Genetics
Recombination
Females homozygous for mei-W68 have virtually no meiotic recombination. Along with mei-P22, this gene is believed to be required for the initiation of double-strand breaks during meiosis. The absence of meiotic crossing over, and therefore chiasmata, cause a high frequencies of meiotic nondisjunction. Nondisjunction occurs at the first meiotic division and only nonexchange chromosomes nondisjoin. Confocal analysis of mei-W68 stage 14 ovaries shows that metaphase arrest is abolished; presumeably due to the lack of chiasmata.
Recombination Nodules
Ultrastructural analysis of pachytene in mei-W68 females demonstrates synaptonemal complex formation is normal (McKim et al. 1998). Thus, synapsis can form in the absence of double strand breaks and meiotic recombination. Consistent with the recombination phenotype, recombination nodules are not observed in mei-W68 mutants.
Mitotic Cells
Baker et al. found that mei-W68 mutants effect mitotic chromosome stability. Lutken and Baker observed an increase in recombination in the male germline. This has also been repeated in mei-W68/Df males (Graham and McKim, unpublished results). In addition, mei-W68 homozygotes are not sensitive to MMS or hydroxyurea. The fact that small clusters of crossovers are recovered from mei-W68 females, suggesting there are pre-meiotic events.
Alleles
The original allele was probably recovered spontaneously (A.T.C. Carpenter, per. comm.). A second weaker allele was isolated by Lindsley. McKim and Hyashi-Hagihara (1998) have isolated a series of mei-W68 deletions.
Gene product
The mei-W68 locus was cloned based on its genetic map position (see Figure). The transcript is found in the germarium as well as a variety of developmental stages from embryo to adult, showing the gene product is not meiosis specific. An unusual property of this transcript is it is within the first intron of another gene. These two genes share sequences in the first exon. The transcribed region also appears to be a warm spot for P-element insertions (4 so far from the genome project, see Figure). In fact, the surrounding region has been sequenced by the genome project and several other P-elements cluster in this region (see Figure of P1 clone DS07982).
The 37.1 kD predicted protein is homologous to the Spo11 gene from S. cerevisiae and the topoisomerase II topo6B isolated from several archea bacteria.
