OIKOMONAS, A DISTINCTIVE ZOOFLAGELLATE RELATED TO CHRYSOMONADS

We have examined the common soil zooflagellate Oikomonas mutabilis KEN T in the light and electron microscope and sequenced its 18s rRNA gene . It has tubular mitochondrial cristae and a single anterior tinsel ci lium clothed in a unilateral row of bipartite (or possibly tripartite) hairs (retronemes) with single terminal filaments, as well with simpl e non-tubular hairs; it is therefore clearly a heterokont. It differs from previously studied colourless chrysomonads in the apparent absenc e of a vestigial leucoplast with eyespot, in the probably bipartite re tronemes, in the absence of a vestigial smooth cilium, and in the unil ateral arrangement of the retronemes. Scales appear to be absent. Phyl ogenetic analysis of its 18s rRNA sequence, however, clearly shows tha t it is most closely related to the chrysomonads, but it is not specif ically related to Paraphysomonas. On most trees it is the sister group to chrysomonads; though its sequence is, therefore, significantly div ergent from those of all genuine chrysomonads, with the maximum parsin omy method it can group weakly with Ochromonas. A possible relationshi p with Ochromonas is also suggested by the presence of numerous large cortical vacuoles which fluoresce brightly when exposed to ultraviolet or blue light. Bright short wavelength light kills the cells in a few seconds. Oikomonas appears to be a third independent example of plast id loss and the consequent secondary origin of zooflagellates within t he Heterokonta: our present analysis suggests that Pseudofungi (oomyce tes and hyphochytriomycetes) also evolved from ochristan algae, by the loss of chloroplasts, thus providing evidence for at]east four separa te losses of chloroplasts within Heterokonta. We create a new order (O ikomonadales), validate the class Oikomonadea for Oikomonas, and inclu de it with the classes Chrysomonadea, Eustigmatophycea, and Raphidomon adea in the superclass Limnistia, for which we present a revised class ification.