Nuclear fusion-defective phenocopies in Chlamydomonas reinhardtii: mating-type functions for meiosis can act through the cytoplasm.

Nuclear fusion in newly formed Chlamydomonas reinhardtii zygotes can be inhibited by drugs that affect microtubule stability, which include colchicine, amiprophosmethyl, oryzalin, and taxol. This inhibition can be monitored genetically by the production of haploid meiotic products from conjugations between haploid and diploid parents. Such zygotes would normally produce aneuploid progeny. Inhibition of nuclear fusion by colchicine requires treatment of gametic cells both before conjugation and after formation of the zygotes. These results suggest that nuclear fusion requires dynamic microtubules. Treated zygotes formed from a haploid-diploid mating can produce six spores, but only four spores germinate to form viable haploid colonies. No contribution from the nuclear genome of the haploid parent is recovered, whereas all loci from the diploid parent are recovered. The four viable products from the diploid parent of inhibited zygotes show normal segregation of loci located on linkage groups segregating according to Mendelian laws. Levels of meiotic recombination were tested for pairs of loci on linkage groups XVIII and XIX and found to be unchanged by inhibition of nuclear fusion. Thus, similar to Saccharomyces cerevisiae, C. reinhardtii mating-type functions necessary for nuclear fusion are not nuclear limited and can act through the cytoplasm. Inhibition of nuclear fusion can be used to analyze diploid Chlamydomonas that cannot enter meiosis. This technique permits direct analysis of dominant mutations, dominant suppressors and enhancers, and new alleles of identified loci that have been isolated in diploid strains.