Alternation of haploid and diploid is a common feature of the sexual reproduction. Before establishment of such sexual haploidization through meiosis and sexual diploidization through fertilization, I hypothesize the asexual cycle alternating asexual haploidization and asexual diploidization, which deserves to be called primitive sexual reproduction.
To elucidate the timing of commitment to autogamy in Paramecium bursaria, cells were treated with an autogamy-inducing agent (1.25% methyl cellulose) for 0, 1, 2, 3, 5, and 24 h, and the duration of treatment required for the induction of autogamy was examined 24-25.5 h after the onset of treatment. Many cells treated for 0-2 h were at the vegetative stage, but many cells treated for 3-24 h were at stages from the third pre-zygotic division to the third post-zygotic division and normally underwent autogamy. Therefore, treatment with methyl cellulose for 3 h or more is considered necessary for the induction of autogamy. However, some of the cells treated for 3 h were at the stage of developing either a swollen and spindle-shaped micronucleus or an early round-shaped micronucleus, which is indicative of the pre-meiotic S phase. This suggests that if cells proceed beyond the stage of an early round-shaped micronucleus, they normally undergo autogamy, but if not, they terminate at the stage of either a swollen and spindle-shaped micronucleus or an early round-shaped micronucleus. Consequently, cells of P. bursaria are committed to autogamy just after the stage of an early round-shaped micronucleus (the pre-meiotic S phase).
To analyze cell-cell adhesion in Paramecium caudatum, we examined methyl cellulose-induced autogamous cells for their ability to adhere. It was found that the autogamous cells have cell-cell adhesion ability. In addition, we examined the proportion of adhering (conjugating) pairs derived from autogamous cells at various stages (0.5, 1, 2, 4, 6, 9 and 12 h after the onset of induction of autogamy). The proportion of adhering cells reached a maximum about 2 h after the onset of induction of autogamy, and thereafter, rapidly decreased by 4 h after the onset, eventually reaching almost zero 6 h after the onset. These results suggest that there is a transient expression or activation of molecule(s) involved in cell-cell adhesion around 2 h after the onset of induction of autogamy.