A switch in the cellular basis of skeletogenesis in late-stage sea urchin larvae

Localization and origins of juvenile skeletogenic cells in the sea urchin Lytechinuspictus

The development of the sea urchin larval body plan is well understood from extensive studies of embryonic patterning. However, fewer studies have investigated the late larval stages during which the unique pentaradial adult body plan develops. Previous work on late larval development highlights major tissue changes leading up to metamorphosis, but the location of specific cell types during juvenile development is less understood. Here, we improve on technical limitations by applying highly sensitive hybridization chain reaction fluorescent in situ hybridization (HCR-FISH) to the fast-developing and transparent sea urchin Lytechinus pictus, with a focus on skeletogenic cells. First, we show that HCR-FISH can be used in L. pictus to precisely localize skeletogenic cells in the rudiment. In doing so, we provide a detailed staging scheme for the appearance of skeletogenic cells around the rudiment prior to and during biomineralization and show that many skeletogenic cells unassociated with larval rods localize outside of the rudiment prior to localizing inside. Second, we show that downstream biomineralization genes have similar expression patterns during larval and juvenile skeletogenesis, suggesting some conservation of skeletogenic mechanisms during development between stages. Third, we find co-expression of blastocoelar and skeletogenic cell markers around juvenile skeleton located outside of the rudiment, which is consistent with data showing that cells from the non-skeletogenic mesoderm embryonic lineage contribute to the juvenile skeletogenic cell lineage. This work sets the foundation for subsequent studies of other cell types in the late larva of L. pictus to better understand juvenile body plan development, patterning, and evolution.

Shared regulatory function of non-genomic thyroid hormone signaling in echinoderm skeletogenesis

Thyroid hormones are crucial regulators of metamorphosis and development in bilaterians, particularly in chordate deuterostomes. Recent evidence suggests a role for thyroid hormone signaling, principally via 3,5,3',5'-Tetraiodo-L-thyronine (T4), in the regulation of metamorphosis, programmed cell death and skeletogenesis in echinoids (sea urchins and sand dollars) and sea stars. Here, we test whether TH signaling in skeletogenesis is a shared trait of Echinozoa (Echinoida and Holothouroida) and Asterozoa (Ophiourida and Asteroida). We demonstrate dramatic acceleration of skeletogenesis after TH treatment in three classes of echinoderms: sea urchins, sea stars, and brittle stars (echinoids, asteroids, and ophiuroids). Fluorescently labeled thyroid hormone analogues reveal thyroid hormone binding to cells proximal to regions of skeletogenesis in the gut and juvenile rudiment. We also identify, for the first time, a potential source of thyroxine during gastrulation in sea urchin embryos. Thyroxine-positive cells are present in tip of the archenteron. In addition, we detect thyroid hormone binding to the cell membrane and nucleus during metamorphic development in echinoderms. Immunohistochemistry of phosphorylated MAPK in the presence and absence of TH-binding inhibitors suggests that THs may act via phosphorylation of MAPK (ERK1/2) to accelerate initiation of skeletogenesis in the three echinoderm groups. Together, these results indicate that TH regulation of mesenchyme cell activity via integrin-mediated MAPK signaling may be a conserved mechanism for the regulation of skeletogenesis in echinoderm development. In addition, TH action via a nuclear thyroid hormone receptor may regulate metamorphic development. Our findings shed light on potentially ancient pathways of thyroid hormone activity in echinoids, ophiuroids, and asteroids, or on a signaling system that has been repeatedly co-opted to coordinate metamorphic development in bilaterians.

Thyroid hormone-induced cell death in sea urchin metamorphic development

Thyroid hormones (THs) are important regulators of development, metabolism and homeostasis in metazoans. Specifically, they have been shown to regulate the metamorphic transitions of vertebrates and invertebrates alike. Indirectly developing sea urchin larvae accelerate the formation of juvenile structures in response to thyroxine (T4) treatment, while reducing their larval arm length. The mechanisms underlying larval arm reduction are unknown and we hypothesized that programmed cell death (PCD) is linked to this process. To test this hypothesis, we measured larval arm retraction in response to different THs (T4, T3, rT3, Tetrac) and assessed cell death in larvae using three different methods (TUNEL, YO-PRO-1 and caspase-3 activity) in the sea urchin Strongylocentrotus purpuratus. We also compared the extent of PCD in response to TH treatment before and after the invagination of the larval ectoderm, which marks the initiation of juvenile development in larval sea urchin species. We found that T4 treatment results in the strongest reduction of larval arms but detected a significant increase of PCD in response to T4, T3 and Tetrac in post-ingression but not pre-ingression larvae. As post-ingression larvae have initiated metamorphic development and therefore allocate resources to both larval and the juvenile structures, these results provide evidence that THs regulate larval development differentially via PCD. PCD in combination with cell proliferation likely has a key function in sea urchin development.

Developmental atlas of the indirect-developing sea urchin Paracentrotus lividus: From fertilization to juvenile stages

The sea urchin Paracentrotus lividus has been used as a model system in biology for more than a century. Over the past decades, it has been at the center of a number of studies in cell, developmental, ecological, toxicological, evolutionary, and aquaculture research. Due to this previous work, a significant amount of information is already available on the development of this species. However, this information is fragmented and rather incomplete. Here, we propose a comprehensive developmental atlas for this sea urchin species, describing its ontogeny from fertilization to juvenile stages. Our staging scheme includes three periods divided into 33 stages, plus 15 independent stages focused on the development of the coeloms and the adult rudiment. For each stage, we provide a thorough description based on observations made on live specimens using light microscopy, and when needed on fixed specimens using confocal microscopy. Our descriptions include, for each stage, the main anatomical characteristics related, for instance, to cell division, tissue morphogenesis, and/or organogenesis. Altogether, this work is the first of its kind providing, in a single study, a comprehensive description of the development of P. lividus embryos, larvae, and juveniles, including details on skeletogenesis, ciliogenesis, myogenesis, coelomogenesis, and formation of the adult rudiment as well as on the process of metamorphosis in live specimens. Given the renewed interest for the use of sea urchins in ecotoxicological, developmental, and evolutionary studies as well as in using marine invertebrates as alternative model systems for biomedical investigations, this study will greatly benefit the scientific community and will serve as a reference for specialists and non-specialists interested in studying sea urchins.

Lessons from a transcription factor: Alx1 provides insights into gene regulatory networks, cellular reprogramming, and cell type evolution

The skeleton-forming cells of sea urchins and other echinoderms have been studied by developmental biologists as models of cell specification and morphogenesis for many decades. The gene regulatory network (GRN) deployed in the embryonic skeletogenic cells of euechinoid sea urchins is one of the best understood in any developing animal. Recent comparative studies have leveraged the information contained in this GRN, bringing renewed attention to the diverse patterns of skeletogenesis within the phylum and the evolutionary basis for this diversity. The homeodomain-containing transcription factor, Alx1, was originally shown to be a core component of the skeletogenic GRN of the sea urchin embryo. Alx1 has since been found to be key regulator of skeletal cell identity throughout the phylum. As such, Alx1 is currently serving as a lens through which multiple developmental processes are being investigated. These include not only GRN organization and evolution, but also cell reprogramming, cell type evolution, and the gene regulatory control of morphogenesis. This review summarizes our current state of knowledge concerning Alx1 and highlights the insights it is yielding into these important developmental and evolutionary processes.

Gene regulation of adult skeletogenesis in starfish and modifications during gene network co-option

The larval skeleton of the echinoderm is believed to have been acquired through co-option of a pre-existing gene regulatory network (GRN); that is, the mechanism for adult skeleton formation in the echinoderm was deployed in early embryogenesis during echinoderm diversification. To explore the evolutionary changes that occurred during co-option, we examined the mechanism for adult skeletogenesis using the starfish Patiria pectinifera. Expression patterns of skeletogenesis-related genes (vegf, vegfr, ets1/2, erg, alx1, ca1, and clect) suggest that adult skeletogenic cells develop from the posterior coelom after the start of feeding. Treatment with inhibitors and gene knockout using transcription activator-like effector nucleases (TALENs) suggest that the feeding-nutrient sensing pathway activates Vegf signaling via target of rapamycin (TOR) activity, leading to the activation of skeletogenic regulatory genes in starfish. In the larval skeletogenesis of sea urchins, the homeobox gene pmar1 activates skeletogenic regulatory genes, but in starfish, localized expression of the pmar1-related genes phbA and phbB was not detected during the adult skeleton formation stage. Based on these data, we provide a model for the adult skeletogenic GRN in the echinoderm and propose that the upstream regulatory system changed from the feeding-TOR-Vegf pathway to a homeobox gene-system during co-option of the skeletogenic GRN.

The evolution of a new cell type was associated with competition for a signaling ligand

There is presently a very limited understanding of the mechanisms that underlie the evolution of new cell types. The skeleton-forming primary mesenchyme cells (PMCs) of euechinoid sea urchins, derived from the micromeres of the 16-cell embryo, are an example of a recently evolved cell type. All adult echinoderms have a calcite-based endoskeleton, a synapomorphy of the Ambulacraria. Only euechinoids have a micromere-PMC lineage, however, which evolved through the co-option of the adult skeletogenic program into the embryo. During normal development, PMCs alone secrete the embryonic skeleton. Other mesoderm cells, known as blastocoelar cells (BCs), have the potential to produce a skeleton, but a PMC-derived signal ordinarily prevents these cells from expressing a skeletogenic fate and directs them into an alternative developmental pathway. Recently, it was shown that vascular endothelial growth factor (VEGF) signaling plays an important role in PMC differentiation and is part of a conserved program of skeletogenesis among echinoderms. Here, we report that VEGF signaling, acting through ectoderm-derived VEGF3 and its cognate receptor, VEGF receptor (VEGFR)-10-Ig, is also essential for the deployment of the skeletogenic program in BCs. This VEGF-dependent program includes the activation of aristaless-like homeobox 1 (alx1), a conserved transcriptional regulator of skeletogenic specification across echinoderms and an example of a “terminal selector” gene that controls cell identity. We show that PMCs control BC fate by sequestering VEGF3, thereby preventing activation of alx1 and the downstream skeletogenic network in BCs. Our findings provide an example of the regulation of early embryonic cell fates by direct competition for a secreted signaling ligand, a developmental mechanism that has not been widely recognized. Moreover, they reveal that a novel cell type evolved by outcompeting other embryonic cell lineages for an essential signaling ligand that regulates the expression of a gene controlling cell identity.

How do new cell types evolve? This study shows that mesoderm cells in sea urchin embryos diversified, at least in part, through a heterochronic shift in the expression of a key transcription factor, which led to competition for a signaling ligand and subsequent gene regulatory independence of the two cell types.

Evolutionary modification of AGS protein contributes to formation of micromeres in sea urchins

Evolution is proposed to result, in part, from acquisition of new developmental programs. One such example is the appearance of the micromeres in a sea urchin that form by an asymmetric cell division at the 4th embryonic cleavage and function as a major signaling center in the embryo. Micromeres are not present in other echinoderms and thus are  considered as a derived feature, yet its acquisition mechanism is unknown. Here, we report that the polarity factor AGS and its associated proteins are responsible for micromere formation. Evolutionary modifications of AGS protein seem to have provided the cortical recruitment and binding of AGS to the vegetal cortex, contributing to formation of micromeres in the sea urchins. Indeed, introduction of sea urchin AGS into the sea star embryo induces asymmetric cell divisions, suggesting that the molecular evolution of AGS protein is key in the transition of echinoderms to micromere formation and the current developmental style of sea urchins not seen in other echinoderms.

From genome to anatomy: The architecture and evolution of the skeletogenic gene regulatory network of sea urchins and other echinoderms

The skeletogenic gene regulatory network (GRN) of sea urchins and other echinoderms is one of the most intensively studied transcriptional networks in any developing organism. As such, it serves as a preeminent model of GRN architecture and evolution. This review summarizes our current understanding of this developmental network. We describe in detail the most comprehensive model of the skeletogenic GRN, one developed for the euechinoid sea urchin Strongylocentrotus purpuratus, including its initial deployment by maternal inputs, its elaboration and stabilization through regulatory gene interactions, and its control of downstream effector genes that directly drive skeletal morphogenesis. We highlight recent comparative studies that have leveraged the euechinoid GRN model to examine the evolution of skeletogenic programs in diverse echinoderms, studies that have revealed both conserved and divergent features of skeletogenesis within the phylum. Last, we summarize the major insights that have emerged from analysis of the structure and evolution of the echinoderm skeletogenic GRN and identify key, unresolved questions as a guide for future work.

Thyroid Hormones Accelerate Initiation of Skeletogenesis via MAPK (ERK1/2) in Larval Sea Urchins (Strongylocentrotus purpuratus)

Thyroid hormones are important regulators of development and metabolism in animals. Their function via genomic and non-genomic actions is well-established in vertebrate species but remains largely elusive among invertebrates. Previous work suggests that thyroid hormones, principally 3,5,3',5'-Tetraiodo-L-thyronine (T4), regulate development to metamorphosis in sea urchins. Here we show that thyroid hormones, including T4, 3,5,3'-triiodo-l-thyronine (T3), and 3,5-Diiodothyronine (T2) accelerate initiation of skeletogenesis in sea urchin gastrulae and pluteus larvae of the sea urchin Strongylocentrotus purpuratus, as measured by skeletal spicule formation. Fluorescently conjugated hormones show T4 binding to primary mesenchyme cells in sea urchin gastrulae. Furthermore, our investigation of TH mediated skeletogenesis shows that Ets1, a transcription factor controlling initiation of skeletogenesis, is a target of activated (phosphorylated) mitogen-activated protein kinase [MAPK; extracellular signal-regulated kinase 1/2 (ERK1/2)]. As well, we show that PD98059, an inhibitor of ERK1/2 MAPK signaling, prevents the T4 mediated acceleration of skeletogenesis and upregulation of Ets1. In contrast, SB203580, an inhibitor of p38 MAPK signaling, did not inhibit the effect of T4. Immunohistochemistry revealed that T4 causes phosphorylation of ERK1/2 in presumptive primary mesenchyme cells and the basal membrane of epithelial cells in the gastrula. Pre-incubation of sea urchin gastrulae with RGD peptide, a competitive inhibitor of TH binding to integrins, inhibited the effect of T4 on skeletogenesis. Together, these experiments provide evidence that T4 acts via a MAPK- (ERK1/2) mediated integrin membrane receptor to accelerate skeletogenesis in sea urchin mesenchyme cells. These findings shed light, for the first time, on a putative non-genomic pathway of TH action in a non-chordate deuterostome and help elucidate the evolutionary history of TH signaling in animals.

Comparative studies on the skeletogenic mesenchyme of echinoids

Skeletogenic mesenchyme cells in echinoids are suitable for studying developmental mechanisms, and have been used extensively. Most of these studies have been performed on species in the order Camarodonta, which are modern echinoids (subclass Euechinoidea) and are considered "model" echinoid species. In contrast, species belonging to other orders are studied less frequently, especially investigations of their molecular developmental biology such as gene regulatory networks. Recent studies on mesenchyme development in non-camarodont species suggest that these species are potential sources of comparative information to elucidate the mechanisms underlying skeletogenic mesenchyme development. In this review, the importance of using comparative data to understand development and evolution is discussed.

Essential elements for translation: the germline factor Vasa functions broadly in somatic cells

Vasa is a conserved RNA-helicase found in the germ lines of all metazoans tested. Whereas Vasa presence is often indicated as a metric for germline determination in animals, it is also expressed in stem cells of diverse origin. Recent research suggests, however, that Vasa has a much broader function, including a significant role in cell cycle regulation. Results herein indicate that Vasa is utilized widely, and often induced transiently, during development in diverse somatic cells and adult precursor tissues. We identified that Vasa in the sea urchin is essential for: (1) general mRNA translation during embryogenesis, (2) developmental re-programming upon manipulations to the embryo and (3) larval wound healing. We also learned that Vasa interacted with mRNAs in the perinuclear area and at the spindle in an Importin-dependent manner during cell cycle progression. These results suggest that, when present, Vasa functions are essential to contributing to developmental regulation.

The echinoderm larval skeleton as a possible model system for experimental evolutionary biology

The evolution of various body plans results from the acquisition of novel structures as well as the loss of existing structures. Some novel structures necessitate multiple evolutionary steps, requiring organisms to overcome the intermediate steps, which might be less adaptive or neutral. To examine this issue, echinoderms might provide an ideal experimental system. A larval skeleton is acquired in some echinoderm lineages, such as sea urchins, probably via the co-option of the skeletogenic machinery that was already established to produce the adult skeleton. The acquisition of a larval skeleton was found to require multiple steps and so provides a model experimental system for reproducing intermediate evolutionary stages. The fact that echinoderm embryology has been studied with various natural populations also presents an advantage.

SM30 protein function during sea urchin larval spicule formation

A central issue in better understanding the process of biomineralization is to elucidate the function of occluded matrix proteins present in mineralized tissues. A potent approach to addressing this issue utilizes specific inhibitors of expression of known genes. Application of antisense oligonucleotides that specifically suppress translation of a given mRNA are capable of causing aberrant biomineralization, thereby revealing, at least in part, a likely function of the protein and gene under investigation. We have applied this approach to study the possible function(s) of the SM30 family of proteins, which are found in spicules, teeth, spines, and tests of Strongylocentrotus purpuratus as well as other euechinoid sea urchins. It is possible using the anti-SM30 morpholino-oligonucleotides (MO's) to reduce the level of these proteins to very low levels, yet the development of skeletal spicules in the embryo shows little or no aberration. This surprising result requires re-thinking about the role of these, and possibly other occluded matrix proteins.

Autonomy in specification of primordial germ cells and their passive translocation in the sea urchin

The process of germ line determination involves many conserved genes, yet is highly variable. Echinoderms are positioned at the base of Deuterostomia and are crucial to understanding these evolutionary transitions, yet the mechanism of germ line specification is not known in any member of the phyla. Here we demonstrate that small micromeres (SMics), which are formed at the fifth cell division of the sea urchin embryo, illustrate many typical features of primordial germ cell (PGC) specification. SMics autonomously express germ line genes in isolated culture, including selective Vasa protein accumulation and transcriptional activation of nanos; their descendants are passively displaced towards the animal pole by secondary mesenchyme cells and the elongating archenteron during gastrulation; Cadherin (G form) has an important role in their development and clustering phenotype; and a left/right integration into the future adult anlagen appears to be controlled by a late developmental mechanism. These results suggest that sea urchin SMics share many more characteristics typical of PGCs than previously thought, and imply a more widely conserved system of germ line development among metazoans.

Morphogenesis in sea urchin embryos: linking cellular events to gene regulatory network states

Gastrulation in the sea urchin begins with ingression of the primary mesenchyme cells (PMCs) at the vegetal pole of the embryo. After entering the blastocoel the PMCs migrate, form a syncitium, and synthesize the skeleton of the embryo. Several hours after the PMCs ingress the vegetal plate buckles to initiate invagination of the archenteron. That morphogenetic process occurs in several steps. The nonskeletogenic cells produce the initial inbending of the vegetal plate. Endoderm cells then rearrange and extend the length of the gut across the blastocoel to a target near the animal pole. Finally, cells that will form part of the midgut and hindgut are added to complete gastrulation. Later, the stomodeum invaginates from the oral ectoderm and fuses with the foregut to complete the archenteron. In advance of, and during these morphogenetic events, an increasingly complex input of transcription factors controls the specification and the cell biological events that conduct the gastrulation movements.

Evolutionary crossroads in developmental biology: sea urchins

Embryos of the echinoderms, especially those of sea urchins and sea stars, have been studied as model organisms for over 100 years. The simplicity of their early development, and the ease of experimentally perturbing this development, provides an excellent platform for mechanistic studies of cell specification and morphogenesis. As a result, echinoderms have contributed significantly to our understanding of many developmental mechanisms, including those that govern the structure and design of gene regulatory networks, those that direct cell lineage specification, and those that regulate the dynamic morphogenetic events that shape the early embryo.

The DEAD-box RNA helicase Vasa functions in embryonic mitotic progression in the sea urchin

Vasa is a broadly conserved ATP-dependent RNA helicase that functions in the germ line of organisms from cnidarians to mammals. Curiously, Vasa is also present in the somatic cells of many animals and functions as a regulator of multipotent cells. Here, we report a mitotic function of Vasa revealed in the sea urchin embryo. We found that Vasa protein is present in all blastomeres of the early embryo and that its abundance oscillates with the cell cycle. Vasa associates with the spindle and the separating sister chromatids at metaphase, and then quickly disappears after telophase. Inhibition of Vasa protein synthesis interferes with proper chromosome segregation, arrests cells at M-phase, and delays overall cell cycle progression. Cdk activity is necessary for the proper localization of Vasa, implying that Vasa is involved in the cyclin-dependent cell cycle network, and Vasa is required for the efficient translation of cyclinB mRNA. Our results suggest an evolutionarily conserved role of Vasa that is independent of its function in germ line determination.

Small micromeres contribute to the germline in the sea urchin

Many indirect developing animals create specialized multipotent cells in early development to construct the adult body and perhaps to hold the fate of the primordial germ cells. In sea urchin embryos, small micromeres formed at the fifth division appear to be such multipotent cells: they are relatively quiescent in embryos, but contribute significantly to the coelomic sacs of the larvae, from which the major tissues of the adult rudiment are derived. These cells appear to be regulated by a conserved gene set that includes the classic germline lineage genes vasa, nanos and piwi. In vivo lineage mapping of the cells awaits genetic manipulation of the lineage, but previous research has demonstrated that the germline is not specified at the fourth division because animals are fertile even when micromeres, the parent blastomeres of small micromeres, are deleted. Here, we have deleted small micromeres at the fifth division and have raised the resultant larvae to maturity. These embryos developed normally and did not overexpress Vasa, as did embryos from a micromere deletion, implying the compensatory gene regulatory network was not activated in small micromere-deleted embryos. Adults from control and micromere-deleted embryos developed gonads and visible gametes, whereas small micromere-deleted animals formed small gonads that lacked gametes. Quantitative PCR results indicate that small micromere-deleted animals produce background levels of germ cell products, but not specifically eggs or sperm. These results suggest that germline specification depends on the small micromeres, either directly as lineage products, or indirectly by signaling mechanisms emanating from the small micromeres or their descendants.

Molecular aspects of biomineralization of the echinoderm endoskeleton

Echinoderms possess a rigid endoskeleton composed of calcite and small amounts of occluded organic matrix proteins. The test (i.e., the shell-like structure of adults), spines, pedicellariae, tube feet, and teeth of adults, as well as delicate endoskeletal spicules found in larvae of some classes, are the main skeletal structures. They have been intensively studied for insight into the mechanisms of biomineralization. Recent work on characterization of the mineral phase and occluded proteins in embryonic skeletal spicules shows that these simple-looking structures contain scores of different proteins, and that the mineral phase is composed of amorphous calcium carbonate (ACC), which then transforms to an anhydrous ACC and eventually to calcite. Likewise, the adult tooth shows a similar transition from hydrated ACC to anhydrous ACC to calcite during its formation, and a similar transition is likely occurring during adult spine regeneration. We speculate that: (1) the ACC precursor is a general strategy employed in biomineralization in echinoderms, (2) the numerous occluded proteins play a role in post-secretion formation of the mature biomineralized structure, and (3) proteins with "multi-valent" intrinsically disordered domains are important for formation of occluded matrix structures, and regulation of crucial matrix-mineral interactions, such as ACC to calcite transitions and polymorph selection.

Functional evolution of Ets in echinoderms with focus on the evolution of echinoderm larval skeletons

Convergent evolution of echinoderm pluteus larva was examined from the standpoint of functional evolution of a transcription factor Ets1/2. In sea urchins, Ets1/2 plays a central role in the differentiation of larval skeletogenic mesenchyme cells. In addition, Ets1/2 is suggested to be involved in adult skeletogenesis. Conversely, in starfish, although no skeletogenic cells differentiate during larval development, Ets1/2 is also expressed in the larval mesoderm. Here, we confirmed that the starfish Ets1/2 is indispensable for the differentiation of the larval mesoderm. This result led us to assume that, in the common ancestors of echinoderms, Ets1/2 activates the transcription of distinct gene sets, one for the differentiation of the larval mesoderm and the other for the development of the adult skeleton. Thus, the acquisition of the larval skeleton involved target switching of Ets1/2. Specifically, in the sea urchin lineage, Ets1/2 activated a downstream target gene set for skeletogenesis during larval development in addition to a mesoderm target set. We examined whether this heterochronic activation of the skeletogenic target set was achieved by the molecular evolution of the Ets1/2 transcription factor itself. We tested whether starfish Ets1/2 induced skeletogenesis when injected into sea urchin eggs. We found that, in addition to ectopic induction of mesenchyme cells, starfish Ets1/2 can activate some parts of the skeletogenic pathway in these mesenchyme cells. Thus, we suggest that the nature of the transcription factor Ets1/2 did not change, but rather that some unidentified co-factor(s) for Ets1/2 may distinguish between targets for the larval mesoderm and for skeletogenesis. Identification of the co-factor(s) will be key to understanding the molecular evolution underlying the evolution of the pluteus larvae.

Transfer of a large gene regulatory apparatus to a new developmental address in echinoid evolution

Of the five echinoderm classes, only the modern sea urchins (euechinoids) generate a precociously specified embryonic micromere lineage that ingresses before gastrulation and then secretes the biomineral embryonic skeleton. The gene regulatory network (GRN) underlying the specification and differentiation of this lineage is now known. Many of the same differentiation genes as are used in the biomineralization of the embryo skeleton are also used to make the similar biomineral of the spines and test plates of the adult body. Here, we determine the components of the regulatory state upstream of these differentiation genes that are shared between embryonic and adult skeletogenesis. An abrupt "break point" in the micromere GRN is thus revealed, on one side of which most of the regulatory genes are used in both, and on the other side of which the regulatory apparatus is entirely micromere-specific. This reveals the specific linkages of the micromere GRN forged in the evolutionary process by which the skeletogenic gene batteries were caused to be activated in the embryonic micromere lineage. We also show, by comparison with adult skeletogenesis in the sea star, a distant echinoderm outgroup, that the regulatory apparatus responsible for driving the skeletogenic differentiation gene batteries is an ancient pleisiomorphic aspect of the echinoderm-specific regulatory heritage.