The Determination of Position in Crossing-over. II

Abstract

1. The regional distribution of chiasmata in Drosophila melanogaster can be explained an the assumption that they are formed in a regular time sequence commencing at the centromere. The firnt, or proximal, chiasma forms at a mean distance from the centromere, which is correlated with the length of the chromosome arm. The second chiasma forms at a distance from the first which is probably constant throughout the chromosome complement. This latter distance, the interference distance, is not the Same in size as the former, or differential, distance. The chiasma formation therefore depends an two parameters.2. The chromosome length-chiasma frequency relations in organisms with a large size range, show a curve consisting of two parts. These are the flat lower end, where the chiasma frequency equals one, no matter what the size, and the upper sloping portions where chiasma frequency increases with chromosome length.3. The distinction between the two parts can be explained as depending an (a) the formation of a first chiasma irrespective of the length of the chromosome, but occupying a distance which is positively correlated with the length of the chromosome arm, and (b) the formation of a second and subsequent chiasmata at a mean distance from the directly previous one dependent an interference and constant throughout the chromosomes.4. On the basis of this analysis the sloping upper part of the curve is completely described in terms of the two Parameters, the slope, depending an the distance between the first and second chiasma, and its Position, depending an the mean length of the segment occupied by the first chiasma. These two Parameters should be capable of independent variation. Experimental data show this to be the rase where genotype or environment are changed (in Culex and certain Orthoptera respectively).5. Since the sloping upper portion of the curve does not cut the origin the first chiasma distance and the interference distance are not equal. Consequently chiasma formation must be determined from some fixed point in the chromosome and must follow a regular spacial sequence.6. Thus Drosophila distributior. data and the chromosome length-chiasma frequency data agree in showing the two parameters, the diffarantinl nnrd interference distances. This svstem is fundamental and probably universal where chiasma formation occurs. It may, however, be modified by extraneous influences such as incomplete or irregular pachytene pairing, or chiasma competition. Irregular results are then obtained.