MATERNAL AGING AND SOMATIC CROSSING OVER OF ATTACHED-X CHROMOSOMES

Abstract

Preliminary observations of Drosophila melanogaster stocks carrying either two rod chromosomes or a ring and a rod chromosome, together with the gene markers y and sn3, indicated no statistically significant effect of aging on somatic crossing over. A more refined study was made, using attached-X females (y+.+sn3) derived either from a Muller-doubler stock or from a triploid stock. Females were kept as virgins either 2 days (unaged series; or 10-12 days (aged series; before mating to males which were uniformly 2 days old. Offspring were aged for 2 days after emergence to give equal darkening of the bristles, then frozen at -18[degree]C. Abdomens were mounted in Euparal on a microscope slide and inspected for mosaic spots. Four sets of experiments were carried out, in 2 of which the flies were divided according to autosomal genotype. The data as a whole indicated no effect of maternal aging on the frequency of somatic crossing over, but there were marked differences among cultures in frequency of mosaicism, which were believed to be under genetic rather than en- vironmental control. The total frequency of twin and non-twin spots was lower in the aged than in the unaged series, but the spots in the aged series were somewhat larger. The size-class distribution of mosaic spots seemed to be a more consistent feature of crossing over than frequency of spots. There was a greater incidence of mosaicism in flies heterozygous for the Cy inversion than in their siblings heterozygous for a minute. The authors state that their studies do not confirm Schwartz''s assumptions concerning the effect of aging on crossing over, upon which his two-phase model of crossing over was based.