The Nature of Linkage Variation with Age in Inversion Heterozygotes of Drosophila melanogaster

Abstract

Two kinds of variation in linkage values in Drosophila experiments are well known but poorly understood. One is the change in crossover values with increasing age of the female as shown by Bridges [1]; the other is excessive variation from female to female as pointed out by Gowen [2]. Studies reported in the present paper on an age effect in a special situation may contribute to the interpretation of both problems. They also serve as controls for the irradiation paper which follows [3]. The chief result of aging is a rapid decrease in recombination values during the first six days of egg-laying, particularly at or near the spindle attachment of the chromosome. After that period smaller changes consist, with variations, of a slight rise and second fall. In recognition of this Bridges [4] has defined as a condition for chromosome mapping the use of data from young females. From such data inferences are made as to the amount of actual crossing over which has occurred between genes which are assumed to be heterozygous at the time when the germ cells enter meiosis. That this procedure may overestimate the amount of crossing over has been elaborated by the senior author both by reasoning back from observable clustering of the data [5] and by deducing certain consequences of crossing over premeiotically, in gonial cells [6]. The present experiment shows an age effect and clustering attributable to oogonial crossing over in the same body of data, where meiotic crossovers have been greatly reduced or eliminated by inversions.

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