MATING-TYPE DIFFERENTIATION AND MATE SELECTION IN THE HOMOTHALLIC CHLAMYDOMONAS-MONOICA

By using combinations of phenotypically-distinct - but sexually-compat ible - mutant strains of C. monoica (zym-1, zym-27, and nit-2) and ass aying for zygote genotypes in genetically-mixed mating populations (wh ere gametes of the two parental homothallic strains were present at si milar frequencies), we have found that matings occur preferentially be tween cells of the same genotype. Additional support for an hypothesis of non-random mate selection was provided by using an easily-selectab le genetic marker (sup-1) to verify the frequent occurrence of matings between cells of identical genotype in populations where the selectab le genotype was present at very low relative frequency (10(-2) or 10(- 3)) in a mixed mating population. Direct evidence for nonrandom mate s election was obtained by presenting nitrogen-starved cells with limiti ng nitrate to synchronize gametic differentiation in wild-type strains . Under these conditions, the four, eight, or 16 mitotic daughters rel eased from the same mother sporangium often immediately established ma ting pairs within the group. Thus successive mitotic divisions of a si ngle mother cell yielded progeny of opposite expressed mating-type.