Crossing Over and Inversions in Coadapted Systems

Crossing over between the ST gene arrangement of D. pseudoobscura and various third chromosome inversions has been analyzed and considered in light of the triad hypothesis of Wallace (1953). It has been shown that an inversion's role in a coadapted system cannot be predicted from cytological evidence alone without confirmatory crossover data. This was demonstrated for the Standard/Pikes Peak heterozygote by the presence of only one double crossover in the total of 6105 flies. The relationship of this finding to the formation of stabilized coadapted gene combinations between the ST and PP arrangements is discussed. Since Pikes Peak is a large simple inversion with respect to Standard, two strand double crossovers are expected. This almost complete lack of crossing over cannot be explained on the basis of any simple mechanism of crossover suppression. In addition, data are presented for the crossing over between ST and a newly discovered simple, third chromosome inversion.