THE EVOLUTION OF THRESHOLD TRAITS - A QUANTITATIVE GENETIC-ANALYSIS OF THE PHYSIOLOGICAL AND LIFE-HISTORY CORRELATES OF WING DIMORPHISM IN THE SAND CRICKET

Many traits are phenotypically discrete but polygenically determined. Such traits can be understood using the threshold model of quantitativ e genetics that posits a continuously distributed underlying trait, ca lled the liability, and a threshold of response, individuals above the threshold displaying one morph and individuals below the threshold di splaying the alternate morph. For many threshold traits the liability probably consists of a hormone or a suite of hormones. Previous experi ments have implicated juvenile hormone esterase (JHE), a degratory enz yme of juvenile hormone, as a physiological determinant of vying dimor phism in the crickets Gryllus rubens and C. firmus. The present study uses a half-sib experiment to measure the heritability of JHE in the l ast nymphal stadium of G. firmus and its genetic correlation with fecu ndity, a trait that is itself genetically correlated with wing morph. The phenotypic and genetic parameters are consistent with the hypothes is that JHE is a significant component of the liability. Comparison of sire and dam estimates suggest that nonadditive effects may be import ant. Two models have been proposed to account for the fitness differen ces between morphs: the dichotomy model. which assumes that each morph can be characterized by a particular suite of traits, and the continu ous model, which assumes that the associated fitness traits are correl ated with the Liability rather than the morphs themselves. The latter model predicts that the fitness differences will not be constant but c hange with the morph frequencies, Variation in fecundity and flight mu scle histolysis are shown to be more consistent with the continuous mo del. Data from the present experiment on JHE are inconclusive, but res ults from a previous selection experiment also suggest that variation in JHE is consistent only with the continuous model.