Genetic base in evolution of Oogamy and origin of male-female differentiation

Japanese researchers found the genetics basis of sex dimorphism (two genders) by investigating two closely related species of green algae that practice different forms of sexual reproduction.

Human egg cell

So, how did sex cells modify to eggs and sperm? It seems that a gene underlying a more primitive system of reproduction was likely co-opted during evolution to participate in sex-specific sperm development. 

The Japanese team found a genetic connection between male sexuality of an oogamous multicellular green algae species, Pleodorina starrii, and one of the mating types of a more primitive isogamous unicellular alga Clamydomonas reinhardtii. In C. reinhardtii, isogamy occurs through "plus" (MT+) and "minus" (MT-) mating types. MT- represents a "dominant sex" due to a particular gene, MID ("minus-dominance"), both necessary and sufficient to cause the cells to differentiate as MT- isogametes. However, no sex-specific genes related to MID had been identified in closely related oogamous species.

Human sperm cell

But now the scientists have successfully identified a version of the MID gene in Pleodorina starrii. This "PlestMID" gene is present only in the male genome, and it encodes a protein abundant in the nuclei of mature sperm. This means that P. starrii maleness evolved from the dominant sex (MT-) of its isogamous ancestor. This breakthrough discovery can answer many questions about the evolution of oogamy and the origins of male-female differentiation.