1D-11 Histone modification and programming in newly developed macronucleus after conjugation of Paramecium caudatum

Ciliated protozoan Paramecium caudatum retain two distinct types of nucleus, the germinal micronucleus and the somatic macronucleus. New macronucleus for the next generation is differentiated from a zygotic micronucleus during conjugation, while parental macronucleus undergoes fragmentation into about 50 pieces. The parental macronuclear fragments do not degenerate soon, but last for several cell cycles after conjugation. Here we demonstrate that the new macronucleus is completed by the fourth cell cycle, and then the parental macronuclear fragments are destined to degenerate. In fact, degradation of the parental macronuclear DNA was observed at the fifth cell cycle, suggesting delayed degeneration of the parental fragments. After conjugation, progeny cells enter so-called 'immaturity period', during which cells do not express mating reactivity for 50 fissions. When the 'immature' cells were treated with Trichostatin A, a histone deacetylase inhibitor, they recovered mating competence as if they became 'mature. ' These results strongly suggest involvement of hitone modification in progarmming of the developmental fate of the new macronucleus after conjugation in P. caudatum.