Seasonal interactions between photoperiod and maternal effects determine offspring phenotype in Franklin's gull

When predictable seasonal changes affect offspring fitness, we expect offspring to evolve phenotypes that minimize the costs of seasonal variation in timing of breeding. For species with parental care during embryonic development, offspring receive seasonal cues of the environment from parents that are biased by their parent's fitness (which is not equivalent to offspring fitness). Therefore, mechanisms enabling offspring to detect environmental cues independent of parents should be strongly favoured. 2. We experimentally evaluated the ability of avian embryos to integrate cues of season from photoperiod and maternal environments present in eggs to produce seasonal variation in phenotypes among Franklin's gull (Leucophaeus pipixcan) hatchlings. Eggs were collected early and late in the season and some were separated into their component parts and others were incubated under short (early season) and long (late season) photoperiods. After hatching, we measured the structural size of the chicks and the amount of yolk sac reserves. 3. We found that hatchling size, a phenotype linked with fitness, is sensitive to both egg contents provided by mothers and photoperiod, and development time decreases across the season. The effects of integrating cues of season from eggs and photoperiod on offspring phenotype are complex, and when cues of season from eggs are mismatched with cues of season from photoperiod, alternate phenotypes are created. 4. We also found that seasonal variation in egg size, yolk, albumen or shell content of the eggs do not account for the seasonal maternal egg effect on hatchling size. This seasonal maternal effect could be a result of other egg constituents or reflect heritable variation in timing of breeding that is linked with offspring size. 5. Changes in breeding phenology of adults could result in a mismatch between cues from parents and photoperiod cues of season. For example, if breeding seasons advanced such that late season birds initiate breeding at an early season photoperiod, offspring would then be integrating maternal cues of late season with photoperiod cues of early season and alter their phenotypes. We expect our results to initiate new studies on how vertebrate embryos integrate environmental cues with maternal effects and offspring responses to optimize the expression of offspring phenotype.