Ref ID : 4873
T. Cavalier-Smith, E.E. Chao, C.E. Thompson, and S.L. Hourihane; Oikomonas, a Distinctive Zooflagellate Related to Chrysomonads. Arch.Protistenk 146:273-279, 1995/96
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We have examined the common soil zooflagellate Oikomons mutabilis Kent in the light and electron microscope and sequenced its 18S rRNA gene. It has tubular mitochondrial cristae and a single anterior tinsel cilium clothed in a unilateral row of bipartite (or possibly tripartite) hairs (retronemes) with single terminal filaments, as well with simple non-tubular hairs; it is therefore clearly a heterokont. It differs from previously studied colourless chrysomonads in the apparent absence of a vestigial leucoplast with eyespot, in the probably bipartite retronemes, in the absence of a vestigial smooth cilium, and in the unilateral arrangement of the retronemes. Scales appear to be absent. Phylogenetic analysis of its 18S rRNA sequence, however, clearly shows that it is most closely related to the chrysomonads, but it is not specifically related to Paraphysomonas. On most trees it is the sister group to chrysomonads; though its sequence is, therefore, significantly divergent from those of all genuine chrysomonads, with the maximum parsinomy method it can group weakly with Ochromonas. A possible relationship 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 plastid loss and the consequent secondary origin of zooflagellates within the Heterokonta: our present analysis suggests that Pseudofungi (oomycetes and hyphochytriomycetes) also evolved from ochristan algae, by the loss of chloroplasts, thus providing evidence for at least four separate losses of chloroplasts within Heterokonta. We create a new order (Oikomonadales), validate the class Oikomonadea for Oikomonas, and include it with the classes Chrysomonadea, Eustigmatophycea, and Raphidomonadea in the superclass Limnistia, for which we present a revised classification.