Mutations in the recombination defective (rec) gene were isolated by Rhoda Grell in 1978, because they cause a severe decrease in meiotic crossing over. We cloned the gene and found that it encodes the Drosophila ortholog of MCM8. Previously, eukaryotic genomes were known to encode six related MCM proteins, MCM2-MCM7, believed to function as a heterohexamer or heterododecamer. These MCMs are essential for initiation of DNA replication, and may function as the replicative helicase. MCM8 and MCM9 are found in arthropods and vertebrates. This has led some authors to make the erroneous claim that MCM8 and MCM9 are only found in "higher eukaryotes". However, our phylogenetic analysis showed that there are orthologs of MCM8 and MCM9 in plants and many protists (Blanton et al., 2005), suggesting that they arose early in eukaryotic evolution, but were lost from some lineages. Curiously, there is no MCM9 ortholog in any of the Drosophila species whose genomes has been sequenced. One other meiotic recombination gene, mei-218, encodes a protein that has some sequence similarity to MCMs. However, the similarity is very weak and spans a limited region not including the ATPase domain, so the significance of this similarity is unclear.

To better understand the function of REC in meiotic recombination, we recovered non-crossover gene conversions that occurred within the rosy gene. Using multiple heterologies between the two alleles used, we mapped conversion tract lengths in wild-type flies and in rec mutants. Tracts in rec mutants were significantly shorter than those in wild-type females. We hypothesize that REC facilitates processive synthesis during meiotic DSB repair, and that in the absence of REC, synthesis rarely proceeds far enough to allow formation of a DHJ intermediate. Instead, recombination is completed by synthesis-dependent strand annealing (SDSA), which produces non-crossover progeny only. These results were all published in PLoS Genetics in 2005.

We're now doing physical and biochemical studies with REC, in addition to further investigating the meiotic function through more directed genetic experiments.