and extends to the left side of the cell. The R4 root consists of three microtubules that arise along the left side of the second flagellum basal body. A helical band wraps around two microtubules at the proximal end of the R4 root. Two of the three R4 root microtubules extend along the left side of the second flagellum, curve around to the right side of that flagellum and terminate. No R3 root was found. The orientation of the basal bodies of Hibberdia gen. nov. is similar to that of the Xanthophyceae and Oomycetes. There are apparent homologies in the R1, R2 and R4 roots of Hibberdia and these and other protists, but only Hibberdia lacks a R3 root. Three long flagella are present in preprophase but later one is endocytosized and the axoneme extends to the posterior of the cell. During metaphase the nuclear envelope is more or less intact except at the poles; the flagellar apparatuses are at the poles and the spindle microtubules originate near the basal bodies. Two stages are known in the life history: 1) a capsoidlike state with non‐swimming flagellate cells inside a colonial gel, and 2) a free‐swimming single‐celled monad state. Vegetative cell division occurs in both stages. The flagellar apparatus, the cell division process and the life history combined with the previously described unique light‐harvesting antheraxanthin make H. magna distinct from other algae. A new genus, Hibberdia gen. nov., a new family, Hibberdiaceae fam. nov. and a new order, Hibberdiales, ord. nov. are described.