Cranial sensory placodes are focused areas of the head ectoderm of vertebrates that contribute to the development of the cranial sense organs and their associated ganglia. Placodes have long been considered a key character of vertebrates, and their evolution is proposed to have been essential for the evolution of an active predatory lifestyle by early vertebrates. Despite their importance for understanding vertebrate origins, the evolutionary origin of placodes has remained obscure. Here, we use a panel of molecular markers from the Six, Eya, Pax, Dach, FoxI, COE and POUIV gene families to examine the tunicate Ciona intestinalis for evidence of structures homologous to vertebrate placodes. Our results identify two domains of Ciona ectoderm that are marked by the genetic cascade that regulates vertebrate placode formation. The first is just anterior to the brain, and we suggest this territory is equivalent to the olfactory/adenohypophyseal placodes of vertebrates. The second is a bilateral domain adjacent to the posterior brain and includes cells fated to form the atrium and atrial siphon of adult Ciona. We show this bares most similarity to placodes fated to form the vertebrate acoustico-lateralis system. We interpret these data as support for the hypothesis that sensory placodes did not arise de novo in vertebrates, but evolved from pre-existing specialised areas of ectoderm that contributed to sensory organs in the common ancestor of vertebrates and tunicates.

Many biological systems produce an invariant output when faced with stochastic or environmental variation. This robustness of system output to variation affecting the underlying process may allow for "cryptic" genetic evolution within the system without change in output. We studied variation of cell fate patterning of Caenorhabditis elegans vulva precursors, a developmental system that relies on a simple intercellular signaling network and yields an invariant output of cell fates and lineages among C. elegans wild isolates. We first investigated the system's genetic variation in C. elegans by means of genetic tools and cell ablation to break down its buffering mechanisms. We uncovered distinct architectures of quantitative variation along the Ras signaling cascade, including compensatory variation, and differences in cell sensitivity to induction along the anteroposterior axis. In the unperturbed system, we further found variation between isolates in spatio-temporal dynamics of Ras pathway activity, which can explain the phenotypic differences revealed upon perturbation. Finally, the variation mostly affects the signaling pathways in a tissue-specific manner. We thus demonstrate and characterize microevolution of a developmental signaling network. In addition, our results suggest that the vulva genetic screens would have yielded a different mutation spectrum, especially for Wnt pathway mutations, had they been performed in another C. elegans genetic background.[Keywords: Signaling network; evolution; C. elegans; vulva; robustness; cryptic variation] Supplemental material is available at http://www.genesdev.org.

Biological systems may perform reproducibly to generate invariant outcomes, despite external or internal noise. One example is the C. elegans vulva, in which the final cell fate pattern is remarkably robust. Although this system has been extensively studied and the molecular network underlying cell fate specification is well understood, very little is known in quantitative terms. Here, through pathway dosage modulation and single molecule fluorescence in situ hybridization, we show that the system can tolerate a 4-fold variation in genetic dose of the upstream signaling molecule LIN-3/epidermal growth factor (EGF) without phenotypic change in cell fate pattern. Furthermore, through tissue-specific dosage perturbations of the EGF and Notch pathways, we determine the first-appearing patterning errors. Finally, by combining different doses of both pathways, we explore how quantitative pathway interactions influence system behavior. Our results highlight the feasibility and significance of launching experimental studies of robustness and quantitative network analysis in genetically tractable, multicellular eukaryotes.

SUMMARY Background Biological networks experience quantitative change in response to environmental and evolutionary variation. Computational modeling allows exploration of network parameter space, corresponding to such variations. The intercellular signaling network underlying Caenorhabditis vulva development specifies three fates in a row of six precursor cells, yielding a quasi-invariant 3°-3°-2°-1°-2°-3° cell fate pattern. Two seemingly conflicting verbal models of vulval precursor cell fate specification have been proposed: sequential induction by the EGF-MAP kinase and Notch pathways, or morphogen-based induction by the former. Results To study the mechanistic and evolutionary system properties of this network, we combine experimental studies with computational modeling, using a model that keeps the network architecture constant but varies parameters. We first show that the Delta autocrine loop can play an essential role in 2° fate specification. With this autocrine loop, the same network topology can be quantitatively tuned to use in the six-cell row morphogen-based or sequential patterning mechanisms, which may act singly, cooperatively or redundantly. Moreover, different quantitative tunings of this same network can explain vulval patterning observed experimentally in C. elegans, C. briggsae, C. remanei and C. brenneri. We experimentally validate model predictions, such as interspecific differences in isolated vulva precursor cell behavior and in spatial regulation of Notch activity. Conclusions Our study illustrates how quantitative variation in the same network comprises developmental patterning modes that were considered qualitatively distinct and also accounts for evolution among closely related species.