Antagonistic pleiotropy and the evolution of wing dimorphism in the sand cricket, Gryllus firmus

Abstract

At 30°C the micropterous females of the sand cricket, Gryllus firmus, begin reproduction at an earlier age after eclosion and have a larger cumulative fecundity than macropterous females. These reproductive costs may offset the advantages of being macropterous and hence capable of migration. The evolutionary significance of this phenotypic trade-off, which is characteristic of wing dimorphic insects in general, is contigent on the traits being genetically correlated. The genetic basis of the phenotypic tradeoff between flight capability and reproduction in the sand cricket, Gryllus firmus, was examined by selecting for increased and decreased incidence of macroptery, and measuring the age schedules of fecundity of macropterous and micropterous females in the selected and control lines. The two traits, wing dimorphism and age schedule of reproduction, are shown to be genetically correlated. Although the mean fecundity within the selected populations changed the fecundities of macropterous and micropterous forms remained constant, suggesting that the age schedule of reproduction may itself be a threshold trait with respect to the continuously varying character controlling the expression of wing form. The relevance of antagonistic pleiotropy to the maintenance of genetic variation for wing form and the age schedule of reproduction is discussed.