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form rolled up to ultimately form an ancestral prostomial form, while Small imagined that the ribbon-like form might have been very similar to the contem- porary karyorelictean Kentrophoros , with an elon- gate, flattened \u201cventral\u201d surface, which it used for ingestion. Both these schema suggested that oral ciliature derived from somatic ciliature. Eisler (1992) proposed a contrary model in which the paroral dikinetids of ciliates represent the first ciliature of the ancestral ciliate and that the somatic kinetids derived from this. This model, most recently summarized by Schlegel and Eisler (1996), begins with a similar polyenergid stage as above. However, this \u201cfirst\u201d kinety is interpreted to be a paroral , lying adjacent to a tube-like cytopharyngeal apparatus supported by microtubules, as is found in some dinoflagellates (Fig. 4.2). At this stage, the paroral dikinetids are considered to lie orthogonal to the longitudinal axis of the cell and their postciliary microtubular ribbons extended to support the cytopharynx . In the second stage, somatic Kinety 1 (K1, Fig. 4.2) was derived from these paroral dikinetids by separation and rightward migration of the anterior or rightmost kinetosome of each paroral dikinetid (Step a, Fig. 4.2). Replication of these kinetosomes followed to reconstitute the ancestral dikinetid state (Lynn & Small, 1981). Eisler suggested that multiple repetitions of this process could give rise to multiple somatic kineties (Step b, Fig. 4.2). Alternatively, one could invoke the processes of either torsion and fragmentation (see Small, 1984) or elineation to increase the number of somatic kineties . In a final stage, the adoral or \u201clefthand\u201d oral structures are imagined to derive from the differentiation of somatic kinetids to the left of the oral region (Step c, Fig. 4.2). To support this, Schlegel and Eisler (1996) noted that the adoral ciliature is derived from a somatically-derived anlage in many contemporary ciliates. The paroral model has the advantage that, from the beginning, the ciliate ancestor, like many contemporary dinoflagellates , has a cytopharynx supported by microtubules derived from the \u201cparoral\u201d dikinetids: there is no need to invoke an independent evolution Fig. 4.1. A phylogeny of the ciliates demonstrating the estimated time of divergence of some major lineages as estimated by the divergence rate of small subunit rRNA gene sequences . The upper limit of 1% divergence per 80 million years was used to determine the lengths of the branches on the tree. (from Wright & Lynn, 1997c.) 4. Phylum CILIOPHORA \u2013 Conjugating, Ciliated Protists with Nuclear Dualism 91 92 4. Phylum CILIOPHORA \u2013 Conjugating, Ciliated Protists with Nuclear Dualism of the oral apparatus as in the competing models (see Orias, 1976; Small). Stomatogenesis and mor- phogenesis of the progeny cell or opisthe would have been of a buccokinetal type. Speculations on the evolution of another major feature distinguishing ciliates \u2013 nuclear dualism \u2013 are intriguing. Orias (1991b) used the life cycle of heterokaryotic foraminifera as an analogue to provide a rationale for how a heterophasic life cycle of a ciliate, ancestral to the karyorelicteans , might have evolved. This heterophasic life cycle had an alternation of haploid and diploid generations. From this, Orias developed the possible steps to the evolution of a karyorelictean cell cycle in which nuclear dualism occurs but with divisionless macro- nuclei differentiated at every cell cycle. While we now know that ciliates and foraminiferans are not closely related (Nikolaev et al., 2004), it is still possible that a heterophasic kind of life cycle might have been an intermediate stage in the evolution of ciliate nuclear dualism . Orias (1991a) argued further that macronuclear division must have originated de novo within the ciliates. He noted that macronuclei in most cili- ates fail to divide during the first postzygotic cell cycle following conjugation : more macronuclei are differentiated than is typical and these \u201cexcess\u201d macronuclei are segregated without macronuclear division during postconjugation fissions , reminiscent of the phenomenon that occurs at every cell cycle in karyorelicteans . Macronuclear \u201cdividers\u201d may have had selective advantages: the division process is much faster than the nuclear differentiation proc- ess and so \u201cdividers\u201d would outcompete \u201cnon- dividers\u201d; and dividing macronuclei may have increased the capability to assort intraclonally the genetic diversity of the parents and so increase the probability of generating more fit variants (Orias, 1991a). Why do karyorelicteans still persist? Orias suggested that they may still be more fit in the \u201crefugial\u201d relict environments in which they are often found, and that many karyorelicteans may have compensated for macronuclear polyploidy by increasing the number of macronuclei, so support- ing their increased cell size. As additional evidence that macronuclear divi- sion originated within the ciliates, Orias (1991a) noted that diversity in the modes of karyokinesis suggested at least two independent origins of macronuclear division \u2013 heterotrichs use extrama- cronuclear microtubules and other ciliates use intramacronuclear microtubules . Herrick (1994) argued that macronuclear division may have evolved independently perhaps three or more times, given the diversity of molecular mechanisms underlying macronuclear differentiation . However, Katz (2001) has used current molecular phylogenies of ciliates to argue for a single origin of a differentiating Fig. 4.2. Scheme of evolution of the ancestral ciliate oral and somatic cortex as proposed by Eisler (1992). Step a \u2013 an ancestral flagellate with a cytostome (c) and paroral of dikinetids separates the rightmost kinetosome of each dikinetid (arrowhead) to form somatic Kinety 1 (K1). Step b \u2013 this process is repeated (arrow) a number of times until the entire somatic cortex is covered by somatic kineties (Kn). Step c \u2013 adoral structures derive from the differentiation of somatic kinetids to the left of the cytostome. (Modified from Schlegel & Eisler, 1996.) mechanism relying on trans -acting factors that emanate from the parental macronucleus to influ- ence the differentiation of the developing macro- nuclei. This epigenetic mechanism is conceived to be plastic enough to have generated the molecular genomic diversity that we see in contemporary ciliates. Nevertheless, the number of origins of macronu- clear division is still unsettled. Two models have been proposed. For Model 1, the ancestral ciliate had a dividing macronucleus that lost its capacity to divide in the karyorelicteans . For Model 2, there were two independent origins of macronuclear division from a \u201cnon-dividing\u201d ancestor \u2013 one with extramacronuclear microtubules in the het- erotrich lineage and one with intramacronuclear microtubules in the intramacronucleate lineage (Hammerschmidt et al., 1996; Lynn, 1996a). We currently favor Model 2, which is consistent with the evidence presented by Orias (1991a, 1991b). For it, the macronucleus of intramacronucleate ciliates would \u201cregain\u201d the capacity to use intranu- clear microtubules that continued to be used by the micronucleus during its mitoses while the hetero- trich lineage would have \u201cre-invented\u201d the use of extranuclear microtubules to divide their macronuclei, an invention used also by some dinoflagellates (Perret, Albert, Bordes, & Soyer-Gobillard, 1991). In summary, what would our ancestral ciliate look like? It would have had a pellicle with alveoli underlying the plasma membrane. If the paroral model is used for cortical evolution, the ances- tral ciliate would have possessed a paroral , would have had a ventral oral region , would have had a cytopharynx