Distorted Sex Ratios: A Window into RNAi-Mediated Silencing

Abstract

In 1925, Gershenson started laboratory cultures from 19 female Drosophila obscura that were collected from a forest near Moscow. After recounting his difficulties raising the flies (partial success achieved with a diet of potatoes and fermented raisins), he noted that progeny from most cultures contained an approximately equal sex ratio [1]. Several cultures, however, yielded progeny with highly skewed ratios, such as one group with 87 females and only 7 males. These “deviations from the normal sex-ratio were so considerable that it seemed impossible to explain them by accidental causes,” he wrote. Similar observations had been made by others, but Gershenson went on to perform a number of experiments and reached three important conclusions. First, sex-ratio distortion (referred to hereafter as sex-ratio) was associated with the X chromosome. Second, the expression of the phenotype was sex-limited, because it only occurred in the progeny of males carrying the causal X chromosome. And third, the low numbers of males did not appear to be caused by preferential death of male zygotes or their transformation into females. Rather, he concluded that “the greater part of the spermatozoa determining the development of males do not participate in fertilization”. Because females have two X chromosomes and males are XY, he further suggested that either Y-bearing sperm are less frequently produced by affected males than X-bearing sperm, or that Y-bearing sperm are less capable of achieving fertilization.