Identification of the Minus-Dominance Gene Ortholog in the Mating-Type Locus of Gonium pectorale

Abstract

The evolution of anisogamy/oogamy in the colonial Volvocales might have occurred in an ancestral isogamous colonial organism like Gonium pectorale. The unicellular, close relative Chlamydomonas reinhardtii has a mating-type (MT) locus harboring several mating-type-specific genes, including one involved in mating-type determination and another involved in the function of the tubular mating structure in only one of the two isogametes. In this study, as the first step in identifying the G. pectorale MT locus, we isolated from G. pectorale the ortholog of the C. reinhardtii mating-type-determining minus-dominance (CrMID) gene, which is localized only in the MT À locus. 39-and 59-RACE RT-PCR using degenerate primers identified a CrMID-orthologous 164-amino-acid coding gene (GpMID) containing a leucine-zipper RWP-RK domain near the C-terminal, as is the case with CrMID. Genomic Southern blot analysis showed that GpMID was coded only in the minus strain of G. pectorale. RT-PCR revealed that GpMID expression increased during nitrogen starvation. Analysis of F 1 progeny suggested that GpMID and isopropylmalate dehydratase LEU1S are tightly linked, suggesting that they are harbored in a chromosomal region under recombinational suppression that is comparable to the C. reinhardtii MT locus. However, two other genes present in the C. reinhardtii MT locus are not linked to the G. pectorale LEU1S/MID, suggesting that the gene content of the volvocalean MT loci is not static over time. Inheritance of chloroplast and mitochondria genomes in G. pectorale is uniparental from the plus and minus parents, respectively, as is also the case in C. reinhardtii.

Sex-Specific Posttranslational Regulation of the Gamete Fusogen GCS1 in the Isogamous Volvocine Alga Gonium pectorale

“…The stepwise transition from isogamy to anisogamy/oogamy is considered to be associated with the evolution of multicellularity (31)(32)(33). The minus-and male-specific presence of the minus dominance (MID) genes in isogamous (Chlamydomonas reinhardtii [16] and G. pectorale [18]), anisogamous (Eudorina sp. [20] and Pleodorina starrii [17]), and oogamous (Volvox carteri [19]) species are also indicated.…”

“…The resultant 232-bp fragments were directly sequenced for confirmation. EF-like and GpMID genes were amplified using previously described primer sets (CV_EF1A1-R2 and GpEF1A-INT3-R for EF-like, GPMID_int1F and GPMID_int4R for GpMID) (18) and KOD-Plus-Neo DNA polymerase (Toyobo). The PCR schedule was 2 min at 94°C, followed by 25 cycles of 10 s at 98°C, 30 s at 63°C, and 40 s at 68°C.…”

“…1B). Moreover, in volvocine algae, mating-type minus in isogamous species is homologous to male sex in anisogamous/oogamous species based on the presence of the sex-determining minus dominance (MID) genes in male genomes (16)(17)(18)(19)(20) (Fig. 1A).…”

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

“…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). The MID protein is a member of a large family of presumed but poorlycharacterized transcription factors found primarily in plants that share the motif RWP-RK (102).…”

Green Algae and the Origins of Multicellularity in the Plant Kingdom

“…V. carteri MT is syntenic with Chlamydomonas MT, but its male and female haplotypes are genetically more complex and share several properties with sex chromosomes . The developmental and evolutionary bases for oogamy and anisogamy that are controlled by MT remain to be determined, but may involve the transcription factor MID (minus dominance) that specifies sexual differentiation in Chlamydomonas and is found in either the minus mating type or males of all volvocine algae examined to date (Ferris and Goodenough 1997;Nozaki et al 2006a;Hamaji et al 2008Hamaji et al , 2013.…”