Mutations Affecting O-Glycosylation in Chlamydomonas reinhardtii Cause Delayed Cell Wall Degradation and Sex-Limited Sterility.

We describe a mutation, gag-1, that affects in a temperature-dependent manner a specific type of O-glycosylation in the green alga Chlamydomonas reinhardtii. In the mutant, all the major glycoproteins, in particular cell wall proteins, show a decreased apparent molecular weight in polyacrylamide gels, and their antigenicity is affected. The mutant forms multicellular aggregates (palmelloid colonies) at the restrictive temperature due to the delayed release of the daughter cells from the mother cell wall after mitosis. In addition, the mutation causes sterility by preventing sexual agglutination. In contrast to the other phenotypes, the sterility phenotype is temperature independent, and it is expressed only by cells of the plus mating type. We show that imp-8, a previously described nonagglutinating sex-limited mutation, causes the same glycosylation defect and is allelic to gag-1. Thus, expression of mt+ agglutinability appears to require the specific type of O-glycosylation that is defective in these mutants. More generally, these observations show that a sex-limited phenotype can be caused by a mutation in a gene that is not itself sex limited in its expression.