What we do:

Homologous chromosome pairing

poor homologous synapsis1 (phs1) is the key gene in maize required for promoting pairing between homologous chromosomes and for coordination of meiotic recombination, pairing, and synapsis. In the maize phs1 mutant, chromosomes synapse to nearly the same extent as in wild-type plants but homologous chromosome synapsis is completely replaced by synapsis between non-homologous partners. In addition to controlling pairing, the phs1 gene is required for installation of RAD51 recombination complexes on chromosomes. Double-strand breaks are normally generated in the mutant plants but their repair is delayed. phs1 defines a novel multistep process to coordinate homologous pairing, synapsis, and recombination. phs1 was cloned using Mutator transposon tagging. The gene encodes a novel protein with as of yet unknown function. Interestingly, the PHS1 protein shows a relatively low level of evolutionary conservation, although it contains several short conserved domains.

We are now working to uncover the mechanism by which PHS1 promotes homologous pairing in meiosis. We are also studying other maize genes with mutant phenotypes similar to phs1 that may encode proteins interacting with the PHS1 protein. We are also investigating the phs1 homolog in Arabidopsis to see if the function of the gene is the same as in maize, even though the phs1 gene shows a low level of evolutionary conservation. Our future goal is to identify other genes and pathways affecting homologous chromosome pairing in Arabidopsis and maize.

	Extensive chromosome pairing-like alignment, which involves non-homologous instead of homologous chromosome interactions, in a phs1 mutant meiocyte observed in 3-dimensional fluorescent in situ hybridization (FISH) microscopy. Red – DAPI-stained chromatin, Green – 5S rRNA locus FISH probe, Cyan – telomere FISH probe.
	Installation on meiotic chromosomes of the RAD51 recombination protein involved in repairing meiotic double-strand breaks. Red – chromatin, Green – RAD51 foci.

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