Developmental finite element analysis of cichlid pharyngeal jaws: Quantifying the generation of a key innovation

The legacy and evolvability of Pere Alberch's ideas

Pere Alberch played a pivotal role in shaping the field of evolutionary developmental biology during the 1980s and 1990s. Whereas initially his contributions were sidelined by the empirical advancements of the molecular revolution in developmental and evolutionary biology, his theoretical insights have left a lasting impact on the discipline. This article provides a comprehensive review of the legacy and evolvability of Alberch's ideas in contemporary evo-devo, which included the study of morphogenesis as the proper level of developmental causation, the interplay between developmental constraints and natural selection, the epistemic role of teratologies, the origin of evolutionary novelties and the concept of evolvability.

Functional modularity and mechanical stress shape plastic responses during fish development

The adaptive potential of plastic phenotypes relies on combined developmental responses. We investigated how manipulation of developmental conditions related to foraging mode in the fish Megaleporinus macrocephalus induces plastic responses at different levels: (a) functional modularity of skull bones, (b) biomechanical properties of the chondrocranium using finite element models, (c) bmp4 expression levels, used as a proxy for molecular pathways involved in bone responses to mechanical load. We identified new modules in experimental groups, suggesting increased integration in specific head bone elements associated with the development of subterminal and upturned mouths, which are major features of Megaleporinus plastic morphotypes released in the lab. Plastic responses in head shape involved differences in the magnitude of mechanical stress, which seem restricted to certain chondrocranium regions. Three bones represent a "mechanical unit" related to changes in mouth position induced by foraging mode, suggesting that functional modularity might be enhanced by the way specific regions respond to mechanical load. Differences in bmp4 expression levels between plastic morphotypes indicate associations between molecular signaling pathways and biomechanical responses to load. Our results offer a multilevel perspective of epigenetic factors involved in plastic responses, expanding our knowledge about mechanisms of developmental plasticity that originate novel complex phenotypes.

Phenotype-first hypotheses, spandrels and early metazoan evolution

Against the neo-Darwinian assumption that genetic factors are the principal source of variation upon which natural selection operates, a phenotype-first hypothesis strikes us as revolutionary because development would seem to constitute an independent source of variability. Richard Watson and his co-authors have argued that developmental memory constitutes one such variety of phenotypic variability. While this version of the phenotype-first hypothesis is especially well-suited for the late metazoan context, where animals have a sufficient history of selection from which to draw, appeals to developmental memory seem less plausible in the evolutionary context of the early metazoans. I provide an interpretation of Stuart Newman's account of deep metazoan phylogenesis that suggests that spandrels are, in addition to developmental memory, an important reservoir of phenotypic variability. I conclude by arguing that Gerd Müller's "side-effect hypothesis" is an illuminating generalization of the proposed non-Watsonian version of the phenotype-first hypothesis.

Assessing the Levels of Functional Adaptation: Finite Element Analysis Reveals Species, Hybrid, and Sexual Variation in the Biomechanics of African Cichlid Mandibles

To understand how adaptive divergence emerges it is essential to examine the function of phenotypic traits along a continuum. For vertebrates, the mandible provides a key link with foraging and other important activities which has made it highly relevant for investigations of biomechanical change. Variation in mandible shape is known to correspond with ecology but its function is often only investigated between distinct species. However, for such divergence to occur and be maintained selection likely draws from many sources of biomechanical variation. African cichlids represent an exemplar model for understanding how such processes unfold with mandible variation existing between species, sexes, and is likely generated in nature by the potential for hybridization. We explored such mandible variation through a finite element modelling approach and predicted that hybrids and females would have reduced functional capabilities, the former in line with disruptive selection and the latter due to potential trade-offs incurred by maternal mouthbrooding in Malawian haplochromines. We revealed evidence of structural adaptations between Tropheops ‘Red Cheek’ and Labeotrophues fuelleborni that impacted the dispersion of mechanical stress in ways that matched the foraging of these species. Also, hybrids showed higher stresses relative to both species across the mandible. Sexual dimorphism in stress handling was evident despite minor differences in shape with males showing enhanced load resistance. However, in hybrids it appeared that males were disadvantaged relative to females, and displayed asymmetry in load handling. Together, these results show evidence of species and sex based biomechanical variation, that could be targeted by divergent selection.

Ecological validity in exercise neuroscience research: A systematic investigation

The contribution of cortical processes to adaptive motor behaviour is of great interest in the field of exercise neuroscience. Next to established criteria of objectivity, reliability and validity, ecological validity refers to the concerns of whether measurements and behaviour in research settings are representative of the real world. Because exercise neuroscience investigations using mobile electroencephalography are oftentimes conducted in laboratory settings under controlled environments, methodological approaches may interfere with the idea of ecological validity. This review utilizes an original ecological validity tool to assess the degree of ecological validity in current exercise neuroscience research. A systematic literature search was conducted to identify articles investigating cortical dynamics during goal-directed sports movement. To assess ecological validity, five elements (environment, stimulus, response, body and mind) were assessed on a continuum of artificiality-naturality and simplicity-complexity. Forty-seven studies were included in the present review. Results indicate lowest average ratings for the element of environment. The elements stimulus, body and mind had mediocre ratings, and the element of response had the highest overall ratings. In terms of the type of sport, studies that assessed closed-skill indoor sports had the highest ratings, whereas closed-skill outdoor sports had the lowest overall rating. Our findings identify specific elements that are lacking in ecological validity and areas of improvement in current exercise neuroscience literature. Future studies may potentially increase ecological validity by moving from reductionist, artificial environments towards complex, natural environments and incorporating real-world sport elements such as adaptive responses and competition.

Evolvability under climate change: Bone development and shape plasticity are heritable and correspond with performance in Arctic charr (Salvelinus alpinus)

Environmental conditions can impact the development of phenotypes and in turn the performance of individuals. Climate change, therefore, provides a pressing need to extend our understanding of how temperature will influence phenotypic variation. To address this, we assessed the impact of increased temperatures on ecologically significant phenotypic traits in Arctic charr (Salvelinus alpinus). We raised Arctic charr at 5°C and 9°C to simulate a predicted climate change scenario and examined temperature-induced variation in ossification, bone metabolism, skeletal morphology, and escape response. Fish reared at 9°C exhibited less cartilage and bone development at the same developmental stage, but also higher bone metabolism in localized regions. The higher temperature treatment also resulted in significant differences in craniofacial morphology, changes in the degree of variation, and fewer vertebrae. Both temperature regime and vertebral number affected escape response performance, with higher temperature leading to decreased latency. These findings demonstrate that climate change has the potential to impact development through multiple routes with the potential for plasticity and the release of cryptic genetic variation to have strong impacts on function through ecological performance and survival.

3D Digitization in Functional Morphology: Where is the Point of Diminishing Returns?

Modern computational and imaging methods are revolutionizing the fields of comparative morphology, biomechanics, and ecomorphology. In particular, imaging tools such as X-ray micro computed tomography (µCT) and diffusible iodine-based contrast enhanced CT allow observing and measuring small and/or otherwise inaccessible anatomical structures, and creating highly accurate three-dimensional (3D) renditions that can be used in biomechanical modeling and tests of functional or evolutionary hypotheses. But, do the larger datasets generated through 3D digitization always confer greater power to uncover functional or evolutionary patterns, when compared with more traditional methodologies? And, if so, why? Here, we contrast the advantages and challenges of using data generated via (3D) CT methods versus more traditional (2D) approaches in the study of skull macroevolution and feeding functional morphology in bats. First, we test for the effect of dimensionality and landmark number on inferences of adaptive shifts during cranial evolution by contrasting results from 3D versus 2D geometric morphometric datasets of bat crania. We find sharp differences between results generated from the 3D versus some of the 2D datasets (xy, yz, ventral, and frontal), which appear to be primarily driven by the loss of critical dimensions of morphological variation rather than number of landmarks. Second, we examine differences in accuracy and precision among 2D and 3D predictive models of bite force by comparing three skull lever models that differ in the sources of skull and muscle anatomical data. We find that a 3D model that relies on skull µCT scans and muscle data partly derived from diceCT is slightly more accurate than models based on skull photographs or skull µCT and muscle data fully derived from dissections. However, the benefit of using the diceCT-informed model is modest given the effort it currently takes to virtually dissect muscles from CT scans. By contrasting traditional and modern tools, we illustrate when and why 3D datasets may be preferable over 2D data, and vice versa, and how different methodologies can complement each other in comparative analyses of morphological function and evolution.