Evol Ecol Res 3: 157-177 (2001)     Full PDF if your library subscribes.

Does a negative genetic correlation between wing morph and early fecundity imply a functional constraint in Gryllus firmus?

Gray Stirling,1 Daphne J. Fairbairn,2 Shane Jensen3 and Derek A. Roff 1

1Department of Biology, McGill University, 2Department of Biology, Concordia University and 3Department of Mathematics and Statistics, McGill University, Montreal, Quebec, Canada

Address all correspondence to Gray Stirling, Department of Biology, McGill University, 1205 Avenue Dr Penfield, Montreal, Quebec H3A 1B1, Canada.
e-mail: stirling@bio1.lan.mcgill.ca


We tested the hypothesis that wing muscle size and triglyceride production mediate a life-history trade-off between wing morph (or flight capability) and early fecundity. Roff et al. recently selected for lower number of eggs laid in the first week by Gryllus firmus (sand cricket) and reported a correlated increase in long-winged (and presumably flight-capable) females, but did not test structural or energetic reproductive costs associated with flight capability. We used ovary weight of virgin crickets as an index of their early fecundity and dorsal longitudinal flight muscle (DLM) weight as a quantitative measure of flight muscle status. A preliminary experiment indicated that DLM weight is highly correlated with other measures of DLM status (colour, histolysis state, size). We used a haemolymph assay for acylglycerol concentration as an index of mobile triglyceride derivatives thought to provide energy for flight. Both DLM weight and acylglycerol concentration were negatively related to ovary weight of long-winged crickets, indicating wing muscle size and triglyceride concentrations mediate reproductive costs. In short-winged females, only DLM weight had a negative phenotypic relationship with ovary weight. The heritability of DLM weight and acylglycerol concentration are unknown, so we used the direction and size of standardized differences between base and selected populations to test whether DLM weight and acylglycerol concentration are negatively genetically correlated with ovary weight. Acylglycerol concentration and DLM weight increased in the population selected for low numbers of eggs laid within both wing morphs. Large standardized differences in short-winged females provide strong corroborative evidence of a genetic relationship between early fecundity and flight capability. These results indicate negative genetic correlations between wing morph and early fecundity reflect inherited variation in costs of early reproduction, related to physiological investment in flight capability (flight muscle size and triglyceride allocation), rather than being an arbitrary assumption of a life-history trade-off model.

Keywords: correlated response, flight capability, insects, life-history trade-off, phenotypic costs.

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