Last week, I wrote about Takashi Hamaji’s new paper characterizing the mating-type/sex-determining loci in Eudorina and Yamagishiella. That paper showed that the sex-determining region of anisogamous Eudorina is, surprisingly, considerably smaller than the mating-type loci of isogamous Chlamydomonas, Gonium, or Yamagishiella. Because only one gene, MID, is present in the male version of the sex-determining region in Eudorina, Hamaji and colleagues concluded that
…the evolution of males in volvocine algae might have resulted from altered function of the sex-determining protein MID or its target genes.
I commented that
…we’re still left with two (non-mutually exclusive) possibilities: changes to the MID gene itself may have changed which genes it interacts with (or how it interacts), or there may have been changes in the genes whose expression is controlled by MID.
Now Sa Geng and colleagues have provided at least a partial answer. In a new paper in Development, they swapped versions of MID among different volvocine species (unfortunately, no unpaywalled version of the paper is currently available; I will add a link when I find one). We already knew that MID is necessary and sufficient for male development: genetically male Volvox carteri colonies that have MID expression turned off produce eggs, and genetically female colonies transformed with MID produce sperm packets (“Sex change (in Volvox)”). But that’s MID from the same species. It’s somewhat surprising that a single gene can cause Volvox to switch sexes, but at least Volvox MID evolved side-by-side with the genes whose expression it controls.
What would be really surprising is if MID from other species, species that diverged from the Volvox lineage ~200 million years ago, worked in Volvox. It would be extraordinarily surprising if MID from a species that doesn’t even have males could control their development in Volvox. It won’t work. Waste of time; don’t bother trying.