MEIOTIC DRIVE IN NATURAL POPULATIONS OF DROSOPHILA MELANOGASTER. II. GENETIC VARIATION AT THE SEGREGATION-DISTORTER LOCUS

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Abstract
When a chromosome II carrying an SD allele is made heterozygous with certain structurally normal SD+-bearing second chromosomes in a D. melanogaster male, the SD-bearing chromosome is recovered much more frequently than its SD+-bearing homolog. The extent of the excess recovery of SD (the k value), and the proportion of males showing any specific amount of excess, depends on the source and the history of the SD and SD+-bearing chromosomes. In particular: (1) there are "unstable" SD lines which regularly produce males characterized by k values any-where between 0.50 and 1.00, generation after generation; (2) there are SD+ alleles of various "sensitivities" to any given SD ranging from a completely insensitive allele (producing a k value of 0.50 against any SD) to the very sensitive "standard" SD+ allele with an average k value well above 0.90 in standard backcrosses; (3) the sensitivity of an SD+ allele (as well as the ability to distort of an SD allele) can be changed ("translocal modification") by making the allele in question heterozygous with an insensitive allele for one or more generations (and conversely); and, finally, (4) when certain SD-bearing chromosomes are passed for one generation through a female, so balanced that crossing over in the vicinity of SD does not occur, a proportion of such females produce two types of heterozygous SD sons about equally frequently: (a) those showing normal distortion and (b) those showing no distortion. Females of this type are referred to as "conditioned" and the phenomenon as "conditional distortion." Finally, it is observed that crossing over in the immediate vicinity of the SD locus is very much reduced suggesting, perhaps, that SD is a localized structural modification of some sort, and the various SD states departures from some standard structural condition.

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