The Mains laboratory has previously reported the characterization of the only known meiotic-specific spindle component,
mei-1. This gene can be mutated to disrupt meiosis (as a loss-of-function (lf) mutation) or mitosis (as a ts dominant gain-of-function (gf) mutation). One-cell embryos from
mei-1(lf) homozygotes display abnormal polar bodies and a disruption of the meiotic spindle, however, subsequent mitotic cleavages appear to be unaffected. In contrast to the lf mutations,
mei-1(gf) results in normal meiosis followed by abnormal mitotic spindle orientation at the restrictive temperature. This two-fold genetic behaviour suggests that
mei-1 plays a role in normal meiosis but has a subtle or absent role in mitosis. Our interpretation of the
mei-1(gf) mutation is that its product can still function within normal parameters during meiosis but has inappropriate activity during mitosis. Immunolocalization of wildtype MEI-1 protein indicates it is associated only with the meiotic spindle microtubules, and is concentrated at the poles. The MEI-1 sequence is most similar to katanin, a sea urchin ATPase implicated in severing microtubules at the centrosome during mitosis. It is possible that MEI-1 functions to shorten microtubules during meiosis, however, we have not been able to confirm this by in vitro analysis. Based on genetic and cytological evidence,
mel-26(+) prevents
mei-1 from functioning during mitosis. For instance, removal of
mel-26(+) results in the inappropriate localization of MEI-1 to the mitotic spindle and a shortening of mitotic spindle asters.
mel-26 has been cloned and encodes a novel protein, with the exception of a conserved BTB domain implicated in protein-protein interactions. MEL-26 may function to sequester or degrade MEI-1 product prior to mitotic spindle assembly. Preliminary immunolocalization results indicate that MEL-26 may associate with centrosomes. The
mei-2(lf) phenotype is indistinguishable from
mei-1(lf) and genetic analysis suggests that
mei-2 is an activator of
mei-1. We recently cloned
mei-2 and discovered that is also encodes a novel protein. We have identified the lesions responsible for all four known
mei-2 alleles.
mei-2(
sb31) is a candidate null allele, based on intra-complementation and molecular analysis. This allele dominantly suppresses
mei-1(gf), however, a chromosomal deletion (hDf8) that removes
mei-2 (and should mimic a null) is not able to suppress
mei-1(gf). This result suggests that either
sb31 is not a
mei-2 null, or that the deletion removes
mei-2 and some unknown negative regulator of
mei-1. We are currently testing duplications in this region that subdivide hDf8 in order to identify the putative negative regulator. One candidate is a
cdc25 homologue, which we have shown (in collaboration with Andy Golden and Neville Ashcroft) to give some RNAi phenotypes that are similar to
mei-2;
cdc25RNAi does not disrupt the meiotic spindle structure, but can interfere with the meiotic spindle's association with the anterior cortex to cause meiotic failure. Further molecular characterization of
mei-2, such as immunolocalization, should provide some insight into its function. Our expectation is that MEI-2 may help recruit MEI-1 to the meiotic spindle.