During meiosis, homologous chromosomes must find their appropriate partners and pair with them. This intimate alignment is necessary for homologs to undergo recombination events, which are in turn essential for their proper segregation at the meiosis I division. In most eukaryotes, including C. elegans, meiotic recombination occurs within the context of a proteinaceous structure called the synaptonemal complex (SC) that polymerizes between paired homologs. A long-standing mystery is how chromosome pairing and the formation of the SC are coordinated with the molecular events of meiotic recombination. In budding yeast, meiotic recombination is known to be initiated by the formation of double-strand DNA breaks (DSBs), which are generated by a topoisomerase-like enzyme, Spo11p. In the absence of this protein, there is no recombination, and moreover, chromosomes fail to pair and synapse normally. Because of this and other data, it is believed that chromosome synapsis is dependent on meiotic recombination in S. cerevisiae. Relatively little is known about the mechanism of meiotic recombination in other organisms. Accordingly, a key question is whether double-strand breaks are used to initiate recombination during meiosis in any other eukaryote. We have investigated the meiotic role of a Spo11p homolog, T05E11.4, uncovered by the C. elegans sequencing consortium. PCR screening was used to isolate a deletion allele of this gene, which we have now named
spo-11. We found that
spo-11(
ok79) homozygotes have a severe Him phenotype and produce mostly dead embryos. Examination of their meiotic chromosomes revealed an absence of chiasmata, which is an expected consequence of a failure in meiotic recombination. Using genetic analysis, we confirmed that
spo-11 mutants are completely defective for meiotic crossing over. Based on homology with the yeast protein known to be responsible for generating double-strand breaks, it was likely that the absence of meiotic recombination in
spo-11 worms resulted from a failure to initiate the process. We tested whether DNA breaks induced by subjecting
spo-11 hermaphrodites to gamma-irradiation during meiosis could replace the function of SPO-11, and discovered that both crossing over and progeny viability were partially restored. Thus, we have provided the first evidence from a metazoan organism that meiotic recombination initiates by the same mechanism discovered in budding yeast, and consequently that this DSB mechanism is widely conserved. We have also developed cytological approaches to study chromosome pairing and synaptonemal complex formation in the worm. Using 3- dimensional FISH and high-resolution microscopy, we determined that chromosome pairing is completely unaffected by the
spo-11 deletion. Further, electron microscopic analysis showed that the synaptonemal complex structure in
spo-11 worms is identical to that in wild type. Thus, in contrast to the paradigm that has emerged from studies in budding yeast, we have demonstrated that homologous chromosome synapsis is independent of meiotic recombination in the worm.