Identification of the Minus Dominance Gene Orthologue from Gonium pectorale

The colonial volvocalean algae are a model lineage to study the sex evolution. Gonium pectorale has flattened 8- or 16-celled colonies and produces unicellular isogametes. Although only one of the two conjugating isogametes of C. reinhardtii has a tubular mating structure (TMS), both isogametes of G. pectorale extend a TMS toward each other (bilateral mating papillae). C. reinhardtii has a mating type (mt) locus harboring several mating type-specific genes involved in mating type determination and/or formation of the TMS. Studying G. pectorale mt locus is phylogenetically important, because bilateral mating papillae is the first step in the evolution toward anisogamy/oogamy. As the first step in characterizing the G. pectorale mt locus, we isolated an orthologue of the C. reinhardtii mt- specific minus-dominance (CrMID) gene. Starting with RT-PCR using degenerate primers based on the MIDs from C. reinhardtii, C. incerta, and Pleodorina starrii, we identified a G. pectorale CrMID-orthologous gene (GpMID) encoding 164 amino acids. GpMID contains a RWP-RK domain as is the case with CrMID. The intron-exon structure of GpMID is same as those of CrMID. Southern blotting indicates that GpMID is a single copy gene present only in one of the two mating types. Analysis of F1 progeny correlates mating phenotype with the presence/absence of GpMID and demonstrates that in G. pectorale chloroplast DNA is inherited from the parent lacking GpMID and mitochondrial DNA from the parent containing GpMID, as is the case in C. reinhardtii.