The evolution of embryonic patterning mechanisms in animals

Integrating Complex Life Cycles in Comparative Developmental Biology

The goal of comparative developmental biology is identifying mechanistic differences in embryonic development between different taxa and how these evolutionary changes have led to morphological and organizational differences in adult body plans. Much of this work has focused on direct-developing species in which the adult forms straight from the embryo and embryonic modifications have direct effects on the adult. However, most animal lineages are defined by indirect development, in which the embryo gives rise to a larval body plan and the adult forms by transformation of the larva. Historically, much of our understanding of complex life cycles is viewed through the lenses of ecology and zoology. In this review, we discuss the importance of establishing developmental rather than morphological or ecological criteria for defining developmental mode and explicitly considering the evolutionary implications of incorporating complex life cycles into broad developmental comparisons of embryos across metazoans.

Procuring animals and culturing of eggs and embryos

Echinoderms and especially echinoids have a rich history as model systems for the study of oogenesis, fertilization, and early embryogenesis. The ease of collecting and maintaining adults, as well as in obtaining gametes and culturing large quantities of synchronous embryos, is complemented by the ability to do biochemistry, reverse genetics, embryo manipulations and study gene regulatory networks. The diversity of species and developmental modes as well as unparalleled transparency in early developmental stages also makes echinoderms an excellent system in which to study evolutionary aspects of developmental biology. This chapter provides a practical guide to experimental methods for procuring adults and gametes, achieving synchronous in vitro fertilization, and culturing embryos through early larval stages for several echinoderm species representing four classes (Echinoidea, Asteroidea, Ophiuroidea, and Holothuroidea). We provide specific examples of protocols for obtaining adults and gametes and for culturing embryos of a selected number of species for experimental analysis of their development. The species were chosen to provide breadth across the phylum Echinodermata, as well as to provide practical guidelines for handling some of the more commonly studied species. For each species, we highlight specific advantages, and special note is made of key issues to consider when handling adults, collecting gametes, or setting and maintaining embryo cultures. Finally, information regarding interspecific crosses is provided.

Development of the Sea Star Echinaster (Othilia) brasiliensis, with Inference on the Evolution of Development and Skeletal Plates in Asteroidea

We describe the development and juvenile morphology of the sea star Echinaster (Othilia) brasiliensis in order to explore evolutionary developmental modes and skeletal homologies. This species produces large, buoyant eggs (0.6 ± 0.03 mm diameter), and has a typical lecithotrophic brachiolaria larva. The planktonic brachiolaria larva is formed 2-4 days after fertilization, when cilia cover the surface. Early juveniles are completely formed by 18 days of age. Initial growth is supported by maternal nutrients while the stomach continues to develop until 60 days after fertilization, when juveniles reach about 0.5 mm of radius length. The madreporite was observed 88 days after fertilization. In the youngest juvenile skeleton of E. (O.) brasiliensis, the madreporite and odontophore are homologous to those of other recent, non-paxillosid asteroids, and follow the Late Madreporic Mode. The emergence of plates related to the ambulacral system follows the Ocular Plate Rule. The development and juvenile skeletal morphology of this species are similar to those of the few other studied species in the genus Echinaster. This study corroborates the notion that the mode of development--including a short-lived lecithotrophic brachiolaria larva--in all Echinaster species shares a similar pattern that may be conserved throughout the evolutionary history of the group.

Whole-genome positive selection and habitat-driven evolution in a shallow and a deep-sea urchin

Comparisons of genomic sequence between divergent species can provide insight into the action of natural selection across many distinct classes of proteins. Here, we examine the extent of positive selection as a function of tissue-specific and stage-specific gene expression in two closely-related sea urchins, the shallow-water Strongylocentrotus purpuratus and the deep-sea Allocentrotus fragilis, which have diverged greatly in their adult but not larval habitats. Genes that are expressed specifically in adult somatic tissue have significantly higher dN/dS ratios than the genome-wide average, whereas those in larvae are indistinguishable from the genome-wide average. Testis-specific genes have the highest dN/dS values, whereas ovary-specific have the lowest. Branch-site models involving the outgroup S. franciscanus indicate greater selection (ω_FG) along the A. fragilis branch than along the S. purpuratus branch. The A. fragilis branch also shows a higher proportion of genes under positive selection, including those involved in skeletal development, endocytosis, and sulfur metabolism. Both lineages are approximately equal in enrichment for positive selection of genes involved in immunity, development, and cell–cell communication. The branch-site models further suggest that adult-specific genes have experienced greater positive selection than those expressed in larvae and that ovary-specific genes are more conserved (i.e., experienced greater negative selection) than those expressed specifically in adult somatic tissues and testis. Our results chart the patterns of protein change that have occurred after habitat divergence in these two species and show that the developmental or functional context in which a gene acts can play an important role in how divergent species adapt to new environments.

Morphological evolution and embryonic developmental diversity in metazoa

Most studies of pattern formation and morphogenesis in metazoans focus on a small number of model species, despite the fact that information about a wide range of species and developmental stages has accumulated in recent years. By contrast, this article attempts to use this broad knowledge base to arrive at a classification of developmental types through which metazoan body plans are generated. This classification scheme pays particular attention to the diverse ways by which cell signalling and morphogenetic movements depend on each other, and leads to several testable hypotheses regarding morphological variation within and between species, as well as metazoan evolution.

Comparative embryology without a microscope: using genomic approaches to understand the evolution of development

Until recently, understanding developmental conservation and change has relied on embryological comparisons and analyses of single genes. Several studies, including one recently published in BMC Biology, have now taken a genomic approach to this classical problem, providing insights into how selection operates differentially across the life cycle.

Molluscan larvae: Pelagic juveniles or slowly metamorphosing larvae?

Asking the right questions about evolution of development, larval morphology, and life history requires knowledge of ancestral state. Two hypotheses dominate current opinion about the ancestral life cycle of bilaterians: the "larva-first" and the "intercalation" hypotheses. Until recently, the larva-first hypothesis was preeminent. This proposes that the original indirect life cycle of bilaterians included a planktotrophic larva followed by a benthic adult. Phylogenetic evidence suggests that a planktotrophic larva is plesiomorphic for echinoderms. A preponderance of developmental studies on echinoderms may have fostered a tendency to extrapolate conclusions about echinoderm development to other clades, particularly the concept that larval and juvenile/adult bodies are mostly separate entities. However, some of the recent reconstructions of bilaterian phylogeny suggest that nonfeeding larvae may have been ancestral for bilaterians, and these may have been intercalated into a life cycle that was originally direct. I review comparative data on molluscan development that suggests the trochophore-like stage is little more than a gastrula with transient structures (prototroch and apical sensory organ) to allow a temporary planktonic phase during development. Most lineage founder cells of molluscan embryos generate progeny that develop through the veliger stage into structures of the juvenile, which becomes benthic when the prototroch and apical sensory organ are lost. In light of this, the model of separate larval and juvenile bodies with the latter developing from nests of multipotent cells within the larva is inappropriate for molluscs. The intercalation hypothesis may be a better model for interpreting development of molluscs and other lophotrochozoans.

How did indirect development with planktotrophic larvae evolve?

The two main types of theories for the evolution of the biphasic life cycles in marine invertebrates are discussed. The "intercalation" theories propose that the larval stages (planktotrophic or lecithotrophic) have evolved as specializations from the ancestral, direct life cycle. The opposing "terminal addition" theories propose that the ancestor was holopelagic and that the adult stage was added to the life cycle with the pelagic stage retained as a planktotrophic larva. It is emphasized that theories based on hypothetical ancestors that were unable to feed must be rejected. This applies to planula theories based on a compact planula. Various arguments against the theories that consider the feeding larvae as ancestral in the major eumetazoan lineages and in particular against the trochaea theory are discussed and found untenable. It is suggested that the "Cambrian explosion" was actually a rapid Ediacaran radiation of the eubilaterians that was made possible by the evolution of a tubular gut with all the resulting possibilities for new body plans.

Ernest Everett Just (1883-1941)--an early ecological developmental biologist

Ecological developmental biology (Eco-Devo) involves the study of development in its natural environmental context as opposed to the laboratory setting. Ernest E. Just was an early 20th century African-American embryologist who devoted his career to studying the early development of marine invertebrates in the United States and abroad. Through detailed study of the fertilization process, he came to see the cell cortex as playing a central role in development, inheritance, and evolution. This paper, after presenting some of Just's scientific and philosophical contributions, argues that Just was an Eco-Devo biologist. Three lines of evidence are given. First, Just believed that intimate knowledge of the natural history of the marine animal under study--hence, the natural setting in which fertilization occurs--was essential. Second, he stressed the importance of the egg's "normality"--how well its condition in the laboratory corresponds to the natural, fertilizable state. Finally, Just was an organicist, believing that organisms are holistic systems with emergent properties that arise from their organization and complexity. Although other scientists may stand out more clearly as founding architects of Eco-Devo, E. E. Just, with his unwavering insistence on the normality and holistic integrity of the egg cell, was one of its purest adherents.

Comparative methods in developmental biology

The need for a phylogenetic framework is becoming appreciated in many areas of biology. Such a framework has found limited use in developmental studies. Our current research program is therefore directed to applying comparative and phylogenetic methods to developmental data. In this paper, we examine the concepts underlying this work, discuss potential difficulties, and identify some solutions. While developmental biologists frequently make cross-species comparisons, they usually adopt a phenetic approach, whereby degrees of overall similarity in development are sought. Little emphasis is placed on reconstructing the evolutionary divergence in developmental characters. Indeed, developmental biologists have historically concentrated on apparently 'conserved' or 'universal' developmental mechanisms. Thus, there has been little need for phylogenetic methodologies which analyse specialised features shared only within a subset of species (i.e., synapomorphies). We discuss the potential value of such methodologies, and argue that difficulties in adapting them to developmental studies fall into three interlinked areas: One concerns the nature and definition of developmental characters. Another is the difficulty of identifying equivalent developmental stages in different species. Finally the phylogenetic non-independence of developmental characters presents real problems under some protocols. These problems are not resolved. However, it is clear that the application of phylogenetic methodology to developmental data is both necessary and fundamental to research into the relationship between evolution and development.

Larval ectoderm, organizational homology, and the origins of evolutionary novelty

Comprehending the origin of marine invertebrate larvae remains a key domain of research for evolutionary biologists, including the repeated origin of direct developmental modes in echinoids. In order to address the latter question, we surveyed existing evidence on relationships of homology between the ectoderm territories of two closely related sea urchin species in the genus Heliocidaris that differ in their developmental mode. Additionally, we explored a recently articulated idea about homology called 'organizational homology' (Müller 2003. In: Müller GB, Newman SA, editors. Origination of organismal form: beyond the gene in developmental and evolutionary biology. Cambridge, MA: A Bradford Book, The MIT Press. p 51-69. ) in the context of this specific empirical case study. Applying the perspective of organizational homology to our experimental system of congeneric echinoids has led us to a new hypothesis concerning the ectoderm evolution in these species. The extravestibular ectoderm of the direct developer Heliocidaris erythrogramma is a novel developmental territory that arose as a fusion of the oral and aboral ectoderm territories found in indirect developing echinoids such as Heliocidaris tuberculata. This hypothesis instantiates a theoretical principle concerning the origin of developmental modules, 'integration', which has been neglected because the opposite theoretical principle, 'parcellation', is more readily observable in events such as gene duplication and divergence (Wagner 1996. Am Zool 36:36-43).

The embryonic development of the triclad Schmidtea polychroa

Triclad flatworms are well studied for their regenerative properties, yet little is known about their embryonic development. We here describe the embryonic development of the triclaty 120d Schmidtea polychroa, using histological and immunocytochemical analysis of whole-mount preparations and sections. During early cleavage (stage 1), yolk cells fuse and enclose the zygote into a syncytium. The zygote divides into blastomeres that dissociate and migrate into the syncytium. During stage 2, a subset of blastomeres differentiate into a transient embryonic epidermis that surrounds the yolk syncytium, and an embryonic pharynx. Other blastomeres divide as a scattered population of cells in the syncytium. During stage 3, the embryonic pharynx imbibes external yolk cells and a gastric cavity is formed in the center of the syncytium. The syncytial yolk and the blastomeres contained within it are compressed into a thin peripheral rind. From a location close to the embryonic pharynx, which defines the posterior pole, bilaterally symmetric ventral nerve cord pioneers extend forward. Stage 4 is characterized by massive proliferation of embryonic cells. Large yolk-filled cells lining the syncytium form the gastrodermis. During stage 5 the external syncytial yolk mantle is resorbed and the embryonic cells contained within differentiate into an irregular scaffold of muscle and nerve cells. Epidermal cells differentiate and replace the transient embryonic epidermis. Through stages 6-8, the embryo adopts its worm-like shape, and loosely scattered populations of differentiating cells consolidate into structurally defined organs. Our analysis reveals a picture of S. polychroa embryogenesis that resembles the morphogenetic events underlying regeneration.

Echinoderm eggs and embryos: procurement and culture

The protocols outlined here hopefully will provide researchers with healthy, beautiful echinoderm oocytes, eggs, and embryos for experimental use. The large size of echinoderm oocytes and eggs, the ease with which they can be manipulated, and (in many species) their optical clarity, make them an ideal model system for studying not only the events specific to oocyte maturation and fertilization, but also for investigating more general questions regarding cell cycle regulation in an in vivo system. The quick rate at which development proceeds after fertilization to produce transparent embryos and larva makes the echinoderm an advantageous organism for studying deuterostome embryogenesis. Continued use of the echinoderms as model systems will undoubtedly uncover exciting answers to questions regarding fertilization, cell cycle regulation, morphogenesis, and how developmental events are controlled.

Gene expression and larval evolution: changing roles of distal-less and orthodenticle in echinoderm larvae

We describe the expression of the homeobox genes orthodenticle (Otx) and distal-less (Dlx) during the larval development of seven species representing three classes of echinoderms: Holothuroidea, Asteroidea, and Echinoidea. Several expression domains are conserved between species within a single class, including Dlx expression within the brachiolar arms of asteroid larvae and Otx expression within the ciliated bands of holothuroid larvae. Some expression domains are apparently conserved between classes, such as the expression of Dlx within the hydrocoel (left mesocoel) in all three classes. However, several substantial differences in expression domains among taxa were also evident for both genes. Some autapomorphic (unique derived) features of gene expression are phylogenetically associated with autapomorphic structures, such as Dlx expression within the invaginating rudiment of euechinoids. Other autapomorphic gene expression domains are associated with evolutionary shifts in life history from feeding to nonfeeding larval development, such as Otx expression within the ciliated bands of a nonfeeding holothuroid larva. Similar associations between evolutionary changes in morphology and life history mode with changes in regulatory gene expression have also been observed in arthropods, urochordates, and chordates. We predict that recruitment of regulatory genes to a new developmental role is commonly associated with evolutionary changes in morphology and may be particularly common in clades with complex life cycles and diversity of life history modes. Caution should be used when making generalizations about gene expression and function based on a single species, which may not accurately reflect developmental processes and life histories of the phyla to which it belongs.