Synapsis acid chiasma formation in Caenorhabditis elegans require HIM-3, ameiotic chromosome core component that functions in chromosome segregation

Meiotic chromosomes are organized about a proteinaceous core that forms bet ween replicated sister chromatids. We have isolated a Caenorhabditis elegan s gene, him-3, which encodes a meiosis-specific component of chromosome cor es with some similarity to the yeast lateral element protein Hop1p. Antibod ies raised against HIM-3 localize the protein to condensing chromosomes in early prophase I and to the cores of both synapsed and desynapsed chromosom es. In RNA interference experiments, chromosomes appear to condense normall y in the absence of detectable protein but fail to synapse and form chiasma ta, indicating that HIM-3 is essential for these processes. Hypomorphs of h im-3, although being synapsis proficient, show severe reductions in the fre quency of crossing-over, demonstrating that HIM-3 has a role in establishin g normal levels of interhomolog exchange. Him-3 mutants also show defects i n meiotic chromosome segregation and the persistence of the protein at the chromosome core until the metaphase I-anaphase I transition suggests that H IM-3 may play a role in sister chromatid cohesion. The analysis of him-3 pr ovides the first functional description of a chromosome core component in a multicellular organism and suggests that a mechanistic link exists between the early meiotic events of synapsis and recombination, and later events s uch as segregation.