Developmental bias in the evolution and plasticity of beetle horn shape

The degree to which developmental systems bias the phenotypic effects of
environmental and genetic variation, and how these biases affect
evolution, is subject to much debate. Here, we assess whether
developmental variability in horn shape aligns with the phenotypic effects
of plasticity and evolutionary divergence, yielding three salient results.
First, we find that most pathways previously shown to regulate horn length
also affect shape. Second, we find that the phenotypic effects of
manipulating divergent developmental pathways are correlated with each
other as well as multivariate fluctuating asymmetry – a measure of
developmental variability. Third, these effects further aligned with
thermal plasticity, population differences, and macroevolutionary
divergence between sister taxa and more distantly related species.
Collectively, our results support the hypothesis that changes in horn
shape —whether brought about by environmentally plastic responses,
functional manipulations, or evolutionary divergences— converge along
‘developmental lines of least resistance’, i.e., are biased by the
developmental system underpinning horn shape.