A semi-lethal in Drosophila funebris that causes an excess of males

In the course of the genetic work on Drosophila melanogaster cases have been found rather frequently in which the sex ratios showed marked deviations from the usual approximate equality. Those cases in which there is a deficiency of males have been the most frequent, and the explanation of many of these has been worked out by Rawls, Morgan, Bridges, Stark, and others. They are now known to be due to sex-linked lethal genes. Less frequently cases have been noted in which there was a deficiency of females (see Quackenbush, Science vol. 32). In none of these has the explanation hitherto been discovered.

From a culture of Drosophila funebris we obtained one female and 87 males. This female, mated to a few brothers, produced 60 females to 103 males. Descendants of this mating have been inbred for many generations, and have given sex ratios ranging from O : 76 up to approximate equality. There is no obvious relation between the sex ratio of a culture and the pedigree of the parents—in fact, two cultures from the same parents may give quite different sex ratios.

Preliminary experiments indicate that environmental conditions-especially temperature-affect the sex ratio in this line; but we are not yet able to control it at will.

In this race the females frequently have abnormal abdominal bands; but this character appears in the males only very rarely. The evidence indicates that it is this character that is influenced by environmental conditions, and that the very abnormal females do not emerge from their puparia. Dark pupz, evidently dead, are always to be found in cultures that give a significant excess of males. A few of these have been dissected, and have been found to contain dead flies with abnormal abdomens.