The genetic basis of the trade-off between calling and wing morph in males of the cricket Gryllus firmus

Wing dimorphisms exist in a wide range of insects. In wing-dimorphic species one morph is winged, has functional flight muscles (LW), and is flight-capable, whereas the other has reduced wings (SW) and cannot fly. The evolution and maintenance of wing dimorphisms is believed to be due to trade-offs between flight capability and fitness-related traits. Although there are well-established phenotypic trade-offs associated with wing dimorphism in female insects, there only exist two studies that have established a genetic basis to these trade-offs. The present study provides the first evidence for a genetically based trade-off in male insects, specifically in the sand cricket Gryllus firmus. Because they have to expend energy to maintain the flight apparatus (especially flight muscles), LW males are predicted to call less and therefore to attract fewer females. To be of evolutionary significance, call duration, wing morph, and wing muscle condition (size and functionality) should all have measurable heritabilities and all. be genetically correlated. Differences between morphs in male G. firmus in the likelihood of attracting a female were tested in the laboratory using a T-maze where females chose between a LW male and a SW male. Call duration for each male was recorded on the sixth day of adult life. A significant difference in call duration was found between SW and LW males (SW = 0.86 +/- 0.01, LW = 0.64 +/- 0.01 h). SW males attracted significantly more females than did LW males (63% vs. to 37%). All the traits involved in the trade-off had significant heritabilities (call = 0.75 +/- 0.33; wing morph = 0.22 +/- 0.07; muscle weight = 0.38 +/- 0.09) and genetic correlations (call and wing morph = -0.46 +/- 0.20 for SW, -0.68 +/- 0.16 for LW; LW call and muscle weight = -0.80 +/- 0.14). These results provide the first documented evidence that trade-offs between a dimorphic trait and a fitness-related character in males has a genetic basis and hence can be of evolutionary significance.