Previously, we have seen that the eukaryotic versions of RecA (Rad51, Rad51B, Rad51C, Rad51D, DMC1, XRCC2, or XRCC3) were spawned when the domain Eukarya probably existed in a unicellular state. We’ve also seen that removal of Rad51 from the mouse genome was lethal. The same holds true for Rad51B, Rad51C, Rad51D, and XRCC2 (such knock-out experiments with XRCC3 have apparently not been done yet). But removal of DMC1 is not lethal. In fact, the phenotype for such a knock-out mouse is as follows:

Homozygotes for targeted mutations are sterile with failure of homologous pairing in meiotic prophase in males and disrupted oogenesis in embryonic females with absence of germ cells in the adult ovary.

DMC1, which stands for Disruption of Meiotic Control, was originally identified in yeast in the early 1990s. Meiosis is the process by which eukaryotic diploid cells form haploid cells that in turn become gametes. In plants and animals, it is the process that generates ova and sperm/pollen. And in the single-celled yeast, this gene also plays a necessary role in facilitating recombination, guiding homologous chromosomes to cross-over during the very early stages of meiosis. Removal of DMC1 leads to arrests in the early stages of meiosis.

Thus, like Rad51, DMC1 is required for meiosis in plants, animals, and fungi. But unlike Rad51, DMC1 function is restricted to meiosis, as not only indicated by gene disruption experiments, but expression studies that find it to be synthesized only during meiosis. In essence then, DMC1 is a marker for meiosis. It is not necessary for meiosis, as fruit flies and the nematode, C. elegans, have lost their copy. But when it is present, meiosis (or the recent ability to carry out meiosis) is strongly indicated.

So that means we can reasonably estimate when meiosis originated simply by surveying the distribution of DMC1 (as least as a first step in our analysis). Thus, I took DMC1 sequence from fungi and used it to search the data bases, pulling out several examples from distantly related protozoa. For example, here it is from an amoeba. Here it is from Trypanosoma. Here it is from a ciliate (YER179W). And most interestingly, here it is from Giardia, thought to be the most “primitive” eukaryote based on the way its genes so deeply branch in the eukaryotic tree. It seems to be rather ubiquitous among the single-celled eukaryotes.

In other words, DMC1 was spawned very early during the evolution of eukaryotes and its birth may very well have coincided with the birth of Eukarya, thus defining Eukarya. Not only may the unfolding of RecA have facilitated the evolution of the complex genomes seen in metazoans, but it may likewise have spawned meiosis. The echo of the evolution genes repeats itself since it is this evolution gene that gave sex to the biotic world.

The very essence of sex is meiotic recombination - Anne Villeneuve and Ken Hillers