The classification of
Pelomyxa has been the subject of considerable discussion, in recent decades.
Pelomyxa lack
mitochondria, as well as several other organelles usually found in
eukaryote cells (notably,
peroxisomes and
dictyosomes). At one time, they were also believed to lack flagella and to be incapable of
mitosis. As nucleated cells that lacked "nearly every other cell-inclusion of eukaryotes",
Pelomyxa were, for a time, regarded as surviving "proto-Eukaryotes", standing somewhere between the bacteria and the modern cell. In 1973, it was proposed that the ancestors of
Pelomyxa palustris had branched off from the eukaryote line before the advent of mitochondria In 1976, Jean M. Whatley wrote that
Pelomyxa palustris "may justly be considered the most primitive eukaryotic organism living today." As such, the organism was potentially a modern analogue of the ancestral eukaryote that, according to the
theory of serial endosymbiosis, internalized the bacterial symbiont that later evolved into the mitochondria of the modern cell. The species was known to host several bacterial symbionts. While the function of these was unclear, Whatley argued that they might provide a useful evolutionary example, indicating the "ways in which a bacterial mitochondrial transformation might have been attained." The Archamoebae were, in turn, recruited to the new kingdom of
Archezoa, along with other amitochondriate eukaryotes, the
Metamonads and the
Microsporidia. The primitivity of
Pelomyxa came into doubt in 1988, when Joe I. Griffin published a structural study of
Pelomyxa palustris showing that the species does, after all, possess rudimentary flagella, and that it does mitose. Griffin concluded that "
Pelomyxa is neither primitive nor different from related forms, once it is realized that its relatives are amoeboid flagellates." By the end of the decade, it was clear that all members of Cavalier-Smith's Archamoebae were descended from mitochondriate cells. In other words, they were not early-branching or "primitive" eukaryotes at all, but rather "degenerate protists" that had lost organelles their ancestors had possessed. Consequently,
Pelomyxa and the other Archamoeba were reassigned to the phylum
Amoebozoa, under the subphylum
Conosa (shared with the
Mycetozoan slime moulds). Kingdom Archezoa was eliminated. ==Video gallery==