Equilibria for inversions induced by x-rays in isogenic strains of Drosophila pseudoobscura.

ATURAL populations of many species of Drosophila show chromosomal N p o l ymorphisms, maintained presumably by superior fitness of the heterokaryotypes. Because inversions suppress the recombination in structurally heterozygous chromosomes, they may keep together the coadapted gene complexes inside, and in the vicinity of the inverted sections. This view is supported by the €act that in experimental populations frequency equilibria are regularly established between gene arrangements of similar geographic origin, but not between the same gene arrangements if the chromosomes are derived from geographically different populations (DOBZHANSKY 1948; DOBZHANSKY and PAVLOVSKY 1953). On the other hand, the inversions may produce position effects. The breakage points in naturally occurring inversions are not randomly distributed over the chromosomes, and clustering and coincidence of the breakage points in different inversions is not uncommon ( BERNSTEIN and GOLDSCHMIDT 1961 ; KUNZE-MUHL and SPERLICH 1955). A case is known in Drosophila subobscura in which spontaneous and a X-ray induced inversion shared one of their breakage points ( SPERLICH 1959). If an inversion has no selective advantage when it appears, its chances of spreading and becoming established in a population are very small. A favorable position effect could, however, maintain a new gene arrangement, and permit it to accumulate a gene complex coadapted with other gene arrangements in the same populations. The experiments described in this report were made in the hope that they may throw light on the problem of the dynamics of inversion polymorphisms in Drosophila populations.