What is evolutionary novelty? Process versus character based definitions

Two-way exchanges between animal and plant biology, with focus on evo-devo

By definition, biology is the science of all living beings. However, horizons restricted to either plants or animals have characterized the development of life sciences well beyond the emergence of unified perspectives applying to all forms of life, such as the cell theory or the theory of evolution. Separation between botanical and zoological traditions is not destined to go extinct easily, or shortly. Disciplinary isolation is emphasized by institutional contexts such as scientific societies and their congresses, specialist journals, disciplines recognized as teaching subjects and legitimate and fundable research fields. By shaping the personal agendas of individual scientists, this has a strong impact on the development of biology. In some fields, botanical and zoological contributions have long being effectively intertwined, but in many others plant and animal biology have failed to progress beyond a marginal dialogue. Characteristically, the so-called “general biology” and the philosophy of biology are still zoocentric (and often vertebrato- or even anthropocentric). In this article, I discuss legitimacy and fruitfulness of some old lexical and conceptual exchanges between the two traditions (cell, tissue, and embryo). Finally, moving to recent developments, I compare the contributions of plant vs. animal biology to the establishment of evolutionary developmental biology. We cannot expect that stronger integration between the different strands of life sciences will soon emerge by self-organization, but highlighting this persisting imbalance between plant and animal biology will arguably foster progress.

A developmental perspective of homology and evolutionary novelty

The development and evolution of multicellular body plans is complex. Many distinct organs and body parts must be reproduced at each generation, and those that are traceable over long time scales are considered homologous. Among the most pressing and least understood phenomena in evolutionary biology is the mode by which new homologs, or "novelties" are introduced to the body plan and whether the developmental changes associated with such evolution deserve special treatment. In this chapter, we address the concepts of homology and evolutionary novelty through the lens of development. We present a series of case studies, within insects and vertebrates, from which we propose a developmental model of multicellular organ identity. With this model in hand, we make predictions regarding the developmental evolution of body plans and highlight the need for more integrative analysis of developing systems.

Phenotypes to remember: Evolutionary developmental memory capacity and robustness

There is increased awareness of the possibility of developmental memories resulting from evolutionary learning. Genetic regulatory and neural networks can be modelled by analogous formalism raising the important question of productive analogies in principles, processes and performance. We investigate the formation and persistence of various developmental memories of past phenotypes asking how the number of remembered past phenotypes scales with network size, to what extent memories stored form by Hebbian-like rules, and how robust these developmental "devo-engrams" are against networks perturbations (graceful degradation). The analogy between neural and genetic regulatory networks is not superficial in that it allows knowledge transfer between fields that used to be developed separately from each other. Known examples of spectacular phenotypic radiations could partly be accounted for in such terms.

A conceptual framework of evolutionary novelty and innovation

Since 1990 the recognition of deep homologies among metazoan developmental processes and the spread of more mechanistic approaches to developmental biology have led to a resurgence of interest in evolutionary novelty and innovation. Other evolutionary biologists have proposed central roles for behaviour and phenotypic plasticity in generating the conditions for the construction of novel morphologies, or invoked the accessibility of new regions of vast sequence spaces. These approaches contrast with more traditional emphasis on the exploitation of ecological opportunities as the primary source of novelty. This definitional cornucopia reflects differing stress placed on three attributes of novelties: their radical nature, the generation of new taxa, and ecological and evolutionary impact. Such different emphasis has led to conflating four distinct issues: the origin of novel attributes (genes, developmental processes, phenotypic characters), new functions, higher clades and the ecological impact of new structures and functions. Here I distinguish novelty (the origin of new characters, deep character transformations, or new combinations) from innovation, the ecological and evolutionary success of clades. Evidence from the fossil record of macroevolutionary lags between the origin of a novelty and its ecological success demonstrates that novelty may be decoupled from innovation, and only definitions of novelty based on radicality (rather than generativity or consequentiality) can be assessed without reference to the subsequent history of the clade to which a novelty belongs. These considerations suggest a conceptual framework for novelty and innovation, involving: (i) generation of the potential for novelty; (ii) the formation of novel attributes; (iii) refinement of novelties through adaptation; (iv) exploitation of novelties by a clade, which may coincide with a new round of ecological or environmental potentiation; followed by (v) the establishment of innovations through ecological processes. This framework recognizes that there is little empirical support for either the dominance of ecological opportunity, nor abrupt discontinuities (often caricatured as 'hopeful monsters'). This general framework may be extended to aspects of cultural and social innovation.

Evolution of the Mammalian Ear: An Evolvability Hypothesis

Encapsulated within the temporal bone and comprising the smallest elements of the vertebrate skeleton, the ear is key to multiple senses: balance, posture control, gaze stabilization, and hearing. The transformation of the primary jaw joint into the mammalian ear ossicles is one of the most iconic transitions in vertebrate evolution, but the drivers of this complex evolutionary trajectory are not fully understood. We propose a novel hypothesis: The incorporation of the bones of the primary jaw joint into the middle ear has considerably increased the genetic, regulatory, and developmental complexity of the mammalian ear. This increase in the number of genetic and developmental factors may, in turn, have increased the evolutionary degrees of freedom for independent adaptations of the different functional ear units. The simpler ear anatomy in birds and reptiles may be less susceptible to developmental instabilities and disorders than in mammals but also more constrained in its evolution. Despite the tight spatial entanglement of functional ear components, the increased "evolvability" of the mammalian ear may have contributed to the evolutionary success and adaptive diversification of mammals in the vast diversity of ecological and behavioral niches observable today. A brief literature review revealed supporting evidence for this hypothesis.

Temporal flexibility of gene regulatory network underlies a novel wing pattern in flies

Developmental genes can be coopted to generate evolutionary novelties by changing their spatial regulation. However, developmental genes seldom act independently, but rather work in a gene regulatory network (GRN). How is it possible to recruit a single gene from a whole GRN? What are the properties that allow parallel cooptions of the same genes during evolution? Here, we show that a novel engrailed gene expression underlies a novel wing color pattern in flies. We show that cooption is facilitated 1) because of GRN flexibility over development and 2) because every single gene of the GRN has its own functional time window. We suggest these two temporal properties could explain why the same gene can be independently recruited several times during evolution.

Organisms have evolved endless morphological, physiological, and behavioral novel traits during the course of evolution. Novel traits were proposed to evolve mainly by orchestration of preexisting genes. Over the past two decades, biologists have shown that cooption of gene regulatory networks (GRNs) indeed underlies numerous evolutionary novelties. However, very little is known about the actual GRN properties that allow such redeployment. Here we have investigated the generation and evolution of the complex wing pattern of the fly Samoaia leonensis. We show that the transcription factor Engrailed is recruited independently from the other players of the anterior–posterior specification network to generate a new wing pattern. We argue that partial cooption is made possible because 1) the anterior–posterior specification GRN is flexible over time in the developing wing and 2) this flexibility results from the fact that every single gene of the GRN possesses its own functional time window. We propose that the temporal flexibility of a GRN is a general prerequisite for its possible cooption during the course of evolution.

On the co-evolution of surface oxygen levels and animals

Few topics in geobiology have been as extensively debated as the role of Earth's oxygenation in controlling when and why animals emerged and diversified. All currently described animals require oxygen for at least a portion of their life cycle. Therefore, the transition to an oxygenated planet was a prerequisite for the emergence of animals. Yet, our understanding of Earth's oxygenation and the environmental requirements of animal habitability and ecological success is currently limited; estimates for the timing of the appearance of environments sufficiently oxygenated to support ecologically stable populations of animals span a wide range, from billions of years to only a few million years before animals appear in the fossil record. In this light, the extent to which oxygen played an important role in controlling when animals appeared remains a topic of debate. When animals originated and when they diversified are separate questions, meaning either one or both of these phenomena could have been decoupled from oxygenation. Here, we present views from across this interpretive spectrum-in a point-counterpoint format-regarding crucial aspects of the potential links between animals and surface oxygen levels. We highlight areas where the standard discourse on this topic requires a change of course and note that several traditional arguments in this "life versus environment" debate are poorly founded. We also identify a clear need for basic research across a range of fields to disentangle the relationships between oxygen availability and emergence and diversification of animal life.

Developing an integrated understanding of the evolution of arthropod segmentation using fossils and evo-devo

Segmentation is fundamental to the arthropod body plan. Understanding the evolutionary steps by which arthropods became segmented is being transformed by the integration of data from evolutionary developmental biology (evo-devo), Cambrian fossils that allow the stepwise acquisition of segmental characters to be traced in the arthropod stem-group, and the incorporation of fossils into an increasingly well-supported phylogenetic framework for extant arthropods based on genomic-scale datasets. Both evo-devo and palaeontology make novel predictions about the evolution of segmentation that serve as testable hypotheses for the other, complementary data source. Fossils underpin such hypotheses as arthropodization originating in a frontal appendage and then being co-opted into other segments, and segmentation of the endodermal midgut in the arthropod stem-group. Insights from development, such as tagmatization being associated with different modes of segment generation in different body regions, and a distinct patterning of the anterior head segments, are complemented by palaeontological evidence for the pattern of tagmatization during ontogeny of exceptionally preserved fossils. Fossil and developmental data together provide evidence for a short head in stem-group arthropods and the mechanism of its formation and retention. Future breakthroughs are expected from identification of molecular signatures of developmental innovations within a phylogenetic framework, and from a focus on later developmental stages to identify the differentiation of repeated units of different systems within segmental precursors.

Historical and philosophical perspectives on the study of developmental bias

Throughout the recent history of research at the intersection of evolution and development, notions such as developmental constraint, evolutionary novelty, and evolvability have been prominent, but the term "developmental bias" has scarcely been used. And one may even doubt whether a unique and principled definition of bias is possible. I argue that the concept of developmental bias can still play a vital scientific role by means of setting an explanatory agenda that motivates investigation and guides the formulation of integrative explanatory frameworks. Less crucial is a definition that would classify patterns of phenotypic variation and unify variational patterns involving different traits and taxa as all being "bias." Instead, what we should want is a concept that generates intellectual identity across various researchers, and that unites the diverse fields and approaches relevant to the study of developmental bias, from paleontology to behavioral biology. I point to some advantages of conducting research specifically under the label of "developmental bias," compared with employing other, more common terms such as "evolvability."

The evolutionary origin of digit patterning

The evolution of tetrapod limbs from paired fins has long been of interest to both evolutionary and developmental biologists. Several recent investigative tracks have converged to restructure hypotheses in this area. First, there is now general agreement that the limb skeleton is patterned by one or more Turing-type reaction–diffusion, or reaction–diffusion–adhesion, mechanism that involves the dynamical breaking of spatial symmetry. Second, experimental studies in finned vertebrates, such as catshark and zebrafish, have disclosed unexpected correspondence between the development of digits and the development of both the endoskeleton and the dermal skeleton of fins. Finally, detailed mathematical models in conjunction with analyses of the evolution of putative Turing system components have permitted formulation of scenarios for the stepwise evolutionary origin of patterning networks in the tetrapod limb. The confluence of experimental and biological physics approaches in conjunction with deepening understanding of the developmental genetics of paired fins and limbs has moved the field closer to understanding the fin-to-limb transition. We indicate challenges posed by still unresolved issues of novelty, homology, and the relation between cell differentiation and pattern formation.

Developmental push or environmental pull? The causes of macroevolutionary dynamics

Have the large-scale evolutionary patterns illustrated by the fossil record been driven by fluctuations in environmental opportunity, by biotic factors, or by changes in the types of phenotypic variants available for evolutionary change? Since the Modern Synthesis most evolutionary biologists have maintained that microevolutionary processes carrying on over sufficient time will generate macroevolutionary patterns, with no need for other pattern-generating mechanisms such as punctuated equilibrium or species selection. This view was challenged by paleontologists in the 1970s with proposals that the differential sorting and selection of species and clades, and the effects of biotic crises such as mass extinctions, were important extensions to traditional evolutionary theory. More recently those interested in macroevolution have debated the relative importance of abiotic and biotic factors in driving macroevolutionary patterns and have introduced comparative phylogenetic methods to analyze the rates of change in taxonomic diversity. Applying Peter Godfrey-Smith's distinction between distributional explanations and explanations focusing on the origin of variation, most macroevolutionary studies have provided distributional explanations of macroevolutionary patterns. Comparative studies of developmental evolution, however, have implicated the origin of variants as a driving macroevolution force. In particular, the repatterning of gene regulatory networks provides new insights into the origins of developmental novelties. This raises the question of whether macroevolution has been pulled by the generation of environmental opportunity, or pushed by the introduction of new morphologies. The contrast between distributional and origination scenarios has implications for understanding evolutionary novelty and innovation and how macroevolutionary process may have evolved over time.

Simplification, Innateness, and the Absorption of Meaning from Context: How Novelty Arises from Gradual Network Evolution

How does new genetic information arise? Traditional thinking holds that mutation happens by accident and then spreads in the population by either natural selection or random genetic drift. There have been at least two fundamental conceptual problems with imagining an alternative. First, it seemed that the only alternative is a mutation that responds "smartly" to the immediate environment; but in complex multicellulars, it is hard to imagine how this could be implemented. Second, if there were mechanisms of mutation that "knew" what genetic changes would be favored in a given environment, this would have only begged the question of how they acquired that particular knowledge to begin with. This paper offers an alternative that avoids these problems. It holds that mutational mechanisms act on information that is in the genome, based on considerations of simplicity, parsimony, elegance, etc. (which are different than fitness considerations). This simplification process, under the performance pressure exerted by selection, not only leads to the improvement of adaptations but also creates elements that have the capacity to serve in new contexts they were not originally selected for. Novelty, then, arises at the system level from emergent interactions between such elements. Thus, mechanistically driven mutation neither requires Lamarckian transmission nor closes the door on novelty, because the changes it implements interact with one another globally in surprising and beneficial ways. Finally, I argue, for example, that genes used together are fused together; that simplification leads to complexity; and that evolution and learning are conceptually linked.

The cephalopod arm crown: appendage formation and differentiation in the Hawaiian bobtail squid Euprymna scolopes

BACKGROUND

Cephalopods are a highly derived class of molluscs that adapted their body plan to a more active and predatory lifestyle. One intriguing adaptation is the modification of the ventral foot to form a bilaterally symmetric arm crown, which constitutes a true morphological novelty in evolution. In addition, this structure shows many diversifications within the class of cephalopods and therefore offers an interesting opportunity to study the molecular underpinnings of the emergence of phenotypic novelties and their diversification. Here we use the sepiolid Euprymna scolopes as a model to study the formation and differentiation of the decabrachian arm crown, which consists of four pairs of sessile arms and one pair of retractile tentacles. We provide a detailed description of arm crown formation in order to understand the basic morphology and the developmental dynamics of this structure.

RESULTS

We show that the morphological formation of the cephalopod appendages occurs during distinct phases, including outgrowth, elongation, and tissue differentiation. Early outgrowth is characterized by uniform cell proliferation, while the elongation of the appendages initiates tissue differentiation. The latter progresses in a gradient from proximal to distal, whereas cell proliferation becomes restricted to the distal-most end of the arm. Differences in the formation of arms and tentacles exist, with the tentacles showing an expedite growth rate and higher complexity at younger stages.

CONCLUSION

The early outgrowth and differentiation of the E. scolopes arm crown shows similarities to the related, yet derived cephalopod Octopus vulgaris. Parallels in the growth and differentiation of appendages seem to exist throughout the animal kingdom, raising the question of whether these similarities reflect a recruitment of similar molecular patterning pathways.

Novelty and Innovation in the History of Life

The history of life as documented by the fossil record encompasses evolutionary diversifications at scales ranging from the Ediacaran-Cambrian explosion of animal life and the invasion of land by vascular plants, insects and vertebrates to the diversification of flowering plants over the past 100 million years and the radiation of horses. Morphological novelty and innovation has been a recurrent theme. The architects of the modern synthesis of evolutionary theory made three claims about evolutionary novelty and innovation: first, that all diversifications in the history of life represent adaptive radiations; second, that adaptive radiations are driven principally by ecological opportunity rather than by the supply of new morphological novelties, thus the primary questions about novelty and innovation focus on their ecological and evolutionary success; and third, that the rate of morphological divergence between taxa was more rapid early in the history of a clade but slowed over time as ecological opportunities declined. These claims have strongly influenced subsequent generations of evolutionary biologists, yet over the past two decades each has been challenged by data from the fossil record, by the results of comparative phylogenetic analyses and through insights from evolutionary developmental biology. Consequently a broader view of novelty and innovation is required. An outstanding issue for future work is identifying the circumstances associated with different styles of diversification and whether their frequency has changed through the history of life.

Do novel genes drive morphological novelty? An investigation of the nematosomes in the sea anemone Nematostella vectensis

BACKGROUND

The evolution of novel genes is thought to be a critical component of morphological innovation but few studies have explicitly examined the contribution of novel genes to the evolution of novel tissues. Nematosomes, the free-floating cellular masses that circulate through the body cavity of the sea anemone Nematostella vectensis, are the defining apomorphy of the genus Nematostella and are a useful model for understanding the evolution of novel tissues. Although many hypotheses have been proposed, the function of nematosomes is unknown. To gain insight into their putative function and to test hypotheses about the role of lineage-specific genes in the evolution of novel structures, we have re-examined the cellular and molecular biology of nematosomes.

RESULTS

Using behavioral assays, we demonstrate that nematosomes are capable of immobilizing live brine shrimp (Artemia salina) by discharging their abundant cnidocytes. Additionally, the ability of nematosomes to engulf fluorescently labeled bacteria (E. coli) reveals the presence of phagocytes in this tissue. Using RNA-Seq, we show that the gene expression profile of nematosomes is distinct from that of the tentacles and the mesenteries (their tissue of origin) and, further, that nematosomes (a Nematostella-specific tissue) are enriched in Nematostella-specific genes.

CONCLUSIONS

Despite the small number of cell types they contain, nematosomes are distinct among tissues, both functionally and molecularly. We provide the first evidence that nematosomes comprise part of the innate immune system in N. vectensis, and suggest that this tissue is potentially an important place to look for genes associated with pathogen stress. Finally, we demonstrate that Nematostella-specific genes comprise a significant proportion of the differentially expressed genes in all three of the tissues we examined and may play an important role in novel cell functions.

Phenotypic Novelty in EvoDevo: The Distinction Between Continuous and Discontinuous Variation and Its Importance in Evolutionary Theory

The introduction of novel phenotypic structures is one of the most significant aspects of organismal evolution. Yet the concept of evolutionary novelty is used with drastically different connotations in various fields of research, and debate exists about whether novelties represent features that are distinct from standard forms of phenotypic variation. This article contrasts four separate uses for novelty in genetics, population genetics, morphology, and behavioral science, before establishing how novelties are used in evolutionary developmental biology (EvoDevo). In particular, it is detailed how an EvoDevo-specific research approach to novelty produces insight distinct from other fields, gives the concept explanatory power with predictive capacities, and brings new consequences to evolutionary theory. This includes the outlining of research strategies that draw attention to productive areas of inquiry, such as threshold dynamics in development. It is argued that an EvoDevo-based approach to novelty is inherently mechanistic, treats the phenotype as an agent with generative potential, and prompts a distinction between continuous and discontinuous variation in evolutionary theory.

Past climate change on Sky Islands drives novelty in a core developmental gene network and its phenotype

BACKGROUND

A fundamental and enduring problem in evolutionary biology is to understand how populations differentiate in the wild, yet little is known about what role organismal development plays in this process. Organismal development integrates environmental inputs with the action of gene regulatory networks to generate the phenotype. Core developmental gene networks have been highly conserved for millions of years across all animals, and therefore, organismal development may bias variation available for selection to work on. Biased variation may facilitate repeatable phenotypic responses when exposed to similar environmental inputs and ecological changes. To gain a more complete understanding of population differentiation in the wild, we integrated evolutionary developmental biology with population genetics, morphology, paleoecology and ecology. This integration was made possible by studying how populations of the ant species Monomorium emersoni respond to climatic and ecological changes across five 'Sky Islands' in Arizona, which are mountain ranges separated by vast 'seas' of desert. Sky Islands represent a replicated natural experiment allowing us to determine how repeatable is the response of M. emersoni populations to climate and ecological changes at the phenotypic, developmental, and gene network levels.

RESULTS

We show that a core developmental gene network and its phenotype has kept pace with ecological and climate change on each Sky Island over the last ~90,000 years before present (BP). This response has produced two types of evolutionary change within an ant species: one type is unpredictable and contingent on the pattern of isolation of Sky lsland populations by climate warming, resulting in slight changes in gene expression, organ growth, and morphology. The other type is predictable and deterministic, resulting in the repeated evolution of a novel wingless queen phenotype and its underlying gene network in response to habitat changes induced by climate warming.

CONCLUSION

Our findings reveal dynamics of developmental gene network evolution in wild populations. This holds important implications: (1) for understanding how phenotypic novelty is generated in the wild; (2) for providing a possible bridge between micro- and macroevolution; and (3) for understanding how development mediates the response of organisms to past, and potentially, future climate change.

Vertebrate cranial placodes as evolutionary innovations--the ancestor's tale

Evolutionary innovations often arise by tinkering with preexisting components building new regulatory networks by the rewiring of old parts. The cranial placodes of vertebrates, ectodermal thickenings that give rise to many of the cranial sense organs (ear, nose, lateral line) and ganglia, originated as such novel structures, when vertebrate ancestors elaborated their head in support of a more active and exploratory life style. This review addresses the question of how cranial placodes evolved by tinkering with ectodermal patterning mechanisms and sensory and neurosecretory cell types that have their own evolutionary history. With phylogenetic relationships among the major branches of metazoans now relatively well established, a comparative approach is used to infer, which structures evolved in which lineages and allows us to trace the origin of placodes and their components back from ancestor to ancestor. Some of the core networks of ectodermal patterning and sensory and neurosecretory differentiation were already established in the common ancestor of cnidarians and bilaterians and were greatly elaborated in the bilaterian ancestor (with BMP- and Wnt-dependent patterning of dorsoventral and anteroposterior ectoderm and multiple neurosecretory and sensory cell types). Rostral and caudal protoplacodal domains, giving rise to some neurosecretory and sensory cells, were then established in the ectoderm of the chordate and tunicate-vertebrate ancestor, respectively. However, proper cranial placodes as clusters of proliferating progenitors producing high-density arrays of neurosecretory and sensory cells only evolved and diversified in the ancestors of vertebrates.

Limb, tooth, beak: three modes of development and evolutionary innovation of form

The standard model of evolutionary change of form, deriving from Darwin's theory via the Modern Synthesis, assumes a gradualistic reshaping of anatomical structures, with major changes only occurring by many cycles of natural selection for marginal adaptive advantage. This model, with its assertion that a single mechanism underlies both micro- and macroevolutionary change, contains an implicit notion of development which is only applicable in some cases. Here we compare the embryological processes that shape the vertebrate limb bud, the mammalian tooth and the avian beak. The implied notion of development in the standard evolutionary picture is met only in the case of the vertebrate limb, a single-primordium organ with morphostatic shaping, in which cells rearrange in response to signalling centres which are essentially unchanged by cell movement. In the case of the tooth, a single-primordium organ with morphodynamic shaping in which the strengths and relationships between signalling centres is influenced by the cell and tissue movements they induce, and the beak, in which the final form is influenced by the collision and rearrangement of multiple tissue primordia, abrupt appearance of qualitatively different forms (i.e. morphological novelties) can occur with small changes in system parameters induced by a genetic change, or by an environmental factor whose effects can be subsequently canalized genetically. Bringing developmental mechanisms and, specifically, the material properties of tissues as excitable media into the evolutionary picture, demonstrates that gradualistic change for incremental adaptive advantage is only one of the possible modes of morphological evolution.

Old cell, new trick? Cnidocytes as a model for the evolution of novelty

Understanding how new cell types arise is critical for understanding the evolution of organismal complexity. Questions of this nature, however, can be difficult to answer due to the challenge associated with defining the identity of a truly novel cell. Cnidarians (anemones, jellies, and their allies) provide a unique opportunity to investigate the molecular regulation and development of cell-novelty because they possess a cell that is unique to the cnidarian lineage and that also has a very well-characterized phenotype: the cnidocyte (stinging cell). Because cnidocytes are thought to differentiate from the cell lineage that also gives rise to neurons, cnidocytes can be expected to express many of the same genes expressed in their neural "sister" cells. Conversely, only cnidocytes posses a cnidocyst (the explosive organelle that gives cnidocytes their sting); therefore, those genes or gene-regulatory relationships required for the development of the cnidocyst can be expected to be expressed uniquely (or in unique combination) in cnidocytes. This system provides an important opportunity to: (1) construct the gene-regulatory network (GRN) underlying the differentiation of cnidocytes, (2) assess the relative contributions of both conserved and derived genes in the cnidocyte GRN, and (3) test hypotheses about the role of novel regulatory relationships in the generation of novel cell types. In this review, we summarize common challenges to studying the evolution of novelty, introduce the utility of cnidocyte differentiation in the model cnidarian, Nematostella vectensis, as a means of overcoming these challenges, and describe an experimental approach that leverages comparative tissue-specific transcriptomics to generate hypotheses about the GRNs underlying the acquisition of the cnidocyte identity.

The evolutionary history of vertebrate cranial placodes--I: cell type evolution

Vertebrate cranial placodes are crucial contributors to the vertebrate cranial sensory apparatus. Their evolutionary origin has attracted much attention from evolutionary and developmental biologists, yielding speculation and hypotheses concerning their putative homologues in other lineages and the developmental and genetic innovations that might have underlain their origin and diversification. In this article we first briefly review our current understanding of placode development and the cell types and structures they form. We next summarise previous hypotheses of placode evolution, discussing their strengths and caveats, before considering the evolutionary history of the various cell types that develop from placodes. In an accompanying review, we also further consider the evolution of ectodermal patterning. Drawing on data from vertebrates, tunicates, amphioxus, other bilaterians and cnidarians, we build these strands into a scenario of placode evolutionary history and of the genes, cells and developmental processes that underlie placode evolution and development.