The evolution of relative trait size and shape: insights from the genitalia of dung beetles

Abstract

Insects show relatively little genital variation within species compared to extraordinary and often rapid diversification among species. It has been suggested that selection for reproductive isolation through differences in genital shape might explain this phenomenon. This hypothesis predicts that populations diverge faster in genital shape than in genital size. We tested this prediction in males from 10 dung beetle species with known phylogenetic relationships from the genus Onthophagus (Coleoptera: Scarabaeidae), including four species for which we were able to sample multiple populations. Specifically, we compared intra- and interspecific differentiation in shape and relative sizes of genitalia and calculated their respective evolutionary rates. We compared these rates to two similarly sized non-genital traits, the head and the fore-tibia. We found significant intraspecific variation in genital shape in all four species for which multiple populations were sampled, but for three of them we also identified significant relative size variation. We also found that genital shape evolved at higher rates than relative genital size. Genital shape evolved faster than head shape, but not fore-tibia shape. However, shapes of all measured structures evolved faster than their relative size. We discuss the functional constraints that may bias the developmental evolution of relative size and shape of genitalia and other morphological traits.

Condition dependence of phenotypic integration and the evolvability of genitalic traits in a neriid fly

“…[42]). For example, genitalic trait shape evolves faster than somatic trait shape in Onthopagus beetles [43]. We suggest that this difference in evolutionary rates could be related to lower integration among genitalic traits and between the genitalia and body size, resulting in less functional constraint on genitalic evolution.…”

Insulin signalling's role in mediating tissue-specific nutritional plasticity and robustness in the horn-polyphenic beetleOnthophagus taurus

“…Sexually selected traits such as weapons, for example, are often greatly exaggerated in larger individuals owing to their significance in aggressive interactions, yet muted in smaller individuals, and thus commonly exhibit extreme sensitivity to variation in nutrition [5,6]. On the other hand, legs or wings function in strict proportion to overall body size, deviations from which may carry severe fitness penalties, and thus commonly exhibit moderate nutrition sensitivity [7]. Lastly, some traits carry out functions that require a more constant absolute size regardless of overall body size or that of other structures; for example, male genitalia in insects selected to fit a wide range of female genitalia, or the central nervous system in most animals [8][9][10].…”

Alpha shapes: determining 3D shape complexity across morphologically diverse structures

“…Such studies frequently consider genital shape variation intraspecifically, or between morphologically similar sister taxa [ 66 , 67 ]. Yet elsewhere, GMM methods have been applied to broader interspecific samples of genitalia [ 65 , 68 , 69 ], highlighting the applicability of these techniques to quantify shape change in rapidly evolving structures, or those comprised entirely/predominantly of soft tissue [ 70 ].…”