Genetics of development time in a butterfly: predictions from optimality and a test by subspecies crossing

Earlier studies on adaptive plasticity in development time and diapause regulation in the speckled wood butterfly, Pararge aegeria L., have been based on optimality models and have not considered genetics. Nevertheless, they have been successful in predicting patterns observed. From the results of these studies we predicted the genetics of larval and pupal development time, as well as of diapause control, to be polygenic and sex linked. We show that this is the case by crossing a population of the northern subspecies P. a. tircis from southern Sweden, which shows a diapause in some daylengths and is protandrous, with a population of the southern subspecies P. a. aegeria from Madeira. The latter inhabits a much less seasonal environment, develops directly at all daylengths, and is not protandrous. Offspring showed variable and intermediate larval and pupal development times when reared at daylengths inducing diapause in Swedish pure stock. Female offspring were more similar to their mothers in the reciprocal crosses, whereas development time in male offspring was not sensitive to the direction of the cross. This suggests the presence of a sex-linked modifying factor. The results show that the outcome of tests of optimality models can be used to predict genetic systems.