The mechanisms responsible for the fine tuning of development, where t
he wild-type phenotype is reproduced with high fidelity, are not well
understood. The difficulty in approaching this problem is the identifi
cation of mutant phenotypes indicative of a defect in these fine-tunin
g control mechanisms. Evolutionary biologists have used asymmetry as a
measure of developmental homeostasis. The rationale for this was that
, since the same genome controls the development of the left and right
sides of a bilaterally symmetrical organism, departures from symmetry
can be used to measure genetic or environmental perturbations. This p
aper examines the relationship between asymmetry and resistance to org
anophosphorous insecticides in the Australian sheep blowfly, Lucilia c
uprina. A resistance gene, Rop-1, which encodes a carboxylesterase enz
yme, also confers a significant increase in asymmetry. Continued expos
ure of resistant populations to insecticide has selected a dominant su
ppressor of the asymmetry phenotype. Genetic evidence indicates that t
he modifier is the L. cuprina Notch homologue.