No evidence for the evolution of thermal or desiccation tolerance of eggs among populations of Manduca sexta

1. Populations within species often occur across divergent habitats and, under the right conditions, can adapt to local conditions. For adult and larval insects, local adaptation in tolerance traits, e.g. thermoregulation and water conservation, is well documented. Eggs of insects are immobile, non-feeding and relatively small, and therefore may be at greater risk from abiotic threats. Selection, therefore, may favour strategies that allow embryos to be robust to desiccation and thermal stresses. In particular, the canteen hypothesis predicts that eggs in hot and dry environments contain more water, and this extra water protects them from elevated rates of water loss. 2. This study tests the canteen hypothesis by (i) examining geographic patterns of egg size and water content and (ii) measuring egg and neonate performance across a range of manipulated temperatures and humidities. We focused on eggs of the hawkmoth, Manduca sexta (Lepidoptera: Sphingidae). Manduca eggs are larger in xeric regions of the United States - e.g. the southwestern deserts - compared with M. sexta eggs from eastern, mesic regions. 3. Specifically, we asked: do eggs from warmer or drier regions (i) contain more water, (ii) perform better in dry conditions and (iii) tolerate higher temperatures? We also assessed whether desiccation during the egg stage affects subsequent growth of larvae. We tested M. sexta eggs from three environments: Arizona (warm, dry), North Carolina (cool, humid) and a laboratory strain (cool, dry). 4. Although M. sexta eggs from warmer and drier populations were larger and contained more water, these changes did not protect them better from abiotic stress. Eggs from all populations were similarly tolerant of high heat and low humidity. Desiccation during the egg stage also had no long-term effects on hatchling growth rate. 5. Our results provided little support for local adaptation in abiotic stress physiology via the canteen hypothesis. However, low humidity and delayed hatching suggest a different mechanism for desiccation resistance: embryos may modify eggshell conductance in response to internal water status. The geographic cline in egg size may instead arise from selection on any of the three subsequent life stages or may reflect other evolutionary pressures on number-size trade-offs.