A MECHANISM INTRINSIC TO HETEROZYGOUS INVERSIONS AFFECTING OBSERVED RECOMBINATION FREQUENCIES IN ADJACENT REGIONS

Abstract

IT has been generally accepted that recombination in a chromosome segment adjacent to a heterozygous inversion is reduced, often to an extreme degree, as compared with recombination in the same segment in homozygous normal controls. This generalization is based for the most part upon the behavior of such segments observed in females of Drosophila (e.g., STURTEVANT and BEADLE 1936; DOBZHANSKY and EPLING 1948) and upon the a priori consideration that the synaptic difficulties introduced by the inversion should lead to reduced frequencies of exchange, particularly near the break points of the inversion. RHOADES and DEMPSEY (1953) have reported a case in maize, and cited earlier reports of two other cases in the same organism (MORGAN 1950; RUSSELL and BURNHAM 1950), in which regions adjacent to heterozygous inversions showed a recombination frequency as much as fourfold higher than the standard map or control recombination values. RHOADES and DEMPSEY seemed to regard this as a real difference in the behavior of the chromosomes of Drosophila and maize, and briefly discussed possible reasons for this difference. One may note, however, that ALEXANDER (1952) has also reported a 50 percent increase in recombination in a region adjacent to the Plum2 inversion in Drosophila melanogaster. It has occurred to the writer that the normal operation of interference W inversion heterozygotes should, in some cases, result in an observed recombination frequency (measured in zygotes) in such an adjacent region which was higher than the actual frequency of recombination at meiosis. The basis of this effect is, essentially, a tendency toward the selective elimination from the zygotic population of chromatids which are nonrecombinant in the adjacent segment. This paper describes the functioning of the mechanism operating to produce this effect, develops means of estimating the magnitude of the effect in the various kinds of situations which may be encountered, and demonstrates that only a part of the observed difference between maize and Drosophila may be explained by the effect.