Effect of Selection on Whaxy: An Autosomal Dominant Mutation in the Screw- Worm Fly with Recessive Lethal Effects

Abstract

Studies on the Whaxy factor in the screw-worm fly, Cochliomyia hominivorax (Coquerel), indicate that the phenotype is due to a dominant autosomal mutation with recessive lethal effects. Data are presented on the hatchability of eggs, pupal emergence, and segregation of progeny from different crosses involving Whaxy flies. The Whaxy character has been under selection for more than 30 generations. In the earlier generations crosses of Whaxy flies produced the expected 2 Whaxy: 1 normal progeny, and outcrosses produced the expected 50 per cent Whaxy progeny. However, after 22 generations of selection the number of Whaxy progeny in each generation increased significantly above 66 per cent and remained at a level higher than expected. The possibility that selection resulted in a higher viability of the heterozygotes, survival of the homozygotes, or lower viability of the wild type is considered and data from several experiments supporting one or the other alternatives are presented.