Chemical defence in a sawfly: genetic components of variation in relevant life-history traits

Larvae of several tenthredinid sawfly species readily release droplets of haemolymph through their integument when attacked by predators. This defence mechanism via 'bleeding' is characterised by a low integument resistance and a high haemolymph deterrence. Both traits are variable, and negatively correlated among species. We sought to determine if such differences in the propensity to bleed also occur intraspecifically by studying the heritability of traits potentially associated with the bleeding phenomenon in the turnip sawfly Athalia rosae ruficornis Jakovlev (Hymenoptera: Tenthredinidae, Allantinae). For three European populations, heritabilities were estimated in the laboratory in a parent-offspring and a full-sib design for haemolymph deterrence (measured as concentration of sequestered glucosinolate), integument resistance, body mass of eonymph and adult, and developmental time. Within A. rosae, no significant negative phenotypic correlation was found between the two traits directly related to the defence mechanism: integument resistance and haemolymph deterrence. However, the significant heritabilities found for these traits in the full-sib analysis (0.39 and 0.35, respectively, for males in the Swiss population) show that the variation has a genetic component. While full-sib analysis revealed highly significant heritabilities for most traits in all the three populations, parent-offspring regression revealed little or no evidence of heritable variation. Effects of common environment for siblings and variation in the host-plant quality between insect generations are likely to be the main factors explaining these differences. A consequence of such host-plant variation in the wild might be that genetic variation of such chemical defensive traits is largely invisible to natural selection.