Nonrandom Disjunction in Female Meiosis of Drosophila. W.D. Gilliland , J.L. Verburg , C.H. Langley. Evolution and Ecology, Univ California, Davis, Davis, CA.
Female meiosis presents a unique opportunity for inter-chromosomal competition, because only one of the four meiotic products will eventually become the egg pronucleus and be included in future generations. Chromosomal structures that preferentially move to the egg pole exhibit a form of meiotic drive, Non-Random Disjunction (NRD). NRD has been observed in females heterozygous for chromosomal rearrangements which form asymmetric dyads after recombination. The smaller chromosome is present in over half of the gametes produced by those females. The magnitude of the bias (deviation from 50% segregation) varies with particular chromosomal rearrangements, and is altered by certain meiotic mutants. However, direct measurement of NRD is confounded with viability differences among markers, and has never been observed in natural populations.
We are studying NRD in several ways. First, we have measured the influence of different meiotic mutants on levels of NRD in a particular aberration, C(1;Ys)3. This revealed that amounts of certain meiotic genes (e.g. a half-dose of normal nod gene copy) decreased bias, while other genes (e.g. one copy of the tubulin mutant alphaTub67CP40) enhanced bias. This pattern is consistent with the model proposed by Matthies et. al. (1999) for nod and alphaTub67C to interact as opposing forces during meiosis. This interaction suggests C(1;Ys)3 can be used to assay chromosomes it is paired with for NRD variation.
Based on previous work on variation at the nod locus, we have also sequenced 10 alleles of alphaTub67C from D. melanogaster and 1 from D. simulans. We found no segregating amino acid variation, but considerable variation in the 5' UTR and intron, which are required for expression levels in the female germline. As this gene is dosage sensitive, we are assaying effects of these alleles on X and 4th chromosome nondisjunction.