Gametogenesis in the Chlamydomonas reinhardtii minus Mating Type Is Controlled by Two Genes, MID and MTD1

Abstract

In the unicellular algae Chlamydomonas reinhardtii, the plus and minus mating types are controlled by a complex locus, MT, where the dominant MID gene in the MT À locus has been shown to be necessary for expression of minus-specific gamete-specific genes in response to nitrogen depletion. We report studies on MID expression patterns during gametogenesis and on a second gene unique to the MT À locus, MTD1. Vegetative cells express basal levels of MID. An early activation of MID transcription after nitrogen removal, and its sequence similarity to plant RWP-RK proteins involved in nitrogen-responsive processes, suggest that Mid conformation/activity may be nitrogen sensitive. A second stage of MID upregulation correlates with the acquisition of mating ability in minus gametes. Knockdown of MTD1 by RNAi in minus strains results in a failure to differentiate into gametes of either mating type after nitrogen deprivation. We propose that intermediate Mid levels are sufficient to activate MTD1 transcription and to repress plus gametespecific genes and that MTD1 expression in turn allows the threshold-level MID expression needed to turn on minus gamete-specific genes. We further propose that an MTD1-equivalent system, utilizing at least one gene product encoded in the MT 1 locus, is operant during plus gametogenesis.C HLAMYDOMONAS reinhardtii is a flagellated unicellular green alga that has two mating types, plus and minus, determined by the mating type (MT ) loci (MT 1 and MT À ). The center of this $1-Mb locus of recombinational suppression carries translocations and inversions and is called the rearranged (R) domain (Ferris and Goodenough 1994). Both housekeeping and sex-limited genes are found in this region (Ferris et al. 2002), similar to mating-type loci and sex chromosomes in other organisms (Graves 2006). Six unique regions (a-f ) are found within the R domain, three (ac) specific to MT 1 and three (d-f ) specific to MT À . Four genes have been identified in these regions: MTA1 (MT locus, region a) in a, FUS1 (fusion) in c, MTD1 (MT locus, region d) in d, and MID (minus dominance) in f (Ferris et al. 2002). The two MT À -specific genes are the focus of this study.In response to nitrogen starvation, haploid vegetative Chlamydomonas cells differentiate into gametes. Gametes of opposite mating type are able to agglutinate and fuse to form zygotes (Harris 1989). Occasionally, heterozygous mt 1 /mt À diploids form after mating, resume vegetative growth, and differentiate as gametes with N-starvation. The fact that these diploids always mate as minus indicates that minus is dominant to plus (Harris 1989), a phenomenon found to be controlled by the MID gene (Galloway and Goodenough 1985). MID encodes a transcription factor in the RWP-RK family that also includes several proteins in higher plants that are suggested to exert their function during nitrogen limitation (Schauser et al. 1999(Schauser et al. , 2005Borisov et al. 2003).Previous studies revealed that MID is necessary and sufficient to convert wild-type ...

Comparative genomics in Chlamydomonas and Plasmodium identifies an ancient nuclear envelope protein family essential for sexual reproduction in protists, fungi, plants, and vertebrates

“…The only other known minus-specific gene, the minus dominance gene MID, was present in C41 (Fig. 2B), which is characterized by transcripts with higher expression levels in resting minus gametes (MGs), consistent with the known role for MID in specifying mating type early during gametogenesis (Ferris and Goodenough 1997;Lin and Goodenough 2007).…”

Systems-Level Analysis of Nitrogen Starvation-Induced Modifications of Carbon Metabolism in a Chlamydomonas reinhardtii Starchless Mutant

“…Gametes subsequently fuse to form zygotes (reviewed in Pan and Snell, 2000). We noted that SAD1, GSM1, and FUSM, all specific to the minus mating type, are expressed in CC-4349 (Ferris and Goodenough, 1994;Ferris et al, 1996;Lin and Goodenough, 2007). Curiously, these genes were not expressed in either sta6 or in strains complemented for STA6.…”

Green Evolution and Dynamic Adaptations Revealed by Genomes of the Marine Picoeukaryotes Micromonas

“…In Chlamydomonas, sex determination is regulated by a gene called MID (MInus Dominance). The minus (MT-) but not the plus (MT+) mating-type locus carries a MID gene that is expressed early in gametogenesis (87); plus cells carrying a MID transgene differentiate as minus (88); and mutations/deletions of MID cause minus cells to differentiate as plus (87,88). In several related volvocacean algae, MID is also confined to genomes of one mating type (89,90).…”