Nathan M. Young

Deciphering the Palimpsest: Studying the Relationship Between Morphological Integration and Phenotypic Covariation

Organisms represent a complex arrangement of anatomical structures and individuated parts that must maintain functional associations through development. This integration of variation between functionally related body parts and the modular organization of development are fundamental determinants of their evolvability. This is because integration results in the expression of coordinated variation that can create preferred directions for evolutionary change, while modularity enables variation in a group of traits or regions to accumulate without deleterious effects on other aspects of the organism. Using our own work on both model systems (e.g., lab mice, avians) and natural populations of rodents and primates, we explore in this paper the relationship between patterns of phenotypic covariation and the developmental determinants of integration that those patterns are assumed to reflect. We show that integration cannot be reliably studied through phenotypic covariance patterns alone and argue that the relationship between phenotypic covariation and integration is obscured in two ways. One is the superimposition of multiple determinants of covariance in complex systems and the other is the dependence of covariation structure on variances in covariance-generating processes. As a consequence, we argue that the direct study of the developmental determinants of integration in model systems is necessary to fully interpret patterns of covariation in natural populations, to link covariation patterns to the processes that generate them, and to understand their significance for evolutionary explanation.

Development and the evolvability of human limbs

The long legs and short arms of humans are distinctive for a primate, the result of selection acting in opposite directions on each limb at different points in our evolutionary history. This mosaic pattern challenges our understanding of the relationship of development and evolvability because limbs are serially homologous and genetic correlations should act as a significant constraint on their independent evolution. Here we test a developmental model of limb covariation in anthropoid primates and demonstrate that both humans and apes exhibit significantly reduced integration between limbs when compared to quadrupedal monkeys. This result indicates that fossil hominins likely escaped constraints on independent limb variation via reductions to genetic pleiotropy in an ape-like last common ancestor (LCA). This critical change in integration among hominoids, which is reflected in macroevolutionary differences in the disparity between limb lengths, facilitated selection for modern human limb proportions and demonstrates how development helps shape evolutionary change.

Quantitative analyses link modulation of sonic hedgehog signaling to continuous variation in facial growth and shape

Variation is an intrinsic feature of biological systems, yet developmental biology does not frequently address population-level phenomena. Sonic hedgehog (SHH) signaling activity in the vertebrate forebrain and face is thought to contribute to continuous variation in the morphology of the upper jaw, but despite its potential explanatory power, this idea has never been quantitatively assessed. Here, we test this hypothesis with an experimental design that is explicitly focused on the generation and measurement of variation in multivariate shape, tissue growth, cellular behavior and gene expression. We show that the majority of upper jaw shape variation can be explained by progressive changes in the spatial organization and mitotic activity of midfacial growth zones controlled by SHH signaling. In addition, nonlinearity between our treatment doses and phenotypic outcomes suggests that threshold effects in SHH signaling may play a role in variability in midfacial malformations such as holoprosencephaly (HPE). Together, these results provide novel insight into the generation of facial morphology, and demonstrate the value of quantifying variation for our understanding of development and disease.

Mechanisms that underlie co-variation of the brain and face.

The effect of the brain on the morphology of the face has long been recognized in both evolutionary biology and clinical medicine. In this work, we describe factors that are active between the development of the brain and face and how these might impact craniofacial variation. First, there is the physical influence of the brain, which contributes to overall growth and morphology of the face through direct structural interactions. Second, there is the molecular influence of the brain, which signals to facial tissues to establish signaling centers that regulate patterned growth. Importantly, subtle alterations to these physical or molecular interactions may contribute to both normal and abnormal variation. These interactions are therefore critical to our understanding of how a diversity of facial morphologies can be generated both within species and across evolutionary time. genesis 49:177–189, 2011.

Function, ontogeny and canalization of shape variance in the primate scapula

Primates have shoulders adapted to a wide range of locomotor functions from terrestrial pronograde quadrupedalism to highly arboreal suspensory behaviours. The shape of the scapula tightly follows these functional differences. Previous analyses of primate postcrania, including the scapula, indicate that quadrupedal monkeys are less variable than non‐quadrupeds. It was previously suggested that this difference was due to a relationship between the strength of stabilizing selection and the functional demands of the upper limb. Here it is shown that intraspecific scapular shape variance is highly correlated with the degree of committed quadrupedalism. Primates that engage in frequent suspensory behaviours (e.g. apes and ateline monkeys) average twice the amount of shape variance as quadrupeds (e.g. Old World monkeys and Saimiri). Because this difference in intraspecific shape variance is apparent in infants and does not increase or decrease appreciably over ontogeny, it is not likely that differences in postnatal growth, neuromuscular control or environmental factors such as habitat structure/composition are the primary contributors to differences in adult shape variance. Instead variance in embryonic factors that affect the shape/size of the scapula or epigenetic factors associated with muscle attachments are more likely candidates. In particular, the heterogeneous functional demands of the non‐quadrupedal shoulder probably reduce the stringency of stabilizing selection, resulting in the persistence into adulthood of increased amounts of embryonically generated scapular shape variance.

Phenotypic Variability: Its Components, Measurement and Underlying Developmental Processes

Variability contrasts with variation in that variability describes the potential for variation, not simply the expressed variation. The power of studying variability lies in creating a conceptual framework around which the relationship between the genotype and phenotype can be understood. Here, we attempt to demonstrate the importance of phenotypic variability, how it structures variation, and how fundamental developmental processes structure variability. Given the broad scope of this topic, we focus on three widely studied properties of variability: canalization, developmental stability and morphological integration. We have organized the paper to emphasize the importance of differentiating between the theory surrounding these components of phenotypic variability, their measurement and the biological factors surrounding their expression. First, we define these properties of variability, how they relate to each other and to variability as a whole. Second, we summarize the common methods of measurement for canalization, developmental stability and morphological integration and the reasoning behind these methods. Finally, we focus on jaw development as an example of how the basic processes of development affect variability and the resultant variation, with emphasis on how processes at all levels of the organismal hierarchy interact with one another and contribute to phenotypic variability.

Embryonic bauplans and the developmental origins of facial diversity and constraint

A central issue in biology concerns the presence, timing and nature of phylotypic periods of development, but whether, when and why species exhibit conserved morphologies remains unresolved. Here, we construct a developmental morphospace to show that amniote faces share a period of reduced shape variance and convergent growth trajectories from prominence formation through fusion, after which phenotypic diversity sharply increases. We predict in silico the phenotypic outcomes of unoccupied morphospaces and experimentally validate in vivo that observed convergence is not due to developmental limits on variation but instead from selection against novel trajectories that result in maladaptive facial clefts. These results illustrate how epigenetic factors such as organismal geometry and shape impact facial morphogenesis and alter the locus of adaptive selection to variation in later developmental events.

A comparison of the ontogeny of shape variation in the anthropoid scapula: functional and phylogenetic signal.

This article compares ontogenetic shape variation in the scapula of 17 anthropoid species using three-dimensional landmark-based geometric morphometrics. These data are used to investigate functional and phylogenetic signal in the major components of scapular variation and to evaluate the degree to which postnatal growth contributes to interspecific differences in shape. Results indicate that the shape of the infant and adult scapula is primarily associated with positional behavior (e.g., orthograde suspensory nonquadrupeds versus pronograde quadrupeds), but within this functional structure there is phylogenetic signal, particularly at infant stages. Growth most closely correlates with infant/adult shape and locomotor function. These results suggest that the shape of the infant scapula drives the pattern of postnatal scapular growth and adult morphology. As such, variation in postnatal growth is not the primary source of interspecific variation in adult shape. Instead, interspecific differences in scapular morphology are hypothesized to be the result of selection for variation in embryonic developmental processes that affect shape.

Short‐faced mice and developmental interactions between the brain and the face

The length of the face represents an important axis of variation in mammals and especially in primates. Mice with mutations that produce variation along this axis present an opportunity to study the developmental factors that may underlie evolutionary change in facial length. The Crf4 mutant, obtained from the C57BL/6J (wt/wt) background by chemical mutagenesis by the Baylor Mouse Mutagenesis Resource, is reported to have a short‐faced phenotype. As an initial step towards developing this model, we performed 3D geometric morphometric comparisons of Crf4 mice to C57BL/6J wild‐type mice focusing on three stages of face development and morphology – embryonic (GD 9.5–12), neonatal, and adult. Morphometric analysis of adult Crf4 mutants revealed that in addition to a shortened face, these mice exhibit a significant reduction in brain size and basicranial length. These same features also differ at the neonatal stage. During embryonic face formation, only dimensions related to brain growth were smaller, whereas the Crf4 face actually appeared advanced relative to the wild‐type at the same somite stage. These results show that aspects of the Crf4 phenotype are evident as early as embryonic face formation. Based on our anatomical findings we hypothesize that the reduction in facial growth in Crf4 mice is a secondary consequence of reduction in the growth of the brain. If correct, the Crf4 mutant will be a useful model for studying the role of epigenetic interactions between the brain and face in the evolutionary developmental biology of the mammalian craniofacial complex as well as human craniofacial dysmorphology.

Canalization and developmental stability in the Brachyrrhine mouse

The semi‐dominant Br mutation affects presphenoid growth, producing the facial retrognathism and globular neurocranial vault that characterize heterozygotes. We analysed the impact of this mutation on skull shape, comparing heterozygotes to wildtype mice, to determine if the effects are skull‐wide or confined to the sphenoid region targeted by the mutation. In addition, we examined patterns of variability of shape for the skull as a whole and for three regions (basicranium, face and neurocranium). We found that the Br mice differed significantly from wildtype mice in skull shape in all three regions as well as in the shape of the skull as a whole. However, the significant increases in variance and fluctuating asymmetry were found only in the basicranium of mutant mice. These results suggest that the mutation has a significant effect on the underlying developmental architecture of the skull, which produces an increase in phenotypic variability that is localized to the anatomical region in which the mean phenotype is most dramatically affected. These results suggest that the same developmental mechanisms that produce the change in phenotypic mean also produce the change in variance.