Mutational analysis of meiosis: Research in my lab is directed at understanding how meiosis works. In particular, we are interested in understanding how homologous chromosomes pair and exchange genetic material during meiosis, and how this leads to the orderly segregation of the homologs at the first, or reductional, meiotic division. My laboratory studies the fruit fly Drosophila melanogaster. The emphasis of our research is on the regulation and mechanisms of meiotic recombination. Several genes have been identified which are required for the execution of meiotic recombination in Drosophila. Our analysis of these genes has placed them in a pathway defined by four major events (McKim et al. 1996). There are "early" genes such as c(3)G and c(2)M, which are required for the synapsis of homlogs, and mei-P22 and mei-W68, which are required for the initiation of meiotic recombination (double streand break formation). Late recombination genes are required either to determine the sites where crossovers will occur (mei-218) or for the recombination event itself (mei-9) (see Sandler et al 1968; Baker and Carpenter 1972). Implied in this hypothesis is the assumption that meiotic recombination in Drosophila works through a Holliday junction. Our discovery that the mei-W68 gene encodes a Spo11 homolog is consistent with that hypothesis (McKim et al 1998). In yeast, Spo11 is believed to be responsible for creating the double strand break that initiates meiotic recombination. The question of how the genes in this pathway interact is currently under investigation in my laboratory. These questions become even more important in light of recent data from the analysis of these genes showing that meiotic recombination in D. melanogaster is under different genetic controls than in S. cerevisiae (McKim et al. 1998).

Direct observation of meiotic recombination sites: We have used cytological detection of a histone modication to observe the dynamics of double strand break formation in Drosophila meiosis. As first shown in mammalian cells and also in Drosophila, the Histone H2 variant HIS2AV is phosphorylated specifically in response to double strand breaks. We have detected this modification in Drosophila meiotic cells. Our studies show that HIS2AV is phophorylated at the sites of double strand breaks and then rapidly disappears. This occurs after SC formation. In mutants that are defective in DNA repair, HIS2AV phophorylation persists longer in meiosis.

Overviews of the genes we study:

You can see a diagram of the meiotic recombination pathway,

or look up details for the following genes:

c(2)M
mei-W68
mei-P22
mei-218
sub

The approach: In a very general way, our approaches can be summarized as follows:

• Genetics (that is rearing and crossing actual fruit flies): My lab's activities are centered around the analysis of meiotic mutants. By definition, these are genes that cause a significant increase in the frequency of homolog nondisjunction during female meiosis. So fare, most of the genes we have been studying have arisen from screens which detect this phenotype.
• Molecular Biology: Like everyone else, I am trying to clone the genes characterized by the genetic studies. Characterizing the genes may involve the generation of antibodies. In addition, since some of the genes we have cloned do not have homologs in the database, therefore we are characterizing their homologs in species such as D. virilis and D. yakuba.
• Cytology: We use confocal microscopy and immunofluorescence to visualize the chromosomes and other cellular features such as the spindle. This data is useful in describing mutant phenotypes and the immunolocalization of various protein. In the future I plan to make use of FISH to study homolog pairing.