Boy meets girl. Boy and girl fall for each other. Boy and girl generate other boys and girls. Its a tale as old as time. Well, kinda. Evolutionarily speaking, there weren't always boys and girls, just its. Those Its reproduced asexually, an efficient if not always a genetically or evolutionarily advantageous reproductive strategy. The evolution of two sexes that swap genes with each other allows for a more diverse and robust gene pool. Usually a great thing. But how did it get that way, at what point did we go from It to Us?
A new study in the journal Science takes a look at Volvox carteri, a multicellular green algae, for the answer to that very question. The authors of the paper compared the sex determining region of V. carteri to that of Chlamydomonas reinhardtii, a unicellular algal relative. They found that this region differed dramatically between these species. The expansion of this region allows for a greater diversity of genes which code for the production male and female gametes. The gametes of C. reinhardtii look identical, but those of V. carteri are distinctly different (egg and sperm).
The mating locus genes of the two species share many genes, but V. carteri's region is almost five times larger, mainly due to additional genes under the control of either male or female programs. Some of these new, additional genes have counterparts in C. reinhardtii that have nothing to do with sex. V. carteri has taken these genes, incorporated them into the mating locus, and started using it in its sexual reproductive cycle. Specifically, the mating locus gene MAT3 has evolved a new role in sexual differentiation, likely a role in controlling cell division and male/female reproductive development.
Future research will involve Gonium, an evolutionarily intermediate species between Volvox and Chlamydomonas. This intermediate will allow researchers to take a look at those in-between steps in the evolutionary process to better understand how this evolutionary change occurred.
Here's the Science article:
Ferris, Patrick et al. (2010) Evolution of an Expanded Sex-Determining Locus in Volvox. Science: 328 (5976), 351. (DOI: 10.1126/science.1186222)
(image from eebweb.arizona.edu)