| The unicellular green algae Chlamydomonas reinhardtii contains two mating types, plus and minus, controlled by a single complex locus MT. Two MT-specific genes, MID and MTD1, are key regulators of minus gametogenesis. The minus dominance gene MID, necessary to activate minus-specific mating traits and repress plus-specific ones, encodes a protein sharing some conserved amino acids with several nitrogen-sensing RWP-RK proteins, suggesting that Mid conformation/activity might be nitrogen-sensitive. MID is activated quickly (within 30 min) by nitrogen starvation from its basal level in vegetative cells to an intermediate level (Level 1), which is necessary for the activation of MTD1 and to repress plus gamete-specific genes. A threshold level (Level 2) of MID up-regulated by the onset of MTD1 is achieved during late gametogenesis when several minus gamete-specific genes are activated. RNA interference of the novel gene MTD1 results in sterile minus cells, revealing that MTD1 , along with MID, plays an important role in minus gametogenesis.;A sex-limited mutant, iso1, which displays an interesting minus-specific isoagglutination phenotype, was generated by insertional mutagenesis. A gene affected at the 3' UTR by the insertion, ISW2 , was the obvious candidate gene responsible for the phenotype until the later discovery of a second gene, CAN, an antisense transcript of ISW2. The transcriptional and translational levels of ISW2 are affected in the mutant, but it was not possible to establish a direct link between the expression/localization of ISW2/Isw2 and the severity of the isoagglutination phenotype. Ribonuclease protection assays of CAN in different strains yielded no interpretable information. Transformation of both ISW2 and CAN into wt minus cells, trying to mimic the dominant iso1 phenotype, was also unsuccessful. Hence the genetic basis for the iso1 phenotype, and, the reason that the phenotype is sex-limited, remain mysterious.;Two homeoproteins, Gsp1 and Gsm1, located downstream of the gametogenesis networks, are transferred from cytoplasm to nucleus during zygote formation, reinforcing the hypothesis that both are involved in activation of Chlamydomonas zygotic program. |
