Ether treatment of embryos in successive generations results in the progressive increase in penetrance of the bithorm phenocopy in both a massbred and an inbred line of Drosophila melanogaster without artificial selection for the phenocopy. The involvement of cumulative cytoplasmic modifications is suggested by the results of reciprocal crosses between the treated massbred line and its foundation stock, and by the lingering effects observed when ether treatment is withheld in later generations.
Regression analyses showed that the effect of successive generations of ether treatment on penetrance is cumulative and direct, and not mediated by possible effects on viability.
Ether treatment carried out on embryos at four consecutive age intervals in the 14th generation suggests that the effects of long-term etherisation are as follows: an increased tendency to phenocopy at all ages tested; an extension of the critical period during which the phenocopy can be induced; and an increased resistance to the lethal effects of ether in older embryos.
The remarkable consistency in every aspect of the response in the two different lines-one containing at least 20 times as much genetic variation as the other-points to the existence of systematic organismic properties which do not depend on specific alleles in specific genes.
The possible evolutionary significance of our results is discussed.
A preliminary version of the results appeared as part of a lecture delivered at the Third International Conference on Evo- lutionary Biology, Brno, August 1981.