Heterosis in Ddt Resistant and Susceptible Populations

HE phenomenon of heterosis has generally been studied in relation to some Tarbitrarily selected trait such as size, weight, or yield. Heterosis is then regarded as being manifested by an increase in the chosen trait. Although various definitions have been proposed ( GOWEN 1952), heterosis is most commonly and most meaningfully defined as an increase in the chosen trait in crossbred individuals as compared to the better of their more inbred parents. While heterosis is usually considered to affect all of the characters of hybrids, it is actually only measured in relation to the trait or traits selected for study. Recently DOBZHANSKY (in GOWEN 1952) has equated heterosis with adaptive value and has called increased size or vigor “luxuriance” when it does not contribute to fitness, This usage, while it has the advantage of referring always to the same trait, adaptive value, may lead to considerable confusion since it is entirely possible for smaller size, for example, to confer greater adaptive value than large, and hence a direct contradiction between the historical meaning of the word (SHULL 1948) and this new definition will result. The experiments to be reported were designed to test some of the concepts which have grown up around the phenomenon of heterosis.