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.

Methods
To investigate the evolution and development of horn shape, we used a two-dimensional geometric morphometric approach. For each of the data sets described below (fluctuating asymmetry, functional genetic manipulations, environmental plasticity, and evolutionary divergence), we took pictures of beetle head horns using a digital camera (Scion) mounted on a Leica MZ-16 stereomicroscope. The final data set included measurements for several wild-caught and laboratory-reared populations of Onthophagus taurus, one wild-caught population of the closely related Onthophagus illyricus, as well as laboratory-reared individuals of the more distantly related Digitonthophagus gazella.

TpsDig2 was used to quantify horn shape using 4 landmarks and 26 semi-landmarks. The landmarks of all 1,044 specimens were subjected simultaneously to a Procrustes analysis in the R-package geomorph. The position of semi-landmarks was optimized by minimizing bending energy. Centroid size was extracted as a shape-independent measure of overall structural size. See Rohner et al. (doi: 10.1098/rspb.2022.1441) for furhter information.