Lack of response to artificial selection on the slope of reaction norms for seasonal polyphenism in the butterfly Bicyclus anynana.

Abstract

The tropical butterfly Bicyclus anynana shows adaptive phenotypic plasticity in response to wet-dry seasonality. The wet season form (WSF) has a conspicuous wing pattern with large eyespots, whereas the dry season form (DSF) lacks eyespots and therefore has a more cryptic appearance. Temperature is the main factor controlling this difference: rearing larvae at a low (<19 degrees C) temperature in the laboratory results in the DSF, whereas rearing at a high (>23 degrees C) temperature induces the WSF. We applied truncation selection in opposite directions in successive generations reared at two alternating temperatures (18.5 degrees C and 23.5 degrees C) to increase (for two High Plasticity (HP) lines), and decrease (for two Low Plasticity (LP) lines) wing pattern plasticity. Plasticity was assessed by partitioning full-sib families over four rearing temperatures (18.5 degrees C, 20.5 degrees C, 21.5 degrees C and 23.5 degrees C). Several wing pattern elements were measured for which the first principal component (PC1) provides a useful summary. The slopes of reaction norms for PC1 were significantly steeper in the HP lines than in the LP lines; however, the selection lines did not always differ significantly from the unselected stock. The results of crosses between the replicates of the selection lines gave no indication for effects of inbreeding. We argue that high, positive genetic correlations across temperatures retard a response to selection in opposite directions in different environments. This is discussed with respect to potential evolutionary constraints in natural populations in these butterflies.