Chapter 76 Mating and Tetrad Analysis in Chlamydomonas reinhardtii

The ease of detecting mutations in haploid organisms is straightforward; any newly arising recessive mutation is apparent because there is no wild-type allele to mask the mutant phenotype. Thus, large-scale screens and/or selections can be performed for the isolation of mutations affecting the flagella of Chlamydomonas. Two different mating types exist in C. reinhardtii: mating type plus (mt⁺) and mating type minus (mt^-). These mating types behave as alleles at a genetic locus on linkage group VI. Chlamydomonas reinhardtii is a haploid organism. Haploid organisms have several advantages over diploid organisms when dissecting a process by a mutational approach. Mating is initiated when the cells are starved for nutrients and specifically when they are starved for nitrogen. Recognition occurs between cells of opposite mating types at two levels. The initial recognition occurs by interactions between hydroxyprolinerich glycoproteins, agglutinins, in the membrane of the flagella of the two cell types. This process is known as tipping. It leads to an increase in the intracellular levels of cyclic AMP, which stimulates the formation of a fertilization tube on the surface of the mt⁺ cell. Mutations that affect flagellar function, length, or number do not block the ability of C. reinhardtii strains to mate. Short flagella or paralyzed flagella may reduce the efficiency of mating, but do not block mating. Flagella are required for conventional mating procedures. Mutations that produce cells that lack flagella require additional steps to ensure mating. Because the first recognition events involve the tipping of flagella of the two mating types, this step must be bypassed. When meiosis is completed and the meiotic progeny have been collected, they can be analyzed as the progeny from individual meiotic events (a tetrad) or individually (random progeny). One advantage of tetrad analysis is the ability to map a gene with respect to its centromere as well as with respect to other genes.