Polyphenism of visual and chemical secondary sexually-selected wing traits in the butterfly Bicyclus anynana: How different is the intermediate phenotype?

Polyphenism is a type of phenotypic plasticity supposedly adaptive to drastic and recurrent changes in the environment such as seasonal alternation in temperate and tropical regions. The butterfly Bicyclus anynana shows polyphenism with well-described wet and dry seasonal forms in sub-Saharan Africa, displaying striking morphological, physiological and behavioural differences in response to higher or lower developmental temperatures. During the seasonal transition in the wild, the intermediate phenotype co-occurs with wet and dry phenotypes. In this study, we aimed to characterize the secondary sexually-selected wing traits of the intermediate form to infer its potential fitness compared to wet and dry phenotypes. Among the previously described wing morphological traits, we first showed that the area of the fifth eyespot on the ventral hindwing is the most discriminant trait to identify wet, dry and intermediate phenotypes in both sexes. Second, we characterized the intermediate form for two secondary sexually-selected wing traits: the area and UV reflectance of the dorsal forewing pupil and the composition of the male sex pheromone. We showed that values of these two traits are often between those of the wet and dry phenotypes. Third, we observed increasing male sex pheromone production in ageing dry and wet phenotypes. Our results contrast with previous reports of values for sexually-selected traits in wet and dry seasonal forms, which might be explained by differences in rearing conditions or sample size effects among studies. Wet, dry and intermediate phenotypes display redundant sexually dimorphic traits, including sexually-selected traits that can inform about their developmental temperature in sexual interactions.

多型现象是一种表型可塑性，据推测它能适应环境中剧烈且反复的变化，比如温带和热带地区的季节交替。非洲眼蛱蝶（Bicyclus anynana）在撒哈拉以南非洲呈现出多型现象，具有被充分描述的湿季型和干季型，其在较高或较低的发育温度下表现出显著的形态、生理和行为差异。在野外的季节转换期间，中间表型与湿季型和干季型表型共同出现。在这项研究中，我们旨在描述中间型的次级性选择翅部特征，以推断其与湿季型和干季型表型相比的潜在适合度。在先前描述的翅部形态特征中，我们首先表明后翅腹面第五个眼斑的面积是区分雌雄两性湿季型、干季型和中间型表型的最具判别性的特征。其次，我们描述了中间型的两个次级性选择翅部特征：前翅背面眼斑的面积和紫外线反射率以及雄性性信息素的成分。我们表明这两个特征的值通常介于湿季型和干季型的值之间。第三，我们观察到干季型和湿季型随着年龄增长雄性性信息素产量增加。我们的结果与先前关于湿季型和干季型性选择特征值的报告形成对比，这可能是由于不同研究之间饲养条件的差异或样本量效应所致。湿季型、干季型和中间型表型表现出冗余的性二态特征，包括性选择特征，这些特征可以在性互动中反映它们的发育温度。