Ultrastructure and reconstruction of the flagellar apparatus architecture in Ankylochrysis lutea (Chrysophyceae, Sarcinochrysidales)

The ultrastructure of the sarcinochrysidalean alga Ankylochrysis lutea (van der Veer) Billard comb. nov. (Chrysophyceae) is investigated, with particular reference to the three-dimensional structure of the flagellar apparatus. This shows characteristics of heterokont algae, but has a number of unusual features: the R2 root is absent; the R3 root is straight; the ‘bypassing root’ consists of only one microtubule; two transitional plates are present in the flagellar transition region. The homologies of the flagellar apparatus components of A. lutea and two additional sarcinochrysidalean algae, Giraudyopsis stellifera Dangeard and Sarcinochrysis marina Geitler, are discussed. Because the flagellar apparatus of A. lutea differs from both G. stellifera and S. marina, the flagellar apparatus is not a usable character to delimit the order Sarcinochrysidales, which is probably a paraphyletic taxon. The external cell covering and the scale-like structures are unique features of A. lutea and may be diagnostic features of the Sarcinochrysidales sensu stricto sensu O'Kelly. The taxonomic significance of the external cell covering is discussed.