THE ISO1 GENE OF CHLAMYDOMONAS IS INVOLVED IN SEX DETERMINATION

Sexual differentiation in the heterothallic alga Chlamydomonas reinhar dtii is controlled by two mating-type loci, mt(+) and mt(-), which beh ave as a pair of alleles but contain different DNA sequences. A mutati on in the mt minus-linked imp11 gene has been shown previously to conv ert a minus gamete into a pseudo-plus gamete that expresses all the pl us gametic traits except the few encoded by the mt(+) locus. Here we d escribe the isol mutation which is unlinked to the mt(-) locus but is expressed only in minus gametes (sex-limited expression). A population of minus gametes carrying the iso1 mutation behaves as a mixture of m inus and pseudo-plus gametes: the gametes isoagglutinate but they do n ot fuse to form zygotes. Further analysis reveals that individual game tes express either plus or minus traits: a given cell displays one typ e of agglutinin (flagellar glycoprotein used for sexual adhesion) and one type of mating structure. The isol mutation identifies a gene unli nked to the mating-type locus that is involved in sex determination an d the repression of plus-specific genes.