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How Did Males/Sperm Emerge? The origin of the sexes	By Stefan Anitei, Science Editor 20th of December 2006, 09:55 GMT Adjust text size:


We know sex is the engine of the evolution, but ... why are we necessarily differentiated into males and females? Imagine a world of only women reproducing among them ... 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. 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 separation between male and female sexes in animal and plant species is based on the anatomically and genetically distinct gametes, sperm and egg, produced by each sex. The reproduction involving sperm and eggs is known as oogamy. Before oogamy, there was isogamy, a reproduction type without different gender and sex cells: two identical sex cells join to form an embryo. Isogamy passed to oogamy multiple times during the evolution of animals and plants, but how precisely this occurred has not been known. 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. 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.

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