Simultaneous Change in Both Differential and Interference Distances of Chiasmata

Abstract

ON the basis of a serial formation of chiasmata in time beginning from the centromere, the effect of experimental factors on chiasma formation has been interpreted in terms of the two parameters, differential and interference distances1–3. By differential distance is meant the distance of the first chiasma from the centromere, and by interference distance, the length of the chromosome arm between the successive chiasmata. With a change in the differential distance alone, the graph showing the relation between chromosome length (x) and chiasma frequency (y) has a series of parallel lines for different temperatures, intersecting the line y = 1 at their respective differential distances (Fig. 1a). Mather3 interprets White's results4 from temporature experiments as showing this type of effect. On the other hand, if only the interference distance changed, the different lines would radiate from the same point (Fig. 1b). Mather3 again explains the data of Moffett5 on different individuals of Culex as representing this type of change. Apart from these examples by Mather which have become classical in studies on chiasma frequency, there does not seem to have been any other interpretation of results along such lines. Data was therefore extracted from experiments of mine6,7 which had shown that temperature reduced chiasma frequency per pollen mother-cell in the bluebell, Endymion nonscriptus (L) Garcke.