Sexual Dimorphism and Morphological Modularity in Acanthoscelides obtectus (Say, 1831) (Coleoptera: Chrysomelidae): A Geometric Morphometric Approach

Simple Summary

The seed beetle Acanthoscelides obtectus used in this study is a worldwide pest species that inhabits storage facilities and fields of beans. Knowing that sexual dimorphism is very common among insects, we investigated the level of morphological differences between the sexes. Such an approach allowed us to look into the modular organization of this organism. As expected, the females were larger than the males. The observed two modular organization (thorax and abdomen) was sex specific, indicating that reproductive function has the central role in forming the patterns of modularity. It seems that natural selection is driving force for females, while males are influenced more by sexual selection.

Abstract

Sexual dimorphism and specific patterns of development contribute in a great manner to the direction and degree of the sexual differences in body size and shape in many insects. Using a landmark-based geometric morpohometrics approach, we investigated sex-specific morphological size and shape variation in the seed beetle, Acanthoscelides obtectus. We also tested the functional hypothesis of the two morphological modules—thorax and abdomen in both sexes. Female-biased sexual dimorphism in size was shown, while differences in shape were reflected in the wider thorax and abdomen and shorter abdomen in females in comparison to males. The functional hypothesis of a two-module body was confirmed only in females before correction for size, and in both sexes after the allometry correction. Our results indicate that reproductive function has the central role in forming the patterns of modularity. We hypothesize that high morphological integration of the abdomen in females results from intense stabilizing selection, while the more relaxed integration in males is driven by the higher intensity of sexual selection.

Morphological integration of the canine region within the hominine alveolar arch

The early hominin record is characterized by numerous shifts in dental proportions (e.g., canine reduction and megadontia) linked to changes in diet and social behavior. Recent studies suggest that hominins exhibit a reduction in the magnitude of covariation between the anterior and posterior dental components compared with other extant great apes. They point toward, but do not directly test, the relative independence of canine morphology within the hominin alveolar arch. This study focuses specifically on the how the canine region covaries with other regions of the dental arch because the canine region has drastically reduced in size and changed in shape across human evolution. We examine extant primate species most commonly used as a comparative framework for fossil hominin morphology: Gorilla gorilla (n = 27), Pan troglodytes (n = 27), and Homo sapiens (n = 30). We used geometric morphometric methods to test for size and shape covariation between the canine region with other dental regions. We also examined the influence of sexual dimorphism and allometry on intraspecific and interspecific patterns of covariation. The analysis of size and shape covariation between the mandibular canine and other individual tooth regions elucidated complex, species-specific, and sex-specific morphological relationships in the mandibular alveolar arch. There was little evidence to support different patterns of morphological integration between humans on the one hand and nonhuman apes on the other. Canine region morphology was relatively independent from other dental regions across species based on shape and did not significantly covary more with either the incisor or postcanine region in any species. The size correlations between the canine and other dental regions were moderate to high. The species-specific results of this study question the ability to make a priori assumptions about morphological integration in the extant hominin mandibular alveolar arch and its application to the fossil record.

A 3D analysis of growth trajectory and integration during early human prenatal facial growth

Significant shape changes in the human facial skeleton occur in the early prenatal period, and understanding this process is critical for studying a myriad of congenital facial anomalies. However, quantifying and visualizing human fetal facial growth has been challenging. Here, we applied quantitative geometric morphometrics (GM) to high-resolution magnetic resonance images of human embryo and fetuses, to comprehensively analyze facial growth. We utilized non-linear growth estimation and GM methods to assess integrated epigenetic growth between masticatory muscles and associated bones. Our results show that the growth trajectory of the human face in the early prenatal period follows a curved line with three flexion points. Significant antero-posterior development occurs early, resulting in a shift from a mandibular prognathic to relatively orthognathic appearance, followed by expansion in the lateral direction. Furthermore, during this time, the development of the zygoma and the mandibular ramus is closely integrated with the masseter muscle.

Constraints associated with captivity alter craniomandibular integration in wild boar

The domestication process is associated with substantial phenotypic changes through time. However, although morphological integration between biological structures is purported to have a major influence on the evolution of new morphologies, little attention has been paid to the influence of domestication on the magnitude of integration. Here, we assessed the influence of constraints associated with captivity, considered as one of the crucial first steps in the domestication process, on the integration of cranial and mandibular structures. We investigated the craniomandibular integration in Western European Sus scrofa using three-dimensional (3D) landmark-based geometric morphometrics. Our results suggest that captivity is associated with a lower level of integration between the cranium and the mandible. Plastic responses to captivity can thus affect the magnitude of integration of key functional structures. These findings underline the critical need to develop integration studies in the context of animal domestication to better understand the processes accountable for the set-up of domestic phenotypes through time.

Three-dimensional geometric morphometric analysis of the skull of Protoceratops andrewsi supports a socio-sexual signalling role for the ceratopsian frill

Socio-sexual selection is predicted to be an important driver of evolution, influencing speciation, extinction and adaptation. The fossil record provides a means of testing these predictions, but detecting its signature from morphological data alone is difficult. There are, nonetheless, some specific patterns of growth and variation which are expected of traits under socio-sexual selection. The distinctive parietal-squamosal frill of ceratopsian dinosaurs has previously been suggested as a socio-sexual display trait, but evidence for this has been limited. Here, we perform a whole-skull shape analysis of an unprecedentedly large sample of specimens of Protoceratops andrewsi using a high-density landmark-based geometric morphometric approach to test four predictions regarding a potential socio-sexual signalling role for the frill. Three predictions-low integration with the rest of the skull, significantly higher rate of change in size and shape during ontogeny, and higher morphological variance than other skull regions-are supported. One prediction, sexual dimorphism in shape, is not supported, suggesting that sexual differences in P. andrewsi are likely to be small. Together, these findings are consistent with mutual mate choice or selection for signalling quality in more general social interactions, and support the hypothesis that the frill functioned as a socio-sexual signal in ceratopsian dinosaurs.

Quercus species divergence is driven by natural selection on evolutionarily less integrated traits

Functional traits are organismal attributes that can respond to environmental cues, thereby providing important ecological functions. In addition, an organism's potential for adaptation is defined by the patterns of covariation among groups of functionally related traits. Whether an organism is evolutionarily constrained or has the potential for adaptation is based on the phenotypic integration or modularity of these traits. Here, we revisited leaf morphology in two European sympatric white oaks (Quercus petraea (Matt.) Liebl. and Quercus robur L.), sampling 2098 individuals, across much of their geographical distribution ranges. At the phenotypic level, leaf morphology traditionally encompasses discriminant attributes among different oak species. Here, we estimated in situ heritability, genetic correlation, and integration across such attributes. Also, we performed Selection Response Decomposition to test these traits for potential differences in oak species' evolutionary responses. Based on the uncovered functional units of traits (modules) in our study, the morphological module "leaf size gradient" was highlighted among functionally integrated traits. Equally, this module was defined in both oaks as being under "global regulation" in vegetative bud establishment and development. Lamina basal shape and intercalary veins' number were not, or, less integrated within the initially defined leaf functional unit, suggesting more than one module within the leaf traits' ensemble. Since these traits generally show the greatest species discriminatory power, they potentially underwent effective differential response to selection among oaks. Indeed, the selection of these traits could have driven the ecological preferences between the two sympatric oaks growing under different microclimates.

Ecomorphological divergence and trophic resource partitioning in 15 syntopic Indo-Pacific parrotfishes (Labridae: Scarini)

Adaptive diversification is a product of both phylogenetic constraint and ecological opportunity. The species-rich parrotfish genera Scarus and Chlorurus display considerable variation in trophic cranial morphology, but these parrotfishes are often described as generalist herbivores. Recent work has suggested that parrotfish partition trophic resources at very fine spatial scales, raising the question of whether interspecific differences in cranial morphology reflect trophic partitioning. We tested this hypothesis by comparing targeted feeding substrata with a previously published dataset of nine cranial morphological traits. We sampled feeding substrata of 15 parrotfish species at Lizard Island, Great Barrier Reef, Australia, by following individuals until focused biting was observed, then extracting a bite core 22 mm in diameter. Three indices were parameterized for each bite core: substratum taphonomy, maximum turf height and cover of crustose coralline algae. Parrotfish species were spread along a single axis of variation in feeding substrata: successional status of the substratum taphonomy and epilithic and endolithic biota. This axis of trophic variation was significantly correlated with cranial morphology, indicating that morphological disparity within this clade is associated with interspecific partitioning of feeding substrata. Phylogenetic signal and phylomorphospace analyses revealed that the evolution of this clade involved a hitherto-unrecognized level of trophic diversification.

Estimating Phylogenies from Shape and Similar Multidimensional Data: Why It Is Not Reliable

In recent years, there has been controversy whether multidimensional data such as geometric morphometric data or information on gene expression can be used for estimating phylogenies. This study uses simulations of evolution in multidimensional phenotype spaces to address this question and to identify specific factors that are important for answering it. Most of the simulations use phylogenies with four taxa, so that there are just three possible unrooted trees and the effect of different combinations of branch lengths can be studied systematically. In a comparison of methods, squared-change parsimony performed similarly well as maximum likelihood, and both methods outperformed Wagner and Euclidean parsimony, neighbor-joining and UPGMA. Under an evolutionary model of isotropic Brownian motion, phylogeny can be estimated reliably if dimensionality is high, even with relatively unfavorable combinations of branch lengths. By contrast, if there is phenotypic integration such that most variation is concentrated in one or a few dimensions, the reliability of phylogenetic estimates is severely reduced. Evolutionary models with stabilizing selection also produce highly unreliable estimates, which are little better than picking a phylogenetic tree at random. To examine how these results apply to phylogenies with more than four taxa, we conducted further simulations with up to eight taxa, which indicated that the effects of dimensionality and phenotypic integration extend to more than four taxa, and that convergence among internal nodes may produce additional complications specifically for greater numbers of taxa. Overall, the simulations suggest that multidimensional data, under evolutionary models that are plausible for biological data, do not produce reliable estimates of phylogeny. [Brownian motion; gene expression data; geometric morphometrics; morphological integration; squared-change parsimony; phylogeny; shape; stabilizing selection.].

Weak genetic signal for phenotypic integration implicates developmental processes as major regulators of trait covariation

Phenotypic integration is an important metric that describes the degree of covariation among traits in a population, and is hypothesized to arise due to selection for shared functional processes. Our ability to identify the genetic and/or developmental underpinnings of integration is marred by temporally overlapping cell-, tissue- and structure-level processes that serve to continually 'overwrite' the structure of covariation among traits through ontogeny. Here, we examine whether traits that are integrated at the phenotypic level also exhibit a shared genetic basis (e.g. pleiotropy). We micro-CT scanned two hard tissue traits, and two soft tissue traits (mandible, pectoral girdle, atrium and ventricle, respectively) from an F5 hybrid population of Lake Malawi cichlids, and used geometric morphometrics to extract 3D shape information from each trait. Given the large degree of asymmetric variation that may reflect developmental instability, we separated symmetric from asymmetric components of shape variation. We then performed quantitative trait loci (QTL) analysis to determine the degree of genetic overlap between shapes. While we found ubiquitous associations among traits at the phenotypic level, except for a handful of notable exceptions, our QTL analysis revealed few overlapping genetic regions. Taken together, this indicates developmental interactions can play a large role in determining the degree of phenotypic integration among traits, and likely obfuscate the genotype to phenotype map, limiting our ability to gain a comprehensive picture of the genetic contributors responsible for phenotypic divergence.

Fluctuating Asymmetry and Developmental Instability, a Guide to Best Practice

Best practices in studies of developmental instability, as measured by fluctuating asymmetry, have developed over the past 60 years. Unfortunately, they are haphazardly applied in many of the papers submitted for review. Most often, research designs suffer from lack of randomization, inadequate replication, poor attention to size scaling, lack of attention to measurement error, and unrecognized mixtures of additive and multiplicative errors. Here, I summarize a set of best practices, especially in studies that examine the effects of environmental stress on fluctuating asymmetry.

Morphological integration and modularity in the hyperkinetic feeding system of aquatic-foraging snakes

The kinetic skull is a key innovation that allowed snakes to capture, manipulate, and swallow prey exclusively using their heads using the coordinated movement of eight bones. Despite these unique feeding behaviors, patterns of evolutionary integration and modularity within the feeding bones of snakes in a phylogenetic framework have yet to be addressed. Here, we use a dataset of 60 μCT-scanned skulls and high-density geometric morphometric methods to address the origin and patterns of variation and integration in the feeding bones of aquatic-foraging snakes. By comparing alternate superimposition protocols allowing us to analyze the entire kinetic feeding system simultaneously, we find that the feeding bones are highly integrated, driven predominantly by functional selective pressures. The most supported pattern of modularity contains four modules, each associated with distinct functional roles: the mandible, the palatopterygoid arch, the maxilla, and the suspensorium. Further, the morphological disparity of each bone is not linked to its magnitude of integration, indicating that integration within the feeding system does not strongly constrain morphological evolution, and that adequate biomechanical solutions to a wide range of feeding ecologies and behaviors are readily evolvable within the constraint due to integration in the snake feeding system.

Patterns of correlations and locomotor specialization in anuran limbs: association with phylogeny and ecology

As anuran saltatory locomotion has specific functional requirements achieved through certain intra- and inter-limb proportions, we analyzed pattern and degree of morphological integration in limbs of ten anuran species to reveal the relationship of shared developmental programs of serially homologous structures and locomotor specialization. Our main objectives were (1) to examine if morphological and functional differences in forelimb and hindlimb were associated with reduced covariation between limbs, (2) and to reveal patterns of correlation between species and the roles played by evolutionary history (phylogeny) and ecology (lifestyle and habitat use). Species with different locomotor behaviours (walking, jumping, hopping, running, climbing, swimming and burrowing) were used. Partial correlations showed that species shared similar patterns of functionally based morphological integration, with increased correlations in elements within limbs and reduced correlations between limbs. This was mainly based on strong correlations between proximal elements, humerus-radioulna and femur-tibiofibula. To test the influence of phylogenetic relationships and ecological demands we used different matrices (correlation similarity matrix, ecological similarity matrix, matrices of phylogenetic distance and morphological distance). The changes in correlation patterns are shown to be dissociated from phylogeny. On the other hand, they are to some extent shaped by habitat use and locomotion, as the species with similar locomotor behaviour also tend to have stronger similarity in integration patterns. The results from this study provide insight into the processes underlying the evolutionary change of anuran limbs, highlighting function as the main factor that shaped morphological integration of the examined species.

Rocker jaw: Global context for a Polynesian characteristic

Our goal is to describe the global distribution of the "rocker jaw" variant in human populations. Rocker jaw refers to mandibles that lack the antegonial notch, making them unstable on a flat surface. Data were collected by C.G. Turner II on 9,207 individuals from Asia, Europe, the Pacific, and the Americas, and by J.D. Irish on 3,526 individuals from North and South Africa. With a focus on Polynesia, where the trait is most common, frequencies are presented for subdivisions of Oceania, Australasia, Eurasia, the Americas, and Africa. While the rocker jaw is a Polynesian characteristic, the trait is found throughout the world. Within major geographic regions, there are interesting contrasts, for example, (a) the similarity of Jomon and Ainu and their difference from modern Japanese; (b) Aleuts and Northwest Coast Indians are similar and both are distinct from the Inuit and other Native Americans; and (c) North and Sub-Saharan Africans show a regional difference that parallels genetic and dental distinctions. Skeletons in South America that exhibit the rocker jaw have been interpreted as Polynesian voyagers who ventured to the west coast of South America. The rarity of rocker jaw in South American natives supports this view. The rocker jaw can be attributed to the unique basicranium morphology and large upper facial height of Polynesians, which highlights the integrated growth of a functional module (i.e., mastication) of the craniofacial complex. The unusually high frequency of the trait in Polynesians is a product of both function and founder effect/genetic drift.

Phenotypic integration in feliform carnivores: Covariation patterns and disparity in hypercarnivores versus generalists

The skeleton is a complex arrangement of anatomical structures that covary to various degrees depending on both intrinsic and extrinsic factors. Among the Feliformia, many species are characterized by predator lifestyles providing a unique opportunity to investigate the impact of highly specialized hypercarnivorous diet on phenotypic integration and shape diversity. To do so, we compared the shape of the skull, mandible, humerus, and femur of species in relation to their feeding strategies (hypercarnivorous vs. generalist species) and prey preference (predators of small vs. large prey) using three-dimensional geometric morphometric techniques. Our results highlight different degrees of morphological integration in the Feliformia depending on the functional implication of the anatomical structure, with an overall higher covariation of structures in hypercarnivorous species. The skull and the forelimb are not integrated in generalist species, whereas they are integrated in hypercarnivores. These results can potentially be explained by the different feeding strategies of these species. Contrary to our expectations, hypercarnivores display a higher disparity for the skull than generalist species. This is probably due to the fact that a specialization toward high-meat diet could be achieved through various phenotypes. Finally, humeri and femora display shape variations depending on relative prey size preference. Large species feeding on large prey tend to have robust long bones due to higher biomechanical constraints.

Selection on ultrasonic call rate in neonatal rats affects low frequency, but not ultrasonic, vocalizations in adults

In this experiment, we studied a rodent model selected over 57 generations for high or low rates of ultrasonic vocalizations (USVs) during maternal separation as pups. We investigated the influence of this breeding on the adult animals' subsequent vocal output, comparing acoustic variables across developmental stages. We hypothesized that selection on pup USV rate would impact adult USV production without affecting lower frequency calls. Contrary to this hypothesis, we found neither number of USV calls or other acoustic variables to differ among selected adult lines. Instead, we found that pup USV selection mainly affected adults' low-frequency (human-audible) calls. Furthermore, low-frequency vocalizations did not fully fit a predicted correlation between body weight and fundamental frequency: high line males, although the heaviest on average, did not produce the lowest fundamental frequencies. Our findings suggest that selection for early ultrasonic vocal behaviour pleiotropically results in changes in anatomical production mechanisms and/or neural control affecting low-frequency calls.

Genetic correlations in the rhesus macaque dentition

Quantitative genetic analyses can indicate how complex traits respond to natural selection by demonstrating the genetic relationships between features that constrain their evolution. Genetic correlations between dental measurements have been estimated previously in baboons, humans, and tamarins and indicate variable patterns of modularity by tooth type across these taxa. Here, heritabilities of, and genetic correlations between, linear dental measurements were estimated from the Cayo Santiago rhesus macaques (Macaca mulatta). Relationships between the genetic correlation matrix and matrices designed to test hypotheses of modularity by tooth type, region, function, and development were assessed using a random skewers approach. Dental measurements were found to be moderately to highly heritable, with 24 of 28 heritability estimates differing significantly (p < 0.05) from zero. Almost all genetic correlations between dental dimensions were positive. The genetic correlation matrix was most similar to a regionally modular matrix, with distinct anterior and postcanine tooth modules. This pattern is consistent with previous quantitative genetic analyses of baboons and previous phenotypic analyses of cercopithecoid primates. The existence of a genetic module for the canines and honing premolar was not supported. Ongoing selection pressures, rather than strong genetic constraints, are likely necessary to preserve functional relationships between the canines and honing premolar based on these findings. The genetic correlation matrix of the Cayo Santiago rhesus macaques mirrors patterns of phenotypic correlations observed for cercopithecoid primates broadly and demonstrates that genetic contributions to these patterns may be fairly stable over the course of cercopithecoid evolution. The quantitative genetic study of additional taxa will be necessary to determine whether the regional modularity of baboons and macaques, or the integrated pattern of humans and tamarins, is shared more broadly across primates.

A first glimpse at the influence of body mass in the morphological integration of the limb long bones: an investigation in modern rhinoceroses

The appendicular skeleton of tetrapods is a particularly integrated structure due to the shared developmental origin or similar functional constraints exerted on its elements. Among these constraints, body mass is considered strongly to influence its integration but its effect on shape covariation has rarely been addressed in mammals, especially in heavy taxa. Here, we propose to explore the covariation patterns of the long bones in heavy animals and their link to body mass. We investigate the five modern rhinoceros species, which display an important range of bodyweight. We used a 3D geometric morphometric approach to describe the shape covariation of the six bones composing the stylopodium and zeugopodium both among and within species. Our results indicate that the appendicular skeleton of modern rhinos is a strongly integrated structure. At the interspecific level, the shape covariation is roughly similar between all pairs of bones and mainly concerns the muscular insertions related to powerful flexion and extension movements. The forelimb integration appears higher and more related to body mass than that of the hind limb, suggesting a specialization for weight support. The integration of the stylopodium elements does not seem to relate to body mass in our sample, which suggests a greater effect of shared developmental factors. Conversely, the covariation of the zeugopodium bones seems more associated with body mass, particularly for the radius-ulna pair. The fibula appears poorly integrated with other bones, especially within non-Rhinoceros species, which may represent a case of parcellation due to a functional dissociation between the hind limb bones. The exploration of the integration patterns at the intraspecific level also highlights a more prominent effect of age over individual body mass on shape covariation within C. simum. This study lends support to previous hypotheses indicating a link between high body mass and high integration level.

Evolutionary and ontogenetic changes of the anatomical organization and modularity in the skull of archosaurs

Comparative anatomy studies of the skull of archosaurs provide insights on the mechanisms of evolution for the morphologically and functionally diverse species of crocodiles and birds. One of the key attributes of skull evolution is the anatomical changes associated with the physical arrangement of cranial bones. Here, we compare the changes in anatomical organization and modularity of the skull of extinct and extant archosaurs using an Anatomical Network Analysis approach. We show that the number of bones, their topological arrangement, and modular organization can discriminate birds from non-avian dinosaurs, and crurotarsans. We could also discriminate extant taxa from extinct species when adult birds were included. By comparing within the same framework, juveniles and adults for crown birds and alligator (Alligator mississippiensis), we find that adult and juvenile alligator skulls are topologically similar, whereas juvenile bird skulls have a morphological complexity and anisomerism more similar to those of non-avian dinosaurs and crurotarsans than of their own adult forms. Clade-specific ontogenetic differences in skull organization, such as extensive postnatal fusion of cranial bones in crown birds, can explain this pattern. The fact that juvenile and adult skulls in birds do share a similar anatomical integration suggests the presence of a specific constraint to their ontogenetic growth.

Macroevolutionary integration of phenotypes within and across ant worker castes

Phenotypic traits are often integrated into evolutionary modules: sets of organismal parts that evolve together. In social insect colonies, the concepts of integration and modularity apply to sets of traits both within and among functionally and phenotypically differentiated castes. On macroevolutionary timescales, patterns of integration and modularity within and across castes can be clues to the selective and ecological factors shaping their evolution and diversification. We develop a set of hypotheses describing contrasting patterns of worker integration and apply this framework in a broad (246 species) comparative analysis of major and minor worker evolution in the hyperdiverse ant genus Pheidole. Using geometric morphometrics in a phylogenetic framework, we inferred fast and tightly integrated evolution of mesosoma shape between major and minor workers, but slower and more independent evolution of head shape between the two worker castes. Thus, Pheidole workers are evolving as a mixture of intracaste and intercaste integration and rate heterogeneity. The decoupling of homologous traits across worker castes may represent an important process facilitating the rise of social complexity.

Digit identity matters: origin and evolution of sexual dimorphism in the digit lengths of tropidurid lizards

Sexual dimorphism in digit lengths reflects phenotypic evolution mediated by developmental steroids. Differences in the identity of the sexually-dimorphic digit may evolve if the concentrations of sex-steroid receptors in the digit are easily modified and the initial changes have low impact on fitness. Accordingly, sexual dimorphism in digit lengths might initially originate under neutral selective regimes, being subsequently co-opted by embryonic hormonal effects on sensitive traits that are more likely to be targeted by selection. Correlated variation among sexually-dimorphic traits might therefore reflect pleiotropic hormonal modulation during development. Moreover, the identity and trend of the sexually-dimorphic digit might be evolutionarily labile even among closely-related species. We evaluated this model by assessing the identity of sexually-dimorphic digits among 11 Tropiduridae lizard species and inferring evolutionary patterns of sexual dimorphism. Assuming that digit lengths can be used as a proxy for hormonal modulation of steroid-sensitive traits during development, we tested for evolutionary associations among sexual dimorphism of digit lengths, body size and other traits that may be direct targets of sexual selection in the Tropiduridae. Sexual dimorphism in digit lengths is evolutionarily labile in the Tropiduridae, and diversity, instead of conservatism, seems to rule developmental mechanisms underlying the evolution of sexual dimorphism in digit ratios.

Ontogenetic changes in magnitudes of integration in the macaque skull

OBJECTIVES

Magnitudes of morphological integration may constrain or facilitate craniofacial shape variation. The aim of this study was to analyze how the magnitude of integration in the skull of Macaca fascicularis changes throughout ontogeny in relation to developmental and/or functional modules.

MATERIALS AND METHODS

Geometric morphometric methods were used to analyze the magnitude of integration in the macaque cranium and mandible in 80 juvenile and 40 adult M. fascicularis specimens. Integration scores in skull modules were calculated using integration coefficient of variation (ICV) of eigenvalues based on a resampling procedure. Resultant ICV scores between the skull as a whole, and developmental and/or functional modules were compared using Mann-Whitney U tests.

RESULTS

Results showed that most skull modules were more tightly integrated than the skull as a whole, with the exception of the chondrocranium in juveniles without canines, the chondrocranium/face complex and the mandibular corpus in adults, and the mandibular ramus in all juveniles. The chondrocranium/face and face/mandibular corpus complexes were more tightly integrated in juveniles than adults, possibly reflecting the influences of early brain growth/development, and the changing functional demands of infant suckling and later masticatory loading. This is also supported by the much higher integration of the mandibular ramus in adults compared with juveniles.

DISCUSSION

Magnitudes of integration in skull modules reflect developmental/functional mechanisms in M. fascicularis. However, the relationship between "evolutionary flexibility" and developmental/functional mechanisms was not direct or simple, likely because of the complex morphology, multifunctionality, and various ossification origins of the skull.

Ecomorphological divergence and habitat lability in the context of robust patterns of modularity in the cichlid feeding apparatus

BACKGROUND

Adaptive radiations are characterized by extreme and/or iterative phenotypic divergence; however, such variation does not accumulate evenly across an organism. Instead, it is often partitioned into sub-units, or modules, which can differentially respond to selection. While it is recognized that changing the pattern of modularity or the strength of covariation (integration) can influence the range or rate of morphological evolution, the relationship between shape variation and covariation remains unclear. For example, it is possible that rapid phenotypic change requires concomitant changes to the underlying covariance structure. Alternatively, repeated shifts between phenotypic states may be facilitated by a conserved covariance structure. Distinguishing between these scenarios will contribute to a better understanding of the factors that shape biodiversity. Here, we explore these questions using a diverse Lake Malawi cichlid species complex, Tropheops, that appears to partition habitat by depth.

RESULTS

We construct a phylogeny of Tropheops populations and use 3D geometric morphometrics to assess the shape of four bones involved in feeding (mandible, pharyngeal jaw, maxilla, pre-maxilla) in populations that inhabit deep versus shallow habitats. We next test numerous modularity hypotheses to understand whether fish at different depths are characterized by conserved or divergent patterns of modularity. We further examine rates of morphological evolution and disparity between habitats and among modules. Finally, we raise a single Tropheops species in environments mimicking deep or shallow habitats to discover whether plasticity can replicate the pattern of morphology, disparity, or modularity observed in natural populations.

CONCLUSIONS

Our data support the hypothesis that conserved patterns of modularity permit the evolution of divergent morphologies and may facilitate the repeated transitions between habitats. In addition, we find the lab-reared populations replicate many trends in the natural populations, which suggests that plasticity may be an important force in initiating depth transitions, priming the feeding apparatus for evolutionary change.

Evolutionary trends of the conserved neurocranium shape in angel sharks (Squatiniformes, Elasmobranchii)

Elasmobranchii (i.e., sharks, skates, and rays) forms one of the most diverse groups of marine predators. With a fossil record extending back into the Devonian, several modifications in their body plan illustrate their body shape diversity through time. The angel sharks, whose fossil record dates back to the Late Jurassic, some 160 Ma, have a dorsoventrally flattened body, similar to skates and rays. Fossil skeletons of this group show that the overall morphology was well established earlier in its history. By examining the skull shape of well-preserved fossil material compared to extant angel sharks using geometric morphometric methods, within a phylogenetic framework, we were able to determine the conservative skull shape among angel sharks with a high degree of integration. The morphospace occupation of extant angel sharks is rather restricted, with extensive overlap. Most of the differences in skull shape are related to their geographic distribution patterns. We found higher levels of disparity in extinct forms, but lower ones in extant species. Since angel sharks display a highly specialized prey capture behaviour, we suggest that the morphological integration and biogeographic processes are the main drivers of their diversity, which might limit their capacity to display higher disparities since their origin.

Mammal Molar Size Ratios and the Inhibitory Cascade at the Intraspecific Scale

Mammalian molar crowns form a module in which measurements of size for individual teeth within a tooth row covary with one another. Molar crown size covariation is proposed to fit the inhibitory cascade model (ICM) or its variant the molar module component (MMC) model, but the inability of the former model to fit across biological scales is a concern in the few cases where it has been tested in Primates. The ICM has thus far failed to explain patterns of intraspecific variation, an intermediate biological scale, even though it explains patterns at both smaller organ-level and larger between-species biological scales. Studies of this topic in a much broader range of taxa are needed, but the properties of a sample appropriate for testing the ICM at the intraspecific level are unclear. Here, we assess intraspecific variation in relative molar sizes of the cotton mouse, Peromyscus gossypinus, to further test the ICM and to develop recommendations for appropriate sampling protocols in future intraspecific studies of molar size variation across Mammalia. To develop these recommendations, we model the sensitivity of estimates of molar ratios to sample size and simulate the use of composite molar rows when complete ones are unavailable. Similar to past studies on primates, our results show that intraspecific variance structure of molar ratios within the rodent P. gossypinus does not meet predictions of the ICM or MMC. When we extend these analyses to include the MMC, one model does not fit observed patterns of variation better than the other. Standing variation in molar size ratios is relatively constant across mammalian samples containing all three molars. In future studies, analyzing average ratio values will require relatively small minimum sample sizes of two or more complete molar rows. Even composite-based estimates from four or more specimens per tooth position can accurately estimate mean molar ratios. Analyzing variance structure will require relatively large sample sizes of at least 40-50 complete specimens, and composite molar rows cannot accurately reconstruct variance structure of ratios in a sample. Based on these results, we propose guidelines for intraspecific studies of molar size covariation. In particular, we note that the suitability of composite specimens for averaging mean molar ratios is promising for the inclusion of isolated molars and incomplete molar rows from the fossil record in future studies of the evolution of molar modules, as long as variance structure is not a key component of such studies.

A Registration and Deep Learning Approach to Automated Landmark Detection for Geometric Morphometrics

Geometric morphometrics is the statistical analysis of landmark-based shape variation and its covariation with other variables. Over the past two decades, the gold standard of landmark data acquisition has been manual detection by a single observer. This approach has proven accurate and reliable in small-scale investigations. However, big data initiatives are increasingly common in biology and morphometrics. This requires fast, automated, and standardized data collection. We combine techniques from image registration, geometric morphometrics, and deep learning to automate and optimize anatomical landmark detection. We test our method on high-resolution, micro-computed tomography images of adult mouse skulls. To ensure generalizability, we use a morphologically diverse sample and implement fundamentally different deformable registration algorithms. Compared to landmarks derived from conventional image registration workflows, our optimized landmark data show up to a 39.1% reduction in average coordinate error and a 36.7% reduction in total distribution error. In addition, our landmark optimization produces estimates of the sample mean shape and variance-covariance structure that are statistically indistinguishable from expert manual estimates. For biological imaging datasets and morphometric research questions, our approach can eliminate the time and subjectivity of manual landmark detection whilst retaining the biological integrity of these expert annotations.

Modularity and heterochrony in the evolution of the ceratopsian dinosaur frill

The fossil record provides compelling examples of heterochrony at macroevolutionary scales such as the peramorphic giant antlers of the Irish elk. Heterochrony has also been invoked in the evolution of the distinctive cranial frill of ceratopsian dinosaurs such as Triceratops. Although ceratopsian frills vary in size, shape, and ornamentation, quantitative analyses that would allow for testing hypotheses of heterochrony are lacking. Here, we use geometric morphometrics to examine frill shape variation across ceratopsian diversity and within four species preserving growth series. We then test whether the frill constitutes an evolvable module both across and within species, and compare growth trajectories of taxa with ontogenetic growth series to identify heterochronic processes. Evolution of the ceratopsian frill consisted primarily of progressive expansion of its caudal and caudolateral margins, with morphospace occupation following taxonomic groups. Although taphonomic distortion represents a complicating factor, our data support modularity both across and within species. Peramorphosis played an important role in frill evolution, with acceleration operating early in neoceratopsian evolution followed by progenesis in later diverging cornosaurian ceratopsians. Peramorphic evolution of the ceratopsian frill may have been facilitated by the decoupling of this structure from the jaw musculature, an inference that predicts an expansion of morphospace occupation and higher evolutionary rates among ceratopsids as indeed borne out by our data. However, denser sampling of the meager record of early-diverging taxa is required to test this further.

Craniodental and Postcranial Characters of Non-Avian Dinosauria Often Imply Different Trees

Despite the increasing importance of molecular sequence data, morphology still makes an important contribution to resolving the phylogeny of many groups, and is the only source of data for most fossils. Most systematists sample morphological characters as broadly as possible on the principle of total evidence. However, it is not uncommon for sampling to be focused on particular aspects of anatomy, either because characters therein are believed to be more informative, or because preservation biases restrict what is available. Empirically, the optimal trees from partitions of morphological data sets often represent significantly different hypotheses of relationships. Previous work on hard-part versus soft-part characters across animal phyla revealed significant differences in about a half of sampled studies. Similarly, studies of the craniodental versus postcranial characters of vertebrates revealed significantly different trees in about one-third of cases, with the highest rates observed in non-avian dinosaurs. We test whether this is a generality here with a much larger sample of 81 published data matrices across all major dinosaur groups. Using the incongruence length difference test and two variants of the incongruence relationship difference test, we found significant incongruence in about 50% of cases. Incongruence is not uniformly distributed across major dinosaur clades, being highest (63%) in Theropoda and lowest (25%) in Thyreophora. As in previous studies, our partition tests show some sensitivity to matrix dimensions and the amount and distribution of missing entries. Levels of homoplasy and retained synapomorphy are similar between partitions, such that incongruence must partly reflect differences in patterns of homoplasy between partitions, which may itself be a function of modularity and mosaic evolution. Finally, we implement new tests to determine which partition yields trees most similar to those from the entire matrix. Despite no bias across dinosaurs overall, there are striking differences between major groups. The craniodental characters of Ornithischia and the postcranial characters of Saurischia yield trees most similar to the "total evidence" trees derived from the entire matrix. Trees from these same character partitions also tend to be most stratigraphically congruent: a mutual consilience suggesting that those partitions yield more accurate trees. [Dinosauria; homoplasy; partition homogeneity.].

Genomic Perspective on Multivariate Variation, Pleiotropy, and Evolution

Multivariate quantitative genetics provides a powerful framework for understanding patterns and processes of phenotypic evolution. Quantitative genetics parameters, like trait heritability or the G-matrix for sets of traits, can be used to predict evolutionary response or to understand the evolutionary history of a population. These population-level approaches have proven to be extremely successful, but the underlying genetics of multivariate variation and evolutionary change typically remain a black box. Establishing a deeper empirical understanding of how individual genetic effects lead to genetic (co)variation is then crucial to our understanding of the evolutionary process. To delve into this black box, we exploit an experimental population of mice composed from lineages derived by artificial selection. We develop an approach to estimate the multivariate effect of loci and characterize these vectors of effects in terms of their magnitude and alignment with the direction of evolutionary divergence. Using these estimates, we reconstruct the traits in the ancestral populations and quantify how much of the divergence is due to genetic effects. Finally, we also use these vectors to decompose patterns of genetic covariation and examine the relationship between these components and the corresponding distribution of pleiotropic effects. We find that additive effects are much larger than dominance effects and are more closely aligned with the direction of selection and divergence, with larger effects being more aligned than smaller effects. Pleiotropic effects are highly variable but are, on average, modular. These results are consistent with pleiotropy being partly shaped by selection while reflecting underlying developmental constraints.

Evolution of the Mammalian Neck from Developmental, Morpho-Functional, and Paleontological Perspectives

The mammalian neck adopts a variety of postures during daily life and generates numerous head trajectories. Despite its functional diversity, the neck is constrained to seven cervical vertebrae in (almost) all mammals. Given this low number, an unexpectedly high degree of modularity of the mammalian neck has more recently been uncovered. This work aims to review neck modularity in mammals from a developmental, morpho-functional, and paleontological perspective and how high functional diversity evolved in the mammalian neck after the occurrence of meristic limitations. The fixed number of cervical vertebrae and the developmental modularity of the mammalian neck are closely linked to anterior Hox genes expression and strong developmental integration between the neck and other body regions. In addition, basic neck biomechanics promote morpho-functional modularity due to preferred motion axes in the cranio-cervical and cervico-thoracic junction. These developmental and biomechanical determinants result in the characteristic and highly conserved shape variation among the vertebrae that delimits morphological modules. The step-wise acquisition of these unique cervical traits can be traced in the fossil record. The increasing functional specialization of neck modules, however, did not evolve all at once but started much earlier in the upper than in the lower neck. Overall, the strongly conserved modularity in the mammalian neck represents an evolutionary trade-off between the meristic constraints and functional diversity. Although a morpho-functional partition of the neck is common among amniotes, the degree of modularity and the way neck disparity is realized is unique in mammals.

Divergence in Plant Traits and Increased Modularity Underlie Repeated Transitions Between Low and High Elevations in the Andean Genus Leucheria

Understanding why some plant lineages move from one climatic region to another is a mayor goal of evolutionary biology. In the southern Andes plant lineages that have migrated along mountain ranges tracking cold-humid climates coexist with lineages that have shifted repeatedly between warm-arid at low elevations and cold habitats at high elevations. Transitions between habitats might be facilitated by the acquisition of common traits favoring a resource-conservative strategy that copes with drought resulting from either low precipitation or extreme cold. Alternatively, transitions might be accompanied by phenotypic divergence and accelerated evolution of plant traits, which in turn may depend on the level of coordination among them. Reduced integration and evolution of traits in modules are expected to increase evolutionary rates of traits, allowing diversification in contrasting climates. To examine these hypotheses, we conducted a comparative study in the herbaceous genus Leucheria. We reconstructed ancestral habitat states using Maximum Likelihood and a previously published phylogeny. We performed a Phylogenetic Principal Components Analysis on traits, and then we tested the relationship between PC axes, habitat and climate using Phylogenetic Generalized Least Squares (PGLS). Finally, we compared the evolutionary rates of traits, and the levels of modularity among the three main Clades of Leucheria. Our results suggest that the genus originated at high elevations and later repeatedly colonized arid-semiarid shrublands and humid-forest at lower elevations. PGLS analysis suggested that transitions between habitats were accompanied by shifts in plant strategies: cold habitats at high elevations favored the evolution of traits related to a conservative-resource strategy (thicker and dissected leaves, with high mass per area, and high biomass allocation to roots), whereas warm-arid habitats at lower elevations favored traits related to an acquisitive-resource strategy. As expected, we detected higher levels of modularity in the clades that switched repeatedly between habitats, but higher modularity was not associated with accelerated rates of trait evolution.

Ontogenetic shape changes in the pelvis of the Greater Rhea (Aves, Palaeognathae) and their relationships with cursorial locomotion: a geometric morphometric approach

Knowledge of the ontogenetic pattern of morphological features is essential to improve biological interpretations. The study of morphological features of the pelvic girdle and hind limb apparatus throughout growth is an excellent approach to understand how the skeletal morphology and muscles are interrelated during growth in a bird with a specialized mode of locomotion. The Greater Rhea (Rhea americana) is a large cursorial palaeognathous bird with long legs and powerful musculature. The postnatal shape changes of the pelvis of this bird were studied with geometric morphometric techniques, using landmarks and semilandmarks. In addition, regression analyses were used to explore the association between pelvic shape changes with muscle and body mass. The pelvises of 16 specimens of Rhea americana from 1 month old to adulthood were studied in dorsal and lateral views. Noticeable differences in pelvic shape were noted between ages, particularly in lateral view. In young birds, the pre- and post-acetabular ilium was subequal in length, whereas in adults the pre-acetabular ilium became shorter. In dorsal view, the main shape changes observed were the progressive thinning of both ilium portions and the elongation of the vertex craniolateralis ilii from chicks to adulthood. In this view, the only clear differentiation was between young and adult birds. Shape differences were influenced by body mass and pelvic muscles; the post-acetabular muscle mass explained the highest percentage of the variation. The specialized locomotion of Greater Rhea is reflected in their pelvic musculoskeletal system, in which the change to a longer post-acetabular ilium correlates with the growth of the powerful post-acetabular muscles. The actions of these muscles provide the necessary strength to support the body mass, minimize the body swinging movements and propel the body forward during locomotion. Bone morphology is affected by the forces produced by body mass and the muscle activity, demonstrating the presence of common growth mechanisms, which are primordial and gave rise to a functional and properly proportioned adult.

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

The spectacular diversity of insect male genitalia, and their relative insensitivity to the environment, have long puzzled evolutionary biologists and taxonomists. We asked whether the unusual evolvability of male genitalia could be associated with low morphological integration of genitalic traits, by comparison with male somatic traits and female traits. We also asked whether this pattern was robust to variation in resource availability during development, which affects adult condition. To address these questions, we manipulated larval diet quality in a split-brood design and compared levels of integration of male and female genitalic and somatic traits in the neriid fly, Telostylinus angusticollis. We found that male genitalic traits were substantially less integrated than male somatic traits, and less integrated than female genitalic traits. Female genitalic traits were also less integrated than female somatic traits, but the difference was less pronounced than in males. However, integration of male genitalic traits was negatively condition-dependent, with high-condition males exhibiting lower trait integration than low-condition males. Finally, genitalic traits exhibited lower larval diet × family interactions than somatic traits. These results could help explain the unusually high evolvability of male genitalic traits in insects.

The Invariant Nature of a Morphological Character and Character State: Insights from Gene Regulatory Networks

What constitutes a discrete morphological character versus character state has been long discussed in the systematics literature but the consensus on this issue is still missing. Different methods of classifying organismal features into characters and character states (CCSs) can dramatically affect the results of phylogenetic analyses. Here, I show that, in the framework of Markov models, the modular structure of the gene regulatory network (GRN) underlying trait development, and the hierarchical nature of GRN evolution, essentially remove the distinction between morphological CCS, thus endowing the CCS with an invariant property with respect to each other. This property allows the states of one character to be represented as several individual characters and vice versa. In practice, this means that a phenotype can be encoded using a set of characters or just one complex character with numerous states. The representation of a phenotype using one complex character can be implemented in Markov models of trait evolution by properly structuring transition rate matrix.

Copulatory function and development shape modular architecture of genitalia differently in males and females

Genitalia are multitasking structures whose development is mediated by numerous regulatory pathways. This multifactorial nature provides an avenue for multiple sources of selection. As a result, genitalia tend to evolve as modular systems comprising semi-independent subsets of structures, yet the processes that give rise to those patterns are still poorly understood. Here, we ask what are the relative roles of development and function in shaping modular patterns of genitalia within populations and across species of stink-bugs. We found that male genitalia are less integrated, more modular, and primarily shaped by functional demands. In contrast, females show higher integration, lower modularity, and a predominant role of developmental processes. Further, interactions among parts of each sex are more determinant to modularity than those between the sexes, and patterns of modularity are equivalent between and within species. Our results strongly indicate that genitalia have been subjected to sex-specific selection, although male and female genitalia are homologous and functionally associated. Moreover, modular patterns are seemingly constant in the evolutionary history of stink-bugs, suggesting a scenario of multivariate stabilizing selection within each sex. Our study demonstrates that interactions among genital parts of the same sex may be more fundamental to genital evolution than previously thought.

Cranial integration in the fire salamander, Salamandra salamandra (Caudata: Salamandridae)

Phenotypic integration and modularity are concepts that represent the pattern of connectivity of morphological structures within an organism. Integration describes the coordinated variation of traits, and analyses of these relationships among traits often reveals the presence of modules, sets of traits that are highly integrated but relatively independent of other traits. Phenotypic integration and modularity have been studied at both the evolutionary and static level across a variety of clades, although most studies thus far are focused on amniotes, and especially mammals. Using a high-dimensional geometric morphometric approach, we investigated the pattern of cranial integration and modularity of the Italian fire salamander (Salamandra salamandra giglioli). We recovered a highly modular pattern, but this pattern did not support either entirely developmental or functional hypotheses of cranial organisation, possibly reflecting complex interactions amongst multiple influencing factors. We found that size had no significant effect on cranial shape, and that morphological variance of individual modules had no significant relationship with degree of within-module integration. The pattern of cranial integration in the fire salamander is similar to that previously recovered for caecilians, with highly integrated jaw suspensorium and occipital regions, suggesting possible conservation of patterns across lissamphibians.

Evolution of a conspicuous melanin-based ornament in gulls Laridae

Melanin- and carotenoid-based ornaments often signal different aspects of individual quality or similar components of quality under different environmental conditions and, thus, they may become evolutionarily integrated into a composite sexual trait. On the other hand, functionally and developmentally different characters (e.g. coloration characters of different developmental origin) are more likely to evolve independently from each other than more similar traits. Here, we examined evolutionary correlations between the occurrence of a conspicuous melanin-based ornament (hood) and carotenoid-based bare-part ornaments within gull family. We also aimed to identify major ecological, life-history and biogeographical predictors of hood occurrence and reconstruct evolutionary history of this ornament. We found that hood occurrence was associated with red or dark coloration of unfeathered traits (bill and legs), whereas combinations of hood with yellow carotenoid-based coloration of integument were evolutionarily avoided. Also, hood occurrence correlated negatively with the occurrence of other melanin-based plumage character (mantle). Breeding latitude and habitat were identified as major predictors of hood occurrence in gulls, as hoods were recorded more frequently in low-latitude and inland (rather than marine) species. Finally, our analysis provided support for evolutionary lability in hood occurrence, with a dominance of transitions towards hood loss in the evolutionary history of gulls. The results of our study provide one of the first evidence for a correlated evolution of melanin- and carotenoid-based ornaments in an avian lineage, which supports evolutionary modularity of developmentally and functionally different coloration traits.

Morphological consequences of artificial cranial deformation: Modularity and integration

The cranium is an anatomically complex structure. One source of its complexity is due to its modular organization. Cranial modules are distinct and partially independent units that interact substantially during ontogeny thus generating morphological integration. Artificial Cranial Deformation (ACD) occurs when the human skull is intentionally deformed, through the use of different deforming devices applied to the head while it is developing. Hence, ACD provides an interesting example to assess the degree to which biomechanical perturbations of the developing neurocranium impact on the degree of morphological integration in the skull as a whole. The main objective of this study was to assess how ACD affects the morphological integration of the skull. This was accomplished by comparing a sample of non-deformed crania and two sets of deformed crania (i.e. antero-posterior and oblique). Both developmental and static modularity and integration were assessed through Generalized Procrustes Analysis by considering the symmetric and asymmetric components of variation in adults, using 3D landmark coordinates as raw data. The presence of two developmental modules (i.e. viscero and neurocranium) in the skull was tested. Then, in order to understand how ACD affects morphological integration, the covariation pattern between the neuro and viscerocranium was examined in antero-posterior, oblique and non-deformed cranial categories using Partial Least-Squares. The main objective of this study was to assess how ACD affects the morphological integration of the skull. This was accomplished by comparing a sample of deformed (i.e. antero-posterior and oblique) and non-deformed crania. Hence, differences in integration patterns were compared between groups. The obtained results support the modular organization of the human skull in the two analyzed modules. The integration analyses show that the oblique ACD style differentially affects the static morphological integration of the skull by increasing the covariance between neuro and viscerocranium in a more constrained way than in antero-posterior and non-deformed skulls. In addition, the antero-posterior ACD style seems to affect the developmental integration of the skull by directing the covariation pattern in a more defined manner as compared to the other cranial categories.

Ecology and Evolution of Intraspecific Chemodiversity of Plants

An extraordinarily high intraspecific chemical diversity, i.e. chemodiversity, has been found in several plant species, of which some are of major ecological or economic relevance. Moreover, even within an individual plant there is substantial chemodiversity among tissues and across seasons. This chemodiversity likely has pronounced ecological effects on plant mutualists and antagonists, associated foodwebs and, ultimately, biodiversity. Surprisingly, studies on interactions between plants and their herbivores or pollinators often neglect plant chemistry as a level of diversity and phenotypic variation. The main aim of this Research Unit (RU) is to understand the emergence and maintenance of intraspecific chemodiversity in plants. We address the following central questions:

1) How does plant chemodiversity vary across levels, i.e., within individuals, among individuals within populations, and among populations?

2) What are the ecological consequences of intraspecific plant chemodiversity?

3) How is plant chemodiversity genetically determined and maintained?

By combining field and laboratory studies with metabolomics, transcriptomics, genetic tools, statistical data analysis and modelling, we aim to understand causes and consequences of plant chemodiversity and elucidate its impacts on the interactions of plants with their biotic environment. Furthermore, we want to identify general principles, which hold across different species, and develop meaningful measures to describe the fascinating diversity of defence chemicals in plants. These tasks require integrated scientific collaboration of experts in experimental and theoretical ecology, including chemical and molecular ecology, (bio)chemistry and evolution.

Burrowing below ground: interaction between soil mechanics and evolution of subterranean mammals

The evolution of species is governed by complex phenomena in which biological and environmental features may interact dynamically. Subterranean mammals dig tunnels whose diameter minimizes energetic costs during excavations and display anatomical adaptations in order to burrow structurally stable tunnels according to specific features of the soil. These animals weight from less than 50 g up to 1-2 kg, and dig tunnels with diameters from 3 to 15 cm. The use of allometric laws has enabled these data to be correlated. However, since tunnels need to be stable with respect to the geomechanical characteristics of the resident soils, a mathematical treatment linking the admissible dimensions of tunnels to the environment here suggests a mechanically grounded correlation between the body mass of subterranean mammals and the maximum dimensions of tunnels. Remarkably, such theoretical findings reflect very well the empirical allometric relationship and contribute to explain the wide differences observed in body sizes of subterranean mammals. In this respect, a far from ancillary role of environmental mechanics on the morphological evolution of subterranean mammals can be hypothesized.

3D revelation of phenotypic variation, evolutionary allometry, and ancestral states of corolla shape: a case study of clade Corytholoma (subtribe Ligeriinae, family Gesneriaceae)

BACKGROUND

Quantification of corolla shape variations helps biologists to investigate plant diversity and evolution. 3D images capture the genuine structure and provide comprehensive spatial information.

RESULTS

This study applied X-ray micro-computed tomography (µCT) to acquire 3D structures of the corollas of clade Corytholoma and extracted a set of 415 3D landmarks from each specimen. By applying the geometric morphometrics (GM) to the landmarks, the first 4 principal components (PCs) in the 3D shape and 3D form analyses, respectively, accounted for 87.86% and 96.34% of the total variance. The centroid sizes of the corollas only accounted for 5.46% of the corolla shape variation, suggesting that the evolutionary allometry was weak. The 4 morphological traits corresponding to the 4 shape PCs were defined as tube curvature, lobe area, tube dilation, and lobe recurvation. Tube curvature and tube dilation were strongly associated with the pollination type and contained phylogenetic signals in clade Corytholoma. The landmarks were further used to reconstruct corolla shapes at the ancestral states.

CONCLUSIONS

With the integration of µCT imaging into GM, the proposed approach boosted the precision in quantifying corolla traits and improved the understanding of the morphological traits corresponding to the pollination type, impact of size on shape variation, and evolution of corolla shape in clade Corytholoma.

Multifunctional Structures and Multistructural Functions: Integration in the Evolution of Biomechanical Systems

Integration is an essential feature of complex biomechanical systems, with coordination and covariation occurring among and within structural components at time scales that vary from microseconds to deep evolutionary time. Integration has been suggested to both promote and constrain morphological evolution, and the effects of integration on the evolution of structure likely vary by system, clade, historical contingency, and time scale. In this introduction to the 2019 symposium "Multifunctional Structures and Multistructural Functions," we discuss the role of integration among structures in the context of functional integration and multifunctionality. We highlight articles from this issue of Integrative and Comparative Biology that explore integration within and among kinematics, sensory and motor systems, physiological systems, developmental processes, morphometric dimensions, and biomechanical functions. From these myriad examples it is clear that integration can exist at multiple levels of organization that can interact with adjacent levels to result in complex patterns of structural and functional phenotypes. We conclude with a synthesis of major themes and potential future directions, particularly with respect to using multifunctionality, itself, as a trait in evolutionary analyses.

A Morphological Integration Perspective on the Evolution of Dimorphism among Sexes and Social Insect Castes

Many species have evolved alternate phenotypes, thus enabling individuals to conditionally produce phenotypes that are favorable for reproductive success. Examples of this phenomenon include sexual dimorphism, alternative reproductive strategies, and social insect castes. While the evolutionary functions and developmental mechanisms of dimorphic phenotypes have been studied extensively, little attention has focused on the evolutionary covariance between each phenotype. We extend the conceptual framework and methods of morphological integration to hypothesize that dimorphic traits tend to be less integrated between sexes or social castes. In the case of social insects, we describe results from our recent study of an ant genus in which workers have major and minor worker castes that perform different behavioral repertoires in and around the nest. In the case of birds, we describe a new analysis of a family of songbirds that exhibits plumage coloration that can differ greatly between males and females, with apparently independent changes in each sex. Ant head shape, which is highly specialized in each worker caste, was weakly integrated between worker castes, whereas thorax shape, which is more monomorphic, was tightly integrated. Similarly, in birds, we found a negative association between dimorphism and the degree of integration between sexes. We also found that integration decreased in fairy wrens (Malurus) for many feather patches that evolved greater dichromatism. Together, this suggests that the process of evolving increased dimorphism results in a decrease in integration between sexes and social castes. We speculate that once a mechanism for dimorphism evolves, that mechanism can create independent variation in one sex or caste upon which selection may act.

Evolutionary Integration and Modularity in the Archosaur Cranium

Complex structures, like the vertebrate skull, are composed of numerous elements or traits that must develop and evolve in a coordinated manner to achieve multiple functions. The strength of association among phenotypic traits (i.e., integration), and their organization into highly-correlated, semi-independent subunits termed modules, is a result of the pleiotropic and genetic correlations that generate traits. As such, patterns of integration and modularity are thought to be key factors constraining or facilitating the evolution of phenotypic disparity by influencing the patterns of variation upon which selection can act. It is often hypothesized that selection can reshape patterns of integration, parceling single structures into multiple modules or merging ancestrally semi-independent traits into a strongly correlated unit. However, evolutionary shifts in patterns of trait integration are seldom assessed in a unified quantitative framework. Here, we quantify patterns of evolutionary integration among regions of the archosaur skull to investigate whether patterns of cranial integration are conserved or variable across this diverse group. Using high-dimensional geometric morphometric data from 3D surface scans and computed tomography scans of modern birds (n = 352), fossil non-avian dinosaurs (n = 27), and modern and fossil mesoeucrocodylians (n = 38), we demonstrate that some aspects of cranial integration are conserved across these taxonomic groups, despite their major differences in cranial form, function, and development. All three groups are highly modular and consistently exhibit high integration within the occipital region. However, there are also substantial divergences in correlation patterns. Birds uniquely exhibit high correlation between the pterygoid and quadrate, components of the cranial kinesis apparatus, whereas the non-avian dinosaur quadrate is more closely associated with the jugal and quadratojugal. Mesoeucrocodylians exhibit a slightly more integrated facial skeleton overall than the other grades. Overall, patterns of trait integration are shown to be stable among archosaurs, which is surprising given the cranial diversity exhibited by the clade. At the same time, evolutionary innovations such as cranial kinesis that reorganize the structure and function of complex traits can result in modifications of trait correlations and modularity.

Evolution of a multifunctional trait: shared effects of foraging ecology and thermoregulation on beak morphology, with consequences for song evolution

While morphological traits are often associated with multiple functions, it remains unclear how evolution balances the selective effects of different functions. Birds' beaks function not only in foraging but also in thermoregulating and singing, among other behaviours. Studies of beak evolution abound, however, most focus on a single function. Hence, we quantified relative contributions of different functions over an evolutionary timescale. We measured beak shape using geometric morphometrics and compared this trait with foraging behaviour, climatic variables and song characteristics in a phylogenetic comparative study of an Australasian radiation of songbirds (Meliphagidae). We found that both climate and foraging behaviour were significantly correlated with the beak shape and size. However, foraging ecology had a greater effect on shape, and climate had a nearly equal effect on size. We also found that evolutionary changes in beak morphology had significant consequences for vocal performance: species with elongate-shaped beaks sang at higher frequencies, while species with large beaks sang at a slower pace. The evolution of the avian beak exemplifies how morphological traits can be an evolutionary compromise among functions, and suggests that specialization along any functional axis may increase ecological divergence or reproductive isolation along others.

Caste-specific morphological modularity in the ant tribe Camponotini (Hymenoptera, Formicidae)

Background

The morphological structures of organisms form tightly integrated but mutually independent character complexes (modules) linked through common development and function. Even though their abundance, diversity, and complex caste systems make camponotine ants ideal subjects to research developmental modularity and phenotypic integration, no studies investigating these phenomena have been conducted in this taxonomic group. This study attempts to identify and visualize integrated character complexes in 14 taxa from the genera Camponotus and Colobopsis using statistical analyses of morphometry.

Results

The identified modules differ between castes: Minor workers have small heads and long extremities, while major workers have enlarged heads modified for defence, and short, thick appendages; extremities (legs and antennae) are strongly correlated in both worker castes. Gynes show weaker integration of extremities, but a strong correlation of mesosoma and eyes, and highly variable median ocellus size. Gynes infested by mermithid nematodes exhibit reduction of gyne-specific characters and altered patterns of phenotypic integration.

Conclusion

The integrated character complexes described herein can largely be interpreted as functional, caste-specific modules related to behavioural ecology and task allocation within ant colonies. This modular nature of the body plan is hypothesized to facilitate the evolution of novel phenotypes and thus contributes to the tremendous evolutionary success of ants. The study of these modules can help to further elucidate the evolution and ontogeny of castes in camponotine ants, as well as the effects of parasite infestation on the phenotype.