Local adaptation of photoperiodic plasticity maintains life cycle variation within latitudes in a butterfly

The seasonal cycle varies geographically and organisms are under selection to express life cycles that optimally exploit their spatiotemporal habitats. In insects, this often means producing an annual number of generations (voltinism) appropriate to the local season length. Variation in voltinism may arise from variation in environmental factors (e.g., temperature or photoperiod) acting on a single reaction norm shared across populations, but it may also result from local adaptation of reaction norms. However, such local adaptation is poorly explored at short geographic distances, especially within latitudes. Using a combination of common‐garden rearing and life cycle modeling, we have investigated the causal factors behind voltinism variation in Swedish populations of the butterfly Pararge aegeria, focusing on a set of populations that lie within a single degree of latitude but nonetheless differ in season length and voltinism. Despite considerable differences in ambient temperature between populations, modeling suggested that the key determinant of local voltinism was in fact interpopulation differences in photoperiodic response. These include differences in the induction thresholds for winter diapause, as well as differences in photoperiodic regulation of larval development, a widespread but poorly studied phenomenon. Our results demonstrate previously neglected ways that photoperiodism may mediate insect phenological responses to temperature, and emphasize the importance of local adaptation in shaping phenological patterns in general, as well as for predicting the responses of populations to changes in climate. The resources and conditions on which many organisms rely for growth and reproduction vary dramatically over the year. As a result, organisms are under selection to synchronize their life cycles with the seasons (Tauber et al. 1986, Varpe 2017). Life cycle regulation, using seasonal cues, is used to ensure that life cycle events such as reproduction (Sharp 2005), dispersal (Hardie 1987), and dormancy (Ingvarsson et al. 2006) are induced, averted, or adjusted as appropriate. A very common such seasonal cue is photoperiod (Bradshaw and Holzapfel 2007). However, geographic variation in climate, e.g., differences in the length of the favorable season, means that the optimal life cycle will typically not be the same for all populations in a species’ range (Roff 1980, Bradshaw et al. 2004, Phillimore et al. 2010, Burr et al. 2016). Hence, even if locally specific photoperiodic reaction norms theoretically allow for very precise seasonal timing (Hut