MEIOSIS IN THE GRASSHOPPER: CHIASMA FREQUENCY AFTER ELEVATED TEMPERATURE AND X-RAYS

Abstract

Grasshoppers were given an acute dose of X-rays, injected with H3-thymidine and thereafter maintained at 42 °C. Autoradiographic analysis of testicular tubules taken at daily intervals after injection gave meiotic stage durations. The mean chiasma frequency was reduced 4 days after the hoppers were placed at 42°, in both X-rayed and non-irradiated animals, and remained low in the non-irradiated animals throughout the experiment. In the X-rayed animals the chiasma frequency rose to control levels on die 6th day and dropped again thereafter. Analysis of the data indicates a period ending approximately at zygotene during which chiasma frequency can be reduced by high temperature. The chiasma frequency can be increased by irradiating leptotene-zygotene. The end of the heat and X-ray sensitive stages are temporally distinct. Univalents were never produced by the heat treatment. Changes in chiasma frequency are restricted to the four pairs of larger chromosomes where multiple chiasmata are commonly found. Hence, the presence or absence of chiasmata shows a complex interaction with the physical environment.