Relationships between Cell Cycle and Conjugation in 3 Hypotrichs*

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Abstract
SYNOPSIS. Relationships between the cell cycle and the beginning of conjugation were analyzed for 3 hypotrichs: Diophrys scutum, Oxytricha bifaria, and Euplotes crassus. The first 2 species enter conjugation with micronuclei in G1; the latter species with a micronucleus in G2. The 1st micronuclear division of conjugating E. crassus is mitotic. Thus meiotic DNA replication occurs when the cells of each species have already entered the mating process.Cells from asynchronous populations start conjugation with their macronuclei primarily in G1 or more rarely at the beginning of the S stage in a percentage significantly different from that expected on the basis of random mating among all cells in the population. Also, macronuclear replication, when already begun, was blocked in cells undergoing conjugation. Therefore only the G1 or the very early S stages of the cell cycle are compatible with conjugation in the 3 analyzed species.