SEASONAL POLYPHENISM IN WING-MELANIN PATTERN AND THERMOREGULATORY ADAPTATION IN PIERIS BUTTERFLIES

We examine the hypothesis that seasonal polyphenism (response to larval photoperiod and/or temperature) in adult wing-melanic pattern of pierine butterflies is adaptive for thermoregulation in seasonally varying thermal conditions. Previous functional analyses demonstrate how changes in different wing-melanic characters can affect thermoregulatory performance of Pieris (Pontia) occidentalis in warm and cool thermal environments. We develop qualitative predictions about the polyphenic response of these characters to larval photoperiod and/or temperature that would improve thermoregulatory performance in seasonally varying thermal environments. Laboratory experiments in which full sibling families of P. occidentalis were split into long day, warm temperature and short day, cool temperature rearing treatments demonstrated significant agreement between the predicted and observed polyphenic responses for both males and females. The thermoregulatory model failed to predict correctly the observed polyphenic response of melanic characters in the basal and discal cell region of the dorsal forewing. Some melanic characters not expected to affect thermoregulatory performance also showed significant polyphenic responses to rearing environment. Norms of reaction revealed significant differences among families in the polyphenic response of melanic pattern. The absence of significant family correlations in melanic character values across larval environments suggests that the polyphenic expression of melanic pattern in different seasonal environments is not strongly constrained genetically. Analyses of field samples suggest that much of the seasonal variation in melanic pattern observed in the field is the result of polyphenic response to larval-rearing environment. Comparisons of the thermoregulatory behavior and polyphenic response between pierine butterflies (family Pieridae, subfamily Pierinae) and their sister group (subfamily Coliadinae) suggest that seasonal polyphenism in pierines is adaptive for thermoregulation and involves both the ventral and dorsal wing surfaces.