Precise control of jaw length during development is crucial for proper form and function. Previously we have shown that in birds, neural crest mesenchyme (NCM) confers species-specific size and shape to the beak by regulating molecular and histological programs for the induction and deposition of cartilage and bone. Here we reveal that a hitherto unrecognized but similarly essential mechanism for establishing jaw length is the ability of NCM to mediate bone resorption. Osteoclasts are considered the predominant cells that resorb bone, although osteocytes have also been shown to participate in this process. In adults, bone resorption is tightly coupled to bone deposition as a means to maintain skeletal homeostasis. Yet, the role and regulation of bone resorption during growth of the embryonic skeleton have remained relatively unexplored. We compare jaw development in short-beaked quail versus long-billed duck and find that quail have substantially higher levels of enzymes expressed by bone-resorbing cells including tartrate-resistant acid phosphatase (TRAP), Matrix metalloproteinase 13 (Mmp13), and Mmp9. Then, we transplant NCM destined to form the jaw skeleton from quail to duck and generate chimeras in which osteocytes arise from quail donor NCM and osteoclasts come exclusively from the duck host. Chimeras develop quail-like jaw skeletons coincident with dramatically elevated expression of TRAP, Mmp13, and Mmp9. To test for a link between bone resorption and jaw length, we block resorption using a bisphosphonate, osteoprotegerin protein, or an MMP13 inhibitor, and this significantly lengthens the jaw. Conversely, activating resorption with RANKL protein shortens the jaw. Finally, we find that higher resorption in quail presages their relatively lower adult jaw bone mineral density (BMD) and that BMD is also NCM-mediated. Thus, our experiments suggest that NCM not only controls bone resorption by its own derivatives but also modulates the activity of mesoderm-derived osteoclasts, and in so doing enlists bone resorption as a key patterning mechanism underlying the functional morphology and evolution of the jaw.

The periodic stripes and spots that often adorn animals’ coats have been largely viewed as self-organizing patterns, forming through dynamics such as Turing’s reaction-diffusion within the developing skin. Whether preexisting positional information also contributes to the periodicity and orientation of these patterns has, however, remained unclear. We used natural variation in colored stripes of juvenile galliform birds to show that stripes form in a two-step process. Autonomous signaling from the somite sets stripe position by forming a composite prepattern marked by the expression profile of agouti. Subsequently, agouti regulates stripe width through dose-dependent control of local pigment production. These results reveal that early developmental landmarks can shape periodic patterns upstream of late local dynamics, and thus constrain their evolution.

The development of an organism involves the formation of patterns from initially homogeneous surfaces in a reproducible manner. Simulations of various theoretical models recapitulate final states of natural patterns, yet drawing testable hypotheses from those often remains difficult. Consequently, little is known about pattern-forming events. Here, we surveyed plumage patterns and their emergence in Galliformes, ratites, passerines, and penguins, together representing the three major taxa of the avian phylogeny, and built a unified model that not only reproduces final patterns but also intrinsically generates shared and varying directionality, sequence, and duration of patterning. We used in vivo and ex vivo experiments to test its parameter-based predictions. We showed that directional and sequential pattern progression depends on a species-specific prepattern: an initial break in surface symmetry launches a travelling front of sharply defined, oriented domains with self-organising capacity. This front propagates through the timely transfer of increased cell density mediated by cell proliferation, which controls overall patterning duration. These results show that universal mechanisms combining prepatterning and self-organisation govern the timely emergence of the plumage pattern in birds.

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Developing tissues can self-organize into a variety of patterned structures through the stabilization of stochastic fluctuations in their molecular and cellular properties. While molecular factors and cell dynamics contributing to self-organization have been identified in vivo, events channeling self-organized systems such that they achieve stable pattern outcomes remain unknown. Here, we described natural variation in the fidelity of self-organized arrays formed by feather follicle precursors in bird embryos. By surveying skin cells prior to and during tissue self-organization and performing species-specific ex vivo drug treatments and mechanical stress tests, we demonstrated that pattern fidelity depends on the initial amplitude of cell anisotropy in regions of the developing dermis competent to produce a pattern. Using live imaging, we showed that cell shape anisotropy is associated with a limited increase in cell motility for sharp and precisely located primordia formation, and thus, proper pattern geometry. These results evidence a mechanism through which initial tissue properties ensure stability in self-organization and thus, reproducible pattern production.

The color patterns that adorn animals’ coats not only exhibit extensive diversity linked to various ecological functions but also display recurrences in geometry, orientation, or body location. How processes of pattern formation shape such phenotypic trends remains a mystery. Here, we surveyed plumage color patterns in passerine finches displaying extreme apparent variation and identified a conserved set of color domains. We linked these domains to putative embryonic skin regions instructed by early developmental tissues and outlined by the combinatory expression of few genetic markers. We found that this embryonic prepattern is largely conserved in birds displaying drastic color differences in the adult, interspecies variation resulting from the masking or display of each domain depending on their coloration. This work showed that a simple molecular landscape serves as common spatial template to extensive color pattern variation in finches, revealing that early conserved landmarks and molecular pathways are a major cause of phenotypic trends.

SummaryCellular self-organisation can emerge from stochastic fluctuations in properties of a developing tissue1–3. This mechanism explains the production of various motifs seen in nature4–7. However, events channelling its outcomes such that patterns are produced with reproducible precision key to fitness remain unexplored. Here, we compared the dynamic emergence of feather primordia arrays in poultry, finch, emu, ostrich and penguin embryos and correlated inter-species differences in pattern fidelity to the amplitude of dermal cell anisotropy in the un-patterned tissue. Using live imaging and ex vivo perturbations in these species, we showed that cell anisotropy optimises cell motility for sharp and precisely located primordia formation, and thus, proper pattern geometry. These results evidence a mechanism through which collective cellular properties of a developmental pattern system ensure stability in its self-organisation and contribute to its evolution.

Parce qu'ils varient considérablement, les motifs de couleur périodiques qui ornent le pelage des vertébrés ont historiquement servi à étudier la formation et l'évolution des modèles biologiques. Si deux grandes stratégies de formation de motifs, à savoir la signalisation pédagogique et l'autoorganisation, ont été théorisées à partir de travaux numériques et empiriques dans des organismes modèles, l'origine, la nature et le mode d'action des facteurs qui sous-tendent ces stratégies in vivo restent flous. Pour répondre à cette question, notre laboratoire a conçu une méthode basée sur des études opportunistes de la variation naturelle des motifs périodiques du plumage. Nous avons établi un lien entre les éléments communs et variables du motif rayé observé chez les jeunes volailles et l'instruction embryonnaire précoce provenant du somite et les mécanismes tardifs dépendant de la dose qui se produisent pendant le développement de la peau. Ces résultats ont permis de réconcilier les théories sur les motifs, en montrant qu'elles se combinent dans un processus en deux étapes façonnant la variation naturelle d'un motif périodique typique.