Genetic and environmental factors affecting chiasma formation in locusts

By means of crosses with the African Migratory locust it has been demonstrated that various strains carry different segregations of genes which quantitatively condition the formation of chiasmata during meiosis. These genes occur in polygenic series, and in one low frequency strain the polygenes show an epistatic effect for low chiasma frequency, while another low frequency strain seems to have attained this property through gamma radiation. In reciprocal crosses between the lastnamed low and a medium frequency strain there appears to be a maternal effect, with heterosis and epistasis respectively in the reciprocal crosses. In one strain two lines were selected for plasticity and non-plasticity of hopper colour change in small populations, and this selection has resulted in multiple changes in that the two lines now diverge in the three main physical criteria of phase transformation, viz. hopper colour, adult morphometric ratios and chiasma frequencies; one line is now solitarious for these characteristics, the other more gregarious. The genotypes selected out could be related to the ability to utilize the gregarization pheromone.A 10° rise in temperature above 22° C has the effect of increasing chiasma frequencies significantly, but increase in the carbon dioxide content of the atmosphere has no effect. Male and female hoppers produce equivalent amounts of the chiasma-inducing pheromone, while solitarised hoppers do not lose the ability to produce this pheromone. The crucial stage for the action of this pheromone on chromosomes is during the fifth nymphal instar. The phase status of parents appears to affect chiasma frequencies in the offspring, more specifically that of the mother, so that there is further evidence of maternal effect on chiasma frequency.