Commitment to the First Cortical Reorganization During Conjugation in Stylonychia mytilus: An Argument for Homology with Cortical Development During Binary Fission

Abstract

The asexual nature of the first cortical reorganization of conjugation in Stylonychia was analyzed by comparing the effect of amputation performed at different stages of early conjugation to that performed on vegetative cells at different stages of the cell cycle. Amputation of vegetative cells delineated a point of commitment to binary fission at 0.51–0.57 of the cell cycle. Cells amputated before this point were induced to undergo the regenerative mode of asexual development, but those amputated after this point continued with binary fission. In parallel, during conjugation a similar commitment was made around the time of formation of tight mating‐pairs: early conjugants amputated around this time might undergo regeneration, and those operated on after this stage continued with the first cortical reorganization as in typical conjugants. The two mates of a pair might differ in their response to amputation, suggesting that the timing of commitment to the first cortical reorganization is not related to the events of conjugation, but rather is individually determined in the vegetative cycle of the cells before they pair up in mating. These observations provide support for the notion that the first cortical reorganization of conjugants is homologous to the asexual mode of cortical development in dividers, according to the theory of developmental heterochrony in the sexual reproduction of hypotrichs. The timing of commitment to the first cortical reorganization was found to temporally correlate with the entrance of the micronuclei into meiosis. Since the first cortical reorganization can proceed without the micronucleus, this raises the possibility that initiation of micronuclear meiosis is closely coupled with, and may be determined by, the commitment to the first cortical reorganization.

Conjugation in the spirotrich ciliate Halteria grandinella (Müller, 1773) Dujardin, 1841 (Protozoa, Ciliophora) and its phylogenetic implications

“…Accordingly, the interlocking arrangement is considered a convergent apomorphy of halteriids and strobilidiid choreotrichs. As in H. grandinella (this study), the left conjugant of the hypotrich Stylonychia is slightly more posterior than the right (Zou and Ng 1991).…”

Anomalous Cases of Conjugation and Spontaneous Autogamy in Stylonychia mytilus: A Note on the Asexuality of Early Conjugants

“…8-10). This differs from the first cortical reorganization in typical conjugants where a new AZM is not connected at any stage of development to the old AZM, which is ultimately resorbed entirely [ 17,24,301 (see accompanying paper [39]). The primordium for the frontal-ventral-transverse c i m originated from the frontal and ventral c i m and developed at the same latitude as the undulating membrane ( Fig.…”

“…The present observations were made on two stocks of S. mytilus. stock G and stock S5, as in another report [39]. Culture methods were detailed in an accompanying paper by Zou & Ng [38].…”

“…Characteristic stages of the normal sequence of conjugation are illustrated in Fig. 1 of our accompanying paper [39]. The conjugants initially pair up ventral-to-ventral with their frontal regions in contact making a cytoplasmic connection in the region between the undulating membrane and the adoral zone of membranelles of both mates.…”

Physiological Reorganization and Post‐Traumatic Regeneration in Stylonychia mytilus: Reflections on Developmental Constraints and the Evolutionary Origin of Alternative Modes of Asexual Morphogenesis

“…As it extends anteriorly, it bends (perhaps forced to do so) toward the cell's right along the fission zone [35, 391 (see Fig. 2, I11 of accompanying paper, [41]). By contrast during physiological reorganization or regeneration, the cell is not demarcated into two compartments; no developmental boundary exists for the newly assembled AZM to observe, which extends anteriorly, eventually merging with the pre-existing AZM.…”