Studies on differentiation without cleavage in Chaetopterus. Requirement for a definite number of DNA replication cycles shown by aphidicolin pulses.

Aphidicolin, a powerful inhibitor of nuclear DNA replication, has been used to establish the level of polyploidisation required for the realization of the late morphogenetic events (segregation, pseudogastrulation and ciliation) of differentiation without cleavage in Chaetopterus-activated eggs. A parallel has been drawn between cytophotometric estimations of the DNA content and cytoplasmic differentiation in eggs treated with aphidicolin under different experimental conditions: either pulses with aphidicolin were followed by culture in sea water or the eggs were treated with aphidicolin after development had taken place in sea water for various lengths of time. The results suggest that a 'quantal' monasterial cycle might take place 3 h after activation, corresponding presumably to the fourth or fifth replication cycle. Moreover, early DNA replication seems to be more important for morphogenesis than late DNA replication.