Differential cortical degradation in the two members of early conjugant pairs ofOxytricha fallax

Abstract

A differential resorption of the anterior ventral cortical structures is shown to occur in early conjugating pairs of Oxytricha. The member on the pair's left resorbs the collar membranelles and the frontal and right marginal cirri; the member on the pair's right resorbs the central and posterior membranelles (lapel) of the adoral zone of membranelles. The lapel of the left member then aligns with the collar of the right member to form a composite structure which appears with light and scanning electron microscopy to be completely integrated. Consequently, a 4–5‐hour‐old pair contains slightly more than one complete set of anterior ventral organelles (viz., one extra set of undulating membranes) and two complete sets of ventral and anal cirri. Analysis of conjugant pairs in which the two members are joined in abnormal positions suggests that neither the proximity of certain cortical organelles to the region of cytoplasmic bridge formation, the specific pairing position, nor the relocation of certain cortical organelles into a different cortical region is directly responsible for determining which resorption pattern occurs. Furthermore, analysis of the cortical events in conjugating doublets suggests that neither nuclear determination nor differential gene expression can account for the phenomena observed in Oxytricha.