him-5 mutants have a greatly reduced level of crossing over on the X chromosome and an altered distribution of the residual crossovers. This leads to a high level of X-chromosome non-disjunction (the Him phenotype). Crossovers on the autosomes are much less affected, and autosomal non-disjunction is much less frequent than X non-disjunction. Not unexpectedly, given its mutant phenotype,
him-5 mutants appear to carry out most of the early events of prophase I normally, and the desynaptic phenotype is seen only at diakinesis. The predicted HIM-5 protein (encoded by D1086.4) is a small protein with a pI of 10.7. Experiments with two different HIM-5 antibodies indicate that the protein is found exclusively in germline nuclei prior to meiosis and persisting until at least early pachytene. From a yeast two-hybrid screen of the ORFeome library, HIM-5 interacts with perhaps a dozen other proteins. (Additional clones are being analyzed and we expect to have a more definite number at the meeting.) These include a RING finger protein predicted by microarrays to be expressed in the germline and a microtubule-associated protein found with both meiotic and mitotic spindles. Thus, the interaction partners include other chromosomal and germline proteins, which may give us a more complete view of how
him-5 affects germline chromosomes and meiosis.