Evolutionary genetics of seasonal polyphenism in the map butterfly Araschnia levana (Nymphalidae : Lepidoptera)

Araschnia levana shows two spectacularly different seasonal forms: a predom inantly orange spring form with black dots and a predominantly black summer form with a white band. The forms are induced by length of day. We quantif ied differences in wing pattern between the forms and sexes with the help o f image analysis. We used a split family design to analyse the genetic back ground. Each wing pattern element responded in its own way to length of day . Heritabilities within forms were generally high. Genetic correlations acr oss lengths of day, between forms, ranged from around 0 to around 1. Wing p attern may thus rapidly respond to natural selection, but this response is for some traits not independent from selection in the other form. The overa ll heritability for producing a spring or summer form in an environment whe re both forms were produced was very high. There was a tight relationship b etween the length of the 5th larval instar and the adult form produced. We discuss a physiological model which can explain this relationship, and whic h has some interesting implications for the debate on whether genes for pla sticity exist. The overall results are discussed in the light of West-Eberh ard's theory of polyphenism as a first step towards speciation.