Timing of life cycle morphogenesis in synchronous samples of Sterkiella histriomuscorum. II. The sexual pathway.

Isolates of Sterkiella (Oxytrichidae, Stichotrichia, Ciliata) are commonly used to study macronuclear development. These organisms respond to changes in food abundance variably by encystment-excystment, conjugation, cannibalism or rescaling cell size. An isolate of Sterkiella histriomuscorum (previously Oxytricha fallax and O. trifallax) is used because two complementary mating types are available. We provide observations on conjugation in cultures of this isolate. Using synchronous samples of conjugants, the timing of stages of nuclear divisions during conjugation was determined. Following ex-conjugant cultures over time, the onset of clonal aging and senescence is described. Cells become sexually mature after a brief period of "adolescence", during which time selfing is possible. Senescent cultures are less vigorous, unable to conjugate and encyst more readily. Excystment survival decreases with clonal age. These results can serve as reference for long-term cultures of this species and for analysing particular stages of developmental processes during conjugation.