Homozygous females show elevated levels of chromosome nondisjunction of the X chromosome (34.7%).
X chromosome nondisjunction is increased to 51.9% in homozygous XXY females, with 95% of the events involving the two X chromosomes segregating from the Y.
Crossing over is reduced in mei-W68L1/mei-W681 females. Homozygotes lack meiotic crossing over. Premeiotic exchanges occur in homozygous females. mei-W681 is recessive to wild type when nondisjunction frequencies are examined but has dominant effects on crossing over. Homozygous and mei-W68L1/mei-W681 females show normal dynamics of germ-line mitotic divisions. The dynamics of entry into meiosis or exit of the losing pro-oocyte is also normal. The synaptonemal complex appears to be of normal morphology in homozygous females and appears to be between homologs. There are no late recombination nodules and early recombination nodules also appear to be missing. A novel structure ("noodle"), that resembles recombination nodules (RNs) in being adjacent to the central region of the synaptonemal complex, is seen. Noodles are smaller than RNs and are also less dense. The number of noodles increases at least up to the time cytoplasmic flow begins and they persist post-cytoplasmic flow.
Ultrastructural studies of pachytene reveal synaptonemal complex that is normal in structure and length and which undergoes the same changes in length as is observed in wild type as the cells progress through pachytene; chromocentral organization and chromatin condensation are also normal. However, no late recombination nodules were observed in the nine nuclei reconstructed which were between the developmental landmarks which demark their presence in wild type.
Metaphase arrest prevented.
Females homozygous for mei-W68 show a complete absence of meiotic recombination (Baker, unpublished data). A less severe allele (mei-W68L1-Lindsley) reduces exchange to approximately 60% of control levels and also alters the distribution of residual exchanges. Analysis of mitotic chromosome behavior (Baker et al., 1978) suggests that the mei-W68+ gene product is also required in mitotic cells.