Meiotic drive in natural populations of Drosophila melanogaster .7. Conditional segregation distortion – a possible nonallelic conversion

Males, heterozygous for the Segregation-distorter (SD) allele (located in or near the centromeric heterochromatin of the right arm of chromosome 11) and a standard tester second chromosome, regularly produce a preponderance of functional SD-bearing sperm ( SANDLER, HIRAIZUMI and SANDLER 1959). Segregation in heterozygous SD females, on the other hand, is always normal. It has been found, however, that, for certain SD lines, if the SD-bearing chromosome is inherited from the female parent, then, in a fraction of F, male sibships, only one half of the heterozygous SD sons exhibit the phenomenon of segregationdistortion; in the other half of the sons, segregation is normal. In the following generation, all of the males exhibit abnormal ratios irrespective of whether or not their father showed an abnormal ratio. Those females that produce sons. only one half of which distort, are referred to as conditioned; the phenomenon is termed conditional distortion ( SANDLER and HIRAIZUMI 1959)