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Abstract Nutrition-dependent growth of sexual traits is a major contributor to phenotypic diversity, and a large body of research documents insulin signalling as a major regulator of nutritional plasticity. However, findings across studies raise the possibility that the role of individual components within the insulin signalling pathway diverges in function among traits and taxa. Here, we use RNAi-mediated transcript depletion in the gazelle dung beetle to investigate the functions of forkhead box O (Foxo) and two paralogs of the insulin receptor (InR1 and InR2) in shaping nutritional plasticity in polyphenic male head horns, exaggerated fore legs, and weakly nutrition-responsive genitalia. Our functional genetic manipulations led to three main findings: FoxoRNAi reduced the length of exaggerated head horns in large males, while neither InR1 nor InR2 knock-downs resulted in measurable horn phenotypes. These results are similar to those documented previously for another dung beetle (Onthophagus taurus), but in stark contrast to findings in rhinoceros beetles. Secondly, knockdown of Foxo, InR1, and InR2 led to an increase in the intercept or slope of the scaling relationship of genitalia size. These findings are in contrast even to results documented previously for O. taurus. Lastly, while FoxoRNAi reduces male forelegs in D. gazella and O. taurus, the effects of InR1 and InR2 knockdowns diverged across dung beetle species. Our results add to the growing body of literature indicating that despite insulin signalling's conserved role as a regulator of nutritional plasticity, the functions of its components may diversify among traits and species, potentially fuelling the evolution of scaling relationships. 

Abstract Plastic responses to environmental conditions may themselves depend on other environmental conditions, but how such environment-by-environment (E×E) interactions may impact evolution remains unclear. We investigate how temperature shapes the nutritional polyphenism in horn length in a beetle and test whether “allometric plasticity” (a form of E×E) predicts latitudinal differentiation during a rapid range expansion. Rearing populations under common garden conditions demonstrates that increased temperatures reduce the body size threshold separating two male morphs in all populations but also that the magnitude of temperature-dependent changes in allometry diverged across recently established populations. Furthermore, we found a latitudinal increase in the threshold in the species’ exotic range at one of the temperatures, suggesting that allometric plasticity in response to temperature may predict evolved clinal differences. Our findings demonstrate that E×E interactions can be similar in magnitude to G×E interactions and that allometric plasticity and its evolution may impact population’s responses to environmental changes. 

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 beetle 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. 

Abstract Many organisms actively manipulate the environment in ways that feed back on their own development, a process referred to as developmental niche construction. Yet, the role that constructed biotic and abiotic environments play in shaping phenotypic variation and its evolution is insufficiently understood. Here, we assess whether environmental modifications made by developing dung beetles impact the environment‐sensitive expression of secondary sexual traits. Gazelle dung beetles both physically modify their ontogenetic environment and structure their biotic interactions through the vertical inheritance of microbial symbionts. By experimentally eliminating (i) physical environmental modifications and (ii) the vertical inheritance of microbes, we assess the degree to which (sym)biotic and physical environmental modifications shape the exaggeration of several traits varying in their degree and direction of sexual dimorphism. We expected the experimental reduction of a larva's ability to shape its environment to affect trait size and scaling, especially for traits that are sexually dimorphic and environmentally plastic. We find that compromised developmental niche construction indeed shapes sexual dimorphism in overall body size and the absolute sizes of male‐limited exaggerated head horns, the strongly sexually dimorphic fore tibia length and width, as well as the weakly dimorphic elytron length and width. This suggests that environmental modifications affect sex‐specific phenotypic variation in functional traits. However, most of these effects can be attributed to nutrition‐dependent plasticity in size and non‐isometric trait scaling rather than body‐size‐independent effects on the developmental regulation of trait size. Our findings suggest that the reciprocal relationship between developing organisms, their symbionts, and their environment can have considerable impacts on sexual dimorphism and functional morphology. 

Abstract Static allometry is a major component of morphological variation. Much of the literature on the development of allometry investigates how functional perturbations of diverse pathways affect the relationship between trait size and body size. Often, this is done with the explicit objective to identify developmental mechanisms that enable the sensing of organ size and the regulation of relative growth. However, changes in relative trait size can also be brought about by a range of other distinctly different developmental processes, such as changes in patterning or tissue folding, yet standard univariate biometric approaches are usually unable to distinguish among alternative explanations. Here, we utilize geometric morphometrics to investigate the degree to which functional genetic manipulations known to affect thesizeof dung beetle horns also recapitulate the effect of hornshapeallometry. We reasoned that the knockdown phenotypes of pathways governing relative growth should closely resemble shape variation induced by natural allometric variation. In contrast, we predicted that if genes primarily affect alternative developmental processes, knockdown effects should align poorly with shape allometry. We find that the knockdown effects of several genes (e.g.,doublesex, Foxo) indeed closely aligned with shape allometry, indicating that their corresponding pathways may indeed function primarily in the regulation of relative trait growth. In contrast, other knockdown effects (e.g.,Distal‐less,dachs) failed to align with allometry, implicating these pathways in potentially scaling‐independent processes. Our findings moderate the interpretation of studies focusing on trait length and highlight the usefulness of multivariate approaches to study allometry and phenotypic plasticity. 

Context-dependent trait exaggeration is a major contributor to phenotypic diversity. However, the genetic modifiers instructing development across multiple contexts remain largely unknown. We use the arthropod tibia, a hotspot for segmental differentiation, as a paradigm to assess the developmental mechanisms underlying the context-dependent structural exaggeration of size and shape through nutritional plasticity, sexual dimorphism and segmental differentiation. Using an RNAseq approach in the sexually dimorphic and male-polyphenic dung beetle Digitonthophagus gazella , we find that only a small portion (3.7%) of all transcripts covary positively in expression level with trait size across contexts. However, RNAi-mediated knockdown of the conserved sex-determination gene doublesex suggests that it functions as a context-dependent master mediator of trait exaggeration in D. gazella as well as the closely related dung beetle Onthophagus taurus . Taken together, our findings suggest (i) that the gene networks associated with trait exaggeration are highly dependent on the precise developmental context, (ii) that doublesex differentially shapes morphological exaggeration depending on developmental contexts and (iii) that this context-specificity of dsx -mediated trait exaggeration may diversify rapidly. This mechanism may contribute to the resolution of conflict arising from environment-dependent antagonistic selection among sexes and divergent developmental contexts in a wide range of animals. 

Abstract Age and size at maturity are key life‐history components, yet the proximate underpinnings that mediate intra‐ and interspecific variation in life history remain poorly understood. We studied the proximate underpinnings of species differences and nutritionally plastic variation in adult size and development time in four species of dung beetles. Specifically, we investigated how variation in insect growth mediates adult size variation, tested whether fast juvenile growth trades‐off with developmental stability in adult morphology and quantified plastic responses of digestive systems to variation in food quality. Contrary to the common size–development time trade‐off, the largest species exhibited by far the shortest development time. Correspondingly, species diverged strongly in the shape of growth trajectories. Nutritionally plastic adjustments to growth were qualitatively similar between species but differed in magnitude. Although we expected rapid growth to induce developmental costs, neither instantaneous growth rates nor the duration of larval growth were related to developmental stability in the adult. This renders the putative costs of rapid growth enigmatic. We further found that larvae that encounter a challenging diet develop a larger midgut and digest more slowly than animals reared on a more nutritious diet. These data are consistent with the hypothesis that larvae invest into a more effective digestive system when exposed to low‐quality nutrition, but suggest that species may diverge readily in their reliance on these mechanisms. More generally, our data highlight the complex, and often hidden, relationships between immature growth and age and size at maturation even in ecologically similar species. 

Abstract Developmental and evolutionary processes underlying phenotypic variation frequently target several traits simultaneously, thereby causing covariation, or integration, among phenotypes. While phenotypic integration can be neutral, correlational selection can drive adaptive covariation. Especially, the evolution and development of exaggerated secondary sexual traits may require the adjustment of other traits that support, compensate for, or otherwise function in a concerted manner. Although phenotypic integration is ubiquitous, the interplay between genetic, developmental, and ecological conditions in shaping integration and its evolution remains poorly understood. Here, we study the evolution and plasticity of trait integration in the bull‐headed dung beetleOnthophagus tauruswhich is characterized by the polyphenic expression of horned (‘major’) and hornless (‘minor’) male morphs. By comparing populations subject to divergent intensities of mate competition, we tested whether mating system shifts affect integration of traits predicted to function in a morph‐specific manner. We focussed on fore and hind tibia morphology as these appendages are used to stabilize major males during fights, and on wings, as they are thought to contribute to morph‐based differences in dispersal behavior. We found phenotypic integration between fore and hind tibia length and horn length that was stronger in major males, suggesting phenotypic plasticity in integration and potentially secondary sexual trait compensation. Similarly, we observed that fore tibiashapewas also integrated with relative horn length. However, although we found population differentiation in wing and tibia shape and allometry, populations did not differ in integration. Lastly, we detected little evidence for morph differences in integration in either tibia or wing shape, although wing allometries differed between morphs. This contrasts with previous studies documenting intraspecific differentiation in morphology, behavior, and allometry as a response to varying levels of mate competition acrossO. tauruspopulations. We discuss how sexual selection may shape morph‐specific integration, compensation, and allometry across populations.