Interactions between newly developed macronuclei and maternal macronuclei in sexually immature multinucleate exconjugants of Paramecium caudatum

Delayed degradation of parental macronuclear DNA in programmed nuclear death of Paramecium caudatum

“…3C,D). In previous reports, it has been observed that there are an average of about 50 fragments immediately after fragmentation (Mikami, 1979), whereas only 42 fragments were observed in exconjugants that were starved for 48 h after mating (Kimura and Mikami, 2003). This discrepancy seems to reflect a selective reduction of the fragments in the exconjugants, depending on nutrient conditions after conjugation, although we cannot deny that it depends on the different stocks used.…”

“…It was previously demonstrated that about 40 -50 fragments derived from the parental macronucleus during conjugation do not degenerate immediately, but persist until the eighth cell cycle after conjugation (Mikami, 1979;Kimura and Mikami, 2003). During this period, the fragments retain their regeneration competence but cannot divide, with the result that they are segregated into daughter cells.…”

“…The behavior of the parental macronucleus during conjugation in P. caudatum is markedly different from most other ciliates, but not some other Paramecium species. The parental macronucleus is retained in the form of many fragments for a long period, at least until the eighth cell cycle after conjugation (Kimura and Mikami, 2003). In other words, both the parental nucleus and the new macronucleus coexist in the same cytoplasm of the progeny of P. caudatum.…”

“…Taking into account this observation, programming of the developmentally regulated gene expression in the new macronucleus might occur at this critical period of the fifth cell cycle. This is also suggested by the fact that immature clones appeared when the old macronucleus was regenerated by microsurgical removal of the new macronucleus at the fifth and sixth cell cycles, probably reflecting a reprogramming of the parental macronucleus (Kimura and Mikami, 2003). This in turn implies that the programming factors are present in the cytoplasm during the period.…”

“…The macronuclear fragments performed DNA synthesis and increased their volume until the fourth cell cycles after conjugation (Mikami, 1979). These fragments do not degenerate after nuclear differentiation, but rather persist in the progeny for eight cell cycles after conjugation, indicating that the new macronucleus and the parental macronuclear fragments coexist for a lengthy period in the daughter cells (Kimura and Mikami, 2003). The biological significance of the coexistence of the old parental fragments and the new macronuclei is unclear, and at present there is no direct evidence whether the fragmented parental macronucleus is eventually degraded as in other ciliates, in particular the closely related species P. tetraurelia (Berger, 1973).…”