Comparative ultrastructural analysis of Chloromonas and Gloeomonas (Chlorophyceae): tracing the origin of Gloeomonas-specific basal apparatus traits

摘要

On the basis of prior phylogenetic analyses placing Gloeomonas as a lineage within the very diverse genus Chloromonas, it was hypothesized that the morphologically distinct Gloeomonas, bearing widely spaced basal bodies, evolved from ancestral organisms like Chloromonas. In addition, the phylogenetically related Chloromonas (Cr.) rubrifilum, was expected to possess traits intermediate to the two forms. To test these hypotheses, we performed detailed transmission electron microscope analysis on nine species of these genera. The species were divided into two categories on the basis of ultrastructural features: group 1, consisting of four diverse species of Chloromonas and Ixipapillifera, had a V-shaped basal body arrangement, and group 2, of Cr. rubrifilum and four Gloeomonas species, forming a single clade possessing widely spaced basal bodies. Members of group 1 commonly had a simple, flat plate-like distal connecting fiber (dcf), three or sometimes two microtubules in the sinister root, continuous dexter and sinister striated microtubule-associated fibers (SMAFs), and a coarsely striated proximal fiber (CSPF) at the posterior-most regions of the basal bodies and probasal bodies, but lacked proximal and median proximal connecting fibers. Group 2 inherited these features, but had novel traits including the elongation and modification of the dcf, SMAFs, and CSPF that occurred with separation of basal bodies during growth, a dcf-associated layered structure, as well as protruding flagellar collars. The ultrastructural traits of Gloeomonas are interpreted as being evolutionarily modified from an ancestral Chloromonas morphology. Specific ultrastructural features were determined to be useful in characterizing these genera. Cr. rubrifilum of group 2 was not intermediate morphologically, but possessed the traits of Gloeomonas; thus G. rubrifilum comb. nov. was proposed.