A positive correlation between crossing over within heterozygous pericentric inversions and reduced egg hatch of Drosophila females.

XTENSIVE chromosomal polymorphism, probably due to pericentric inverE sions, has been reported in animals as dissimilar as grasshoppers (WHITE 1958) and mice ( OHNO et al. 1966). Inversion polymorphism is also prevalent in natural populations of many Drosophila species, but rarely are the inversions pericentric. The infrequency with which pericentric as compared with paracentric inversions are found in natural populations of Drosophila has been attributed to reduced fertility of females heterozygous for inversions that include the centromere. In Drosophila females heterozygous for paracentric inversions (centromere not included in inversion), dicentric bridges and acentric fragments that result from single crossing over within the inversions remain in the polar nuclei of the female and do not lead to reduced fertility ( STURTEVANT and BEADLE 1936). There is. however, no known mechanism through which duplicated and deficient chromatids resulting from single and certain types of double crossing over within pericentric inversions can be eliminated without leading to reduced fertility. Nevertheless, heterozygous pericentric inversions have been reported in natural populations of Drosophila algonquin (MILLER 1939), Drosophila robusta (CARSON and STALKER 1947), and Drosophila ananassag (FRIERE-MAIA 1954). ALEXANDER (1952) tested females heterozygous for two long pencentric inversions of the second chromosome for egg-hatch reduction in order to determine whether appreciable numbers of aneuploid gametes were produced by crossing over within the inversions. She found that egg-hatch reduction was less than 10% for each inversion tested and concluded that heterozygotes for pericentric inversions may not be at so much of a selective disadvantage as had been supposed. In the present experiments, recombination and egg hatch were compared for females heterozygous either for one of three pericentric inversions or for a paracentric inversion of the metacentric third chromosome of D. melanogaster. As genetic theory predicts, only females heterozygous for the two pencentric inversions in which there were appreciable amounts of crossing over had their fertility reduced (by 25% and 50%).