Living on borrowed oxygen in the goldenrod gall fly ( Eurosta solidaginis )

Hypoxia in plant tissue should affect animals living within them. Gall makers stimulate their plant hosts to produce the gall they inhabit and feed on, and also influence the gall phenotype for other adaptations, such as defence against predators. In the context of adapting to potential gall hypoxia through the extended phenotype of the gall, we studied the prevalence and conditions leading to hypoxia in goldenrod galls induced by Eurosta solidaginis. Larval respirometry data showed only mild tolerance to hypoxia, based on elevated critical oxygen partial pressures (∼10 kPa). Mathematical modelling of gas diffusion into the gall suggested that the extent of prevalent hypoxia is tolerable to the larva, while more severe temporary hypoxia may occur early in the host plant's growth season. As galls mature, the cortex layer that makes up most of the gall volume becomes less metabolically active and more permeable to gas diffusion, which is important in preventing severe hypoxia. While the mature cortex layer is unlikely to have been specifically an adaptation to hypoxia, it could mitigate the need for other trade-offs in gas exchange. Since gall makers can influence gall size and tissue composition, this raises the possibility that, on top of the already established predation trade-offs, the gall maker must balance the hypoxia risk of remaining in less differentiated, cytoplasm-rich tissue with the costs associated with inducing a larger, more mature gall with greater gas diffusion capacity.