The familiar notion of the separate male and female sexes exhibited by animal and plant species is based in part on the anatomically and genetically distinct gametes, sperm and egg, produced by members of each sex. But the evolutionary origin of oogamy—reproduction though joining of distinct sperm and egg cells—is in fact poorly understood. In particular, it has remained unclear how oogamy arose from isogamy, a more simple form of sex in which very similar reproductive cells take on different "mating types" but do not differentiate as distinct sperm and egg. The transition from isogamy to oogamy has apparently occurred multiple times during the evolution of animals, plants, and some algae, but how did such transitions occur"
In their new work, the researchers established a genetic connection between male sexuality of an oogamous multicellular green algae species, Pleodorina starrii, and one of the mating types of its isogamous ancestor, the unicellular alga Clamydomonas reinhardtii.
In C. reinhardtii, isogamous sexual reproduction occurs through "plus" (MT+) and "minus" (MT-) mating types. MT- represents a "dominant sex" because a particular gene, MID ("minus-dominance") of C. reinhardtii is both necessary and sufficient to cause the cells to differentiate as MT- isogametes. How
ever, no sex-specific genes related to MID had been identified in closely related oogamous species. The researchers now report that they 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 localized abundantly in the nuclei of mature sperm. The findings indicate that P. starrii maleness evolved from the dominant sex (MT-) of its isogamous ancestor. This breakthrough in understanding provides an opportunity to address any number of extremely interesting questions regarding evolution of oogamy and the origins of male-female dichotomy.
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