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.

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Recent evidence suggests that ascidian pigment cells are related to neural crest-derived melanocytes of vertebrates. Using live-imaging, we determine a revised cell lineage of the pigment cells in Ciona intestinalis embryos. The neural precursors undergo successive rounds of anterior-posterior (A-P) oriented cell divisions, starting at the blastula 64-cell stage. A previously unrecognized fourth A-P oriented cell division in the pigment cell lineage leads to the generation of the post-mitotic pigment cell precursors. We provide evidence that MEK/ERK signals are required for pigment cell specification until approximately 30 minutes after the final cell division has taken place. Following each of the four A-P oriented cell divisions, ERK1/2 is differentially activated in the posterior sister cells, into which the pigment cell lineage segregates. Eph/ephrin signals are critical during the third A-P oriented cell division to spatially restrict ERK1/2 activation to the posterior daughter cell. Targeted inhibition of Eph/ephrin signals results in, at neurula stages, anterior expansion of both ERK1/2 activation and a pigment cell lineage marker and subsequently, at larval stages, supernumerary pigment cells. We discuss the implications of these findings with respect to the evolution of the vertebrate neural crest.

SUMMARYERK1/2 MAP kinase exhibits a highly dynamic activation pattern in developing embryos, which largely depends on fibroblast growth factor (FGF) signals. In ascidian embryos, FGF-dependent activation of ERK1/2 occurs differentially between sister cells during marginal zone and neural lineage patterning. Selective attenuation of FGF signals by localised ephrin/Eph signals accounts for this differential ERK activation, which controls the binary fate choice of each sibling cell pair. Here, we show that p120 Ras GTPase-activating protein (p120RasGAP) is a crucial mediator of these ephrin/Eph signals. First, inhibition of p120RasGAP has a similar effect to inhibition of ephrin/Eph function during marginal zone and neural patterning. Second, p120RasGAP acts epistatically to ephrin/Eph signals. Third, p120RasGAP physically associates with Eph3 in an ephrin-dependent manner. This study provides the first in vivo evidence that the functional association between Eph and RasGAP controls the spatial extent of FGF-activated ERK.

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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.