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and Hypermastigida (Lophomonas, Staurojoenina, and Trichonympha). These cells either bear karyomastigonts or derive by differential organelle reproduction (simple morphological steps) from those that do (Table 1). When, during evolution of these protists, nuclei were severed from their karyomastigonts, akaryomastigonts were generated (Kirby, 1949). Nuclei, unattached, at least temporarily, to undulipodia were freed to proliferate and occupy central positions in cells. Undulipodia, also freed to proliferate, generated larger, faster-swimming cells in the same evolutionary step.

The karyomastigont is the conspicuous central cytoskeleton in basal members of virtually all archaeprotist lineages [three classes: Archamoeba, Metamonads, and Parabasalia (Brugerolle, 1991)] (Fig. 2). In trichomonads, the karyomastigont, which includes a parabasal body (Golgi complex), coordinates the placement of hydrogenosomes (membrane-bounded bacterial-sized cell inclusions that generate hydrogen). The karyomastigont reproduces as a unit structure. Typically, four attached kinetosomes with rolled sheets of microtubules (the axostyle and its extension the pelta) reproduce as their morphological relationships are retained. Kinetosomes reproduce first, the nucleus divides, and the two groups of kinetosomes separate at the poles of a thin microtubule spindle called the paradesmose. Kinetosomes and associated structures are partitioned to one of the two new karyomastigonts. The other produces components it lacks such as the Golgi complex and axostyle.

Nuclear α-proteobacterial genes were interpreted to have originated from lost or degenerate mitochondria in at least two archaeprotist species [Giardia lamblia (Roger et al., 1998); Trichomonas vaginalis (Roger et al., 1996; Germont et al., 1996)] and in a microsporidian (Sogin, 1997). Hydrogenosomes, at least some types, share common origin with mitochondria. In the hydrogen hypothesis (Martin and Müller, 1998), hydrogenosomes are claimed to be the source of eubacterial genes in amitochondriates. That mitochondria were never acquired in the ancestors we consider more likely than that they were lost in every species of these anaerobic protists. Eubacterial genes in the nucleus that are not from the original spirochete probably were acquired in amitochondriate protists from proteobacterial symbionts other than those of the mitochondrial lineage. Gram-negative bacteria, some of which may be related to ancestors of hydrogenosomes, are rampant as epibionts, endobionts, and even endonuclear symbionts —for example, in Caduceia versatilis (d'Ambrosio et al., 1999).

Karyomastigonts freed (detached from) nuclei independently in many lineages both before and after the acquisition of mitochondria. Calonymphid ancestors of Snyderella released free nuclei before the mitochondrial symbiosis (Dolan et al., 2000), and Chlamydomonas-like ancestors of other chlorophytes such as Acetabularia released the nuclei after the lin-

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