Linking Vertebrate Gene Duplications to the New Head Hypothesis

Vertebrates have diverse morphologies and various anatomical novelties that set them apart from their closest invertebrate relatives. A conspicuous head housing a large brain, paired sense organs, and protected by a skeleton of cartilage and bone is unique to vertebrates and is a defining feature of this taxon. Gans and Northcutt (1980s) proposed that the evolution of this "new head" was dependent on two key developmental innovations: neural crest cells (NCCs) and ectodermal placodes. NCCs are migratory embryonic cells that form bone, cartilage, and neurons in the new head. Based on genome size, Ohno (1970s) proposed a separate hypothesis, stating that vertebrate genome content was quadrupled via two rounds (2R) of whole genome duplications (WGDs), and the surplus of genetic material potentiated vertebrate morphological diversification. While both hypotheses offer explanations for vertebrate success, it is unclear if, and how, the "new head" and "2R" hypotheses are linked. Here, we consider both hypotheses and evaluate the experimental evidence connecting the two. Overall, evidence suggests that while the origin of the NC GRN predates the vertebrate WGDs, these genomic events may have potentiated the evolution of distinct genetic subnetworks in different neural crest subpopulations. We describe the general composition of the NC GRN and posit that its increased developmental modularity facilitated the independent evolution of NC derivatives and the diversification of the vertebrate head skeleton. Lastly, we discuss experimental strategies needed to test whether gene duplications drove the diversification of neural crest derivatives and the "new head".

Comment on "Ultrastructure reveals ancestral vertebrate pharyngeal skeleton in yunnanozoans"

Tian et al. (Research Articles, 8 July 2022, abm2708) hypothesized that yunnanozoans are stem-group vertebrates on the basis of "cellular cartilage", "fibrillin microfibers", and "subchordal rod" associated with the branchial arches of yunnanozoans. However, we reject the presence of cellular cartilage, fibrillin, and the phylogenetic proposal of vertebrate affinities based on ultrastructure and morphology of yunnanozoans from more than 8000 specimens.

The developmental and evolutionary origins of cellular pluripotency in the vertebrate neural crest

Neural crest cells are central to vertebrate development and evolution, endowing vertebrates with a "new head" that resulted in morphological, physiological, and behavioral features that allowed vertebrates to become active predators. One remarkable feature of neural crest cells is their multi-germ layer potential that allows for the formation of both ectodermal (pigmentation, peripheral glia, sensory neurons) and mesenchymal (connective tissue, cartilage/bone, dermis) cell types. Understanding the cellular and evolutionary origins of this broad cellular potential in the neural crest has been a long-standing focus for developmental biologists. Here, we review recent work that has demonstrated that neural crest cells share key features with pluripotent blastula stem cells, including expression of the Yamanaka stem cell factors (Oct3/4, Klf4, Sox2, c-Myc). These shared features suggest that pluripotency is either retained in the neural crest from blastula stages or subsequently reactivated as the neural crest forms. We highlight the cellular and molecular parallels between blastula stem cells and neural crest cells and discuss the work that has led to current models for the cellular origins of broad potential in the crest. Finally, we explore how these themes can provide new insights into how and when neural crest cells and pluripotency evolved in vertebrates and the evolutionary relationship between these populations.

Ultrastructure reveals ancestral vertebrate pharyngeal skeleton in yunnanozoans

Pharyngeal arches are a key innovation that likely contributed to the evolution of the jaws and braincase of vertebrates. It has long been hypothesized that the pharyngeal (branchial) arch evolved from an unjointed cartilaginous rod in vertebrate ancestors such as that in the nonvertebrate chordate amphioxus, but whether such ancestral anatomy existed remains unknown. The pharyngeal skeleton of controversial Cambrian animals called yunnanozoans may contain the oldest fossil evidence constraining the early evolution of the arches, yet its correlation with that of vertebrates is still disputed. By examining additional specimens in previously unexplored techniques (for example, x-ray microtomography, scanning and transmission electron microscopy, and energy dispersive spectrometry element mapping), we found evidence that yunnanozoan branchial arches consist of cellular cartilage with an extracellular matrix dominated by microfibrils, a feature hitherto considered specific to vertebrates. Our phylogenetic analysis provides further support that yunnanozoans are stem vertebrates.

Lamprey lecticans link new vertebrate genes to the origin and elaboration of vertebrate tissues

The evolution of vertebrates from an invertebrate chordate ancestor involved the evolution of new organs, tissues, and cell types. It was also marked by the origin and duplication of new gene families. If, and how, these morphological and genetic innovations are related is an unresolved question in vertebrate evolution. Hyaluronan is an extracellular matrix (ECM) polysaccharide important for water homeostasis and tissue structure. Vertebrates possess a novel family of hyaluronan binding proteins called Lecticans, and studies in jawed vertebrates (gnathostomes) have shown they function in many of the cells and tissues that are unique to vertebrates. This raises the possibility that the origin and/or expansion of this gene family helped drive the evolution of these vertebrate novelties. In order to better understand the evolution of the lectican gene family, and its role in the evolution of vertebrate morphological novelties, we investigated the phylogeny, genomic arrangement, and expression patterns of all lecticans in the sea lamprey (Petromyzon marinus), a jawless vertebrate. Though both P. marinus and gnathostomes each have four lecticans, our phylogenetic and syntenic analyses are most consistent with the independent duplication of one of more lecticans in the lamprey lineage. Despite the likely independent expansion of the lamprey and gnathostome lectican families, we find highly conserved expression of lecticans in vertebrate-specific and mesenchyme-derived tissues. We also find that, unlike gnathostomes, lamprey expresses its lectican paralogs in distinct subpopulations of head skeleton precursors, potentially reflecting an ancestral diversity of skeletal tissue types. Together, these observations suggest that the ancestral pre-duplication lectican had a complex expression pattern, functioned to support mesenchymal histology, and likely played a role in the evolution of vertebrate-specific cell and tissue types.

Latent developmental potential to form limb-like skeletal structures in zebrafish

Changes in appendage structure underlie key transitions in vertebrate evolution. Addition of skeletal elements along the proximal-distal axis facilitated critical transformations, including the fin-to-limb transition that permitted generation of diverse modes of locomotion. Here, we identify zebrafish mutants that form supernumerary long bones in their pectoral fins. These new bones integrate into musculature, form joints, and articulate with neighboring elements. This phenotype is caused by activating mutations in previously unrecognized regulators of appendage patterning, vav2 and waslb, that function in a common pathway. This pathway is required for appendage development across vertebrates, and loss of Wasl in mice causes defects similar to those seen in murine Hox mutants. Concordantly, formation of supernumerary bones requires Hox11 function, and mutations in the vav2/wasl pathway drive enhanced expression of hoxa11b, indicating developmental homology with the forearm. Our findings reveal a latent, limb-like pattern ability in fins that is activated by simple genetic perturbation.

Evolutionary and Developmental Associations of Neural Crest and Placodes in the Vertebrate Head: Insights From Jawless Vertebrates

Neural crest and placodes are key innovations of the vertebrate clade. These cells arise within the dorsal ectoderm of all vertebrate embryos and have the developmental potential to form many of the morphological novelties within the vertebrate head. Each cell population has its own distinct developmental features and generates unique cell types. However, it is essential that neural crest and placodes associate together throughout embryonic development to coordinate the emergence of several features in the head, including almost all of the cranial peripheral sensory nervous system and organs of special sense. Despite the significance of this developmental feat, its evolutionary origins have remained unclear, owing largely to the fact that there has been little comparative (evolutionary) work done on this topic between the jawed vertebrates and cyclostomes—the jawless lampreys and hagfishes. In this review, we briefly summarize the developmental mechanisms and genetics of neural crest and placodes in both jawed and jawless vertebrates. We then discuss recent studies on the role of neural crest and placodes—and their developmental association—in the head of lamprey embryos, and how comparisons with jawed vertebrates can provide insights into the causes and consequences of this event in early vertebrate evolution.

The development, patterning and evolution of neural crest cell differentiation into cartilage and bone

Neural crest cells are a vertebrate-specific migratory, multipotent cell population that give rise to a diverse array of cells and tissues during development. Cranial neural crest cells, in particular, generate cartilage, bone, tendons and connective tissue in the head and face as well as neurons, glia and melanocytes. In this review, we focus on the chondrogenic and osteogenic potential of cranial neural crest cells and discuss the roles of Sox9, Runx2 and Msx1/2 transcription factors and WNT, FGF and TGFβ signaling pathways in regulating neural crest cell differentiation into cartilage and bone. We also describe cranioskeletal defects and disorders arising from gain or loss-of-function of genes that are required for patterning and differentiation of cranial neural crest cells. Finally, we discuss the evolution of skeletogenic potential in neural crest cells and their function as a conduit for intraspecies and interspecies variation, and the evolution of craniofacial novelties.

Neural crest and placode roles in formation and patterning of cranial sensory ganglia in lamprey

Vertebrates possess paired cranial sensory ganglia derived from two embryonic cell populations, neural crest and placodes. Cranial sensory ganglia arose prior to the divergence of jawed and jawless vertebrates, but the developmental mechanisms that facilitated their evolution are unknown. Using gene expression and cell lineage tracing experiments in embryos of the sea lamprey, Petromyzon marinus, we find that in the cranial ganglia we targeted, development consists of placode-derived neuron clusters in the core of ganglia, with neural crest cells mostly surrounding these neuronal clusters. To dissect functional roles of neural crest and placode cell associations in these developing cranial ganglia, we used CRISPR/Cas9 gene editing experiments to target genes critical for the development of each population. Genetic ablation of SoxE2 and FoxD-A in neural crest cells resulted in differentiated cranial sensory neurons with abnormal morphologies, whereas deletion of DlxB in cranial placodes resulted in near-total loss of cranial sensory neurons. Taken together, our cell-lineage, gene expression, and gene editing results suggest that cranial neural crest cells may not be required for cranial ganglia specification but are essential for shaping the morphology of these sensory structures. We propose that the association of neural crest and placodes in the head of early vertebrates was a key step in the organization of neurons and glia into paired sensory ganglia.

Cardiopharyngeal mesoderm origins of musculoskeletal and connective tissues in the mammalian pharynx

Cardiopharyngeal mesoderm (CPM) gives rise to muscles of the head and heart. Using genetic lineage analysis in mice, we show that CPM develops into a broad range of pharyngeal structures and cell types encompassing musculoskeletal and connective tissues. We demonstrate that CPM contributes to medial pharyngeal skeletal and connective tissues associated with both branchiomeric and somite-derived neck muscles. CPM and neural crest cells (NCC) make complementary mediolateral contributions to pharyngeal structures, in a distribution established in the early embryo. We further show that biallelic expression of the CPM regulatory gene Tbx1, haploinsufficient in 22q11.2 deletion syndrome patients, is required for the correct patterning of muscles with CPM-derived connective tissue. Our results suggest that CPM plays a patterning role during muscle development, similar to that of NCC during craniofacial myogenesis. The broad lineage contributions of CPM to pharyngeal structures provide new insights into congenital disorders and evolution of the mammalian pharynx.

The Spine: A Strong, Stable, and Flexible Structure with Biomimetics Potential

From its first appearance in early vertebrates, the spine evolved the function of protecting the spinal cord, avoiding excessive straining during body motion. Its stiffness and strength provided the basis for the development of the axial skeleton as the mechanical support of later animals, especially those which moved to the terrestrial environment where gravity loads are not alleviated by the buoyant force of water. In tetrapods, the functions of the spine can be summarized as follows: protecting the spinal cord; supporting the weight of the body, transmitting it to the ground through the limbs; allowing the motion of the trunk, through to its flexibility; providing robust origins and insertions to the muscles of trunk and limbs. This narrative review provides a brief perspective on the development of the spine in vertebrates, first from an evolutionary, and then from an embryological point of view. The paper describes functions and the shape of the spine throughout the whole evolution of vertebrates and vertebrate embryos, from primordial jawless fish to extant animals such as birds and humans, highlighting its fundamental features such as strength, stability, and flexibility, which gives it huge potential as a basis for bio-inspired technologies.

An ancestral role for Semaphorin3F-Neuropilin signaling in patterning neural crest within the new vertebrate head

The origin of the vertebrate head is one of the great unresolved issues in vertebrate evolutionary developmental biology. Although many of the novelties in the vertebrate head and pharynx derive from the neural crest, it is still unknown how early vertebrates patterned the neural crest within the ancestral body plan they inherited from invertebrate chordates. Here, using a basal vertebrate, the sea lamprey, we show that homologs of Semaphorin3F (Sema3F) ligand and its Neuropilin (Nrp) receptors show complementary and dynamic patterns of expression that correlate with key periods of neural crest development (migration and patterning of cranial neural crest-derived structures). Using CRISPR/Cas9-mediated mutagenesis, we demonstrate that lamprey Sema3F is essential for patterning of neural crest-derived melanocytes, cranial ganglia and the head skeleton, but is not required for neural crest migration or patterning of trunk neural crest derivatives. Based on comparisons with jawed vertebrates, our results suggest that the deployment of Nrp-Sema3F signaling, along with other intercellular guidance cues, was pivotal in allowing early vertebrates to organize and pattern cranial neural crest cells into many of the hallmark structures that define the vertebrate head.

Roles for FGF in lamprey pharyngeal pouch formation and skeletogenesis highlight ancestral functions in the vertebrate head

A defining feature of vertebrates (craniates) is a pronounced head supported and protected by a cellularized endoskeleton. In jawed vertebrates (gnathostomes), the head skeleton is made of rigid three-dimensional elements connected by joints. By contrast, the head skeleton of modern jawless vertebrates (agnathans) consists of thin rods of flexible cellular cartilage, a condition thought to reflect the ancestral vertebrate state. To better understand the origin and evolution of the gnathostome head skeleton, we have been analyzing head skeleton development in the agnathan, lamprey. The fibroblast growth factors FGF3 and FGF8 have various roles during head development in jawed vertebrates, including pharyngeal pouch morphogenesis, patterning of the oral skeleton and chondrogenesis. We isolated lamprey homologs of FGF3, FGF8 and FGF receptors and asked whether these functions are ancestral features of vertebrate development or gnathostome novelties. Using gene expression and pharmacological agents, we found that proper formation of the lamprey head skeleton requires two phases of FGF signaling: an early phase during which FGFs drive pharyngeal pouch formation, and a later phase when they directly regulate skeletal differentiation and patterning. In the context of gene expression and functional studies in gnathostomes, our results suggest that these roles for FGFs arose in the first vertebrates and that the evolution of the jaw and gnathostome cellular cartilage was driven by changes developmentally downstream from pharyngeal FGF signaling.

RNA interference technology to control pest sea lampreys--a proof-of-concept

The parasitic sea lamprey (Petromyzon marinus) has caused extensive losses to commercial fish stocks of the upper Great Lakes of North America. Methods of controlling the sea lamprey include trapping, barriers to prevent migration, and use of a chemical lampricide (3-trifluoromethyl-4-nitrophenol) to kill the filter-feeding larvae. Concerns about the non-specificity of these methods have prompted continued development of species-specific methods to control lampreys outside their native range. In this study, we considered the utility of RNA interference to develop a sea lamprey-specific lampricide. Injection of six different short interfering, double-stranded RNAs (siRNAs) into lamprey embryos first confirmed that the siRNAs could reduce the targeted transcript levels by more than 50%. Two size classes of lamprey larvae were then fed the siRNAs complexed with liposomes, and three of the siRNAs (targeting elongation factor 1α, calmodulin, and α-actinin) reduced transcript levels 2.5, 3.6, and 5.0–fold, respectively, within the lamprey midsections. This is not only the first demonstration of RNAi in lampreys, but it is also the first example of delivery of siRNAs to a non-mammalian vertebrate through feeding formulations. One of the siRNA treatments also caused increased mortality of the larvae following a single feeding of siRNAs, which suggests that prolonged or multiple feedings of siRNAs could be used to kill filter-feeding larvae within streams, following development of a slow-release formulation. The genes targeted in this study are highly conserved across many species, and only serve as a proof-of-concept demonstration that siRNAs can be used in lampreys. Given that RNA interference is a sequence-specific phenomenon, it should be possible to design siRNAs that selectively target gene sequences that are unique to sea lampreys, and thus develop a technology to control these pests without adversely affecting non-target species.

The lamprey: a jawless vertebrate model system for examining origin of the neural crest and other vertebrate traits

Lampreys are a group of jawless fishes that serve as an important point of comparison for studies of vertebrate evolution. Lampreys and hagfishes are agnathan fishes, the cyclostomes, which sit at a crucial phylogenetic position as the only living sister group of the jawed vertebrates. Comparisons between cyclostomes and jawed vertebrates can help identify shared derived (i.e. synapomorphic) traits that might have been inherited from ancestral early vertebrates, if unlikely to have arisen convergently by chance. One example of a uniquely vertebrate trait is the neural crest, an embryonic tissue that produces many cell types crucial to vertebrate features, such as the craniofacial skeleton, pigmentation of the skin, and much of the peripheral nervous system (Gans and Northcutt, 1983). Invertebrate chordates arguably lack unambiguous neural crest homologs, yet have cells with some similarities, making comparisons with lampreys and jawed vertebrates essential for inferring characteristics of development in early vertebrates, and how they may have evolved from nonvertebrate chordates. Here we review recent research on cyclostome neural crest development, including research on lamprey gene regulatory networks and differentiated neural crest fates.

Body wall development in lamprey and a new perspective on the origin of vertebrate paired fins

Classical hypotheses regarding the evolutionary origin of paired appendages propose transformation of precursor structures (gill arches and lateral fin folds) into paired fins. During development, gnathostome paired appendages form as outgrowths of body wall somatopleure, a tissue composed of somatic lateral plate mesoderm (LPM) and overlying ectoderm. In amniotes, LPM contributes connective tissue to abaxial musculature and forms ventrolateral dermis of the interlimb body wall. The phylogenetic distribution of this character is uncertain because lineage analyses of LPM have not been generated in anamniotes. We focus on the evolutionary history of the somatopleure to gain insight into the tissue context in which paired fins first appeared. Lampreys diverged from other vertebrates before the acquisition of paired fins and provide a model for investigating the preappendicular condition. We present vital dye fate maps that suggest the somatopleure is eliminated in lamprey as the LPM is separated from the ectoderm and sequestered to the coelomic linings during myotome extension. We also examine the distribution of postcranial mesoderm in catshark and axolotl. In contrast to lamprey, our findings support an LPM contribution to the trunk body wall of these taxa, which is similar to published data for amniotes. Collectively, these data lead us to hypothesize that a persistent somatopleure in the lateral body wall is a gnathostome synapomorphy, and the redistribution of LPM was a key step in generating the novel developmental module that ultimately produced paired fins. These embryological criteria can refocus arguments on paired fin origins and generate hypotheses testable by comparative studies on the source, sequence, and extent of genetic redeployment.

New perspectives on pharyngeal dorsoventral patterning in development and evolution of the vertebrate jaw

Patterning of the vertebrate facial skeleton involves the progressive partitioning of neural-crest-derived skeletal precursors into distinct subpopulations along the anteroposterior (AP) and dorsoventral (DV) axes. Recent evidence suggests that complex interactions between multiple signaling pathways, in particular Endothelin-1 (Edn1), Bone Morphogenetic Protein (BMP), and Jagged-Notch, are needed to pattern skeletal precursors along the DV axis. Rather than directly determining the morphology of individual skeletal elements, these signals appear to act through several families of transcription factors, including Dlx, Msx, and Hand, to establish dynamic zones of skeletal differentiation. Provocatively, this patterning mechanism is largely conserved from mouse and zebrafish to the jawless vertebrate, lamprey. This implies that the diversification of the vertebrate facial skeleton, including the evolution of the jaw, was driven largely by modifications downstream of a conversed pharyngeal DV patterning program.

Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension

Convergent extension driven by mediolateral intercalation of chondrocytes is a key process that contributes to skeletal growth and morphogenesis. While progress has been made in deciphering the molecular mechanism that underlies this process, the involvement of mechanical load exerted by muscle contraction in its regulation has not been studied. Using the zebrafish as a model system, we found abnormal pharyngeal cartilage morphology in both chemically and genetically paralyzed embryos, demonstrating the importance of muscle contraction for zebrafish skeletal development. The shortening of skeletal elements was accompanied by prominent changes in cell morphology and organization. While in control the cells were elongated, chondrocytes in paralyzed zebrafish were smaller and exhibited a more rounded shape, confirmed by a reduction in their length-to-width ratio. The typical columnar organization of cells was affected too, as chondrocytes in various skeletal elements exhibited abnormal stacking patterns, indicating aberrant intercalation. Finally, we demonstrate impaired chondrocyte intercalation in growth plates of muscle-less Sp(d) mouse embryos, implying the evolutionary conservation of muscle force regulation of this essential morphogenetic process.Our findings provide a new perspective on the regulatory interaction between muscle contraction and skeletal morphogenesis by uncovering the role of muscle-induced mechanical loads in regulating chondrocyte intercalation in two different vertebrate models.

Evolution of the bone gene regulatory network

Current fossil, embryological and genetic data shed light on the evolution of the gene regulatory network (GRN) governing bone formation. The key proteins and genes involved in skeletogenesis are well accepted. We discuss when these essential components of the GRN evolved and propose that the Runx genes, master regulators of skeletogenesis, functioned in early cartilages well before they were co-opted to function in the making of bone. Two rounds of whole genome duplication, together with additional tandem gene duplications, created a genetic substrate for segregation of one GRN into several networks regulating the related tissues of cartilage, bone, enamel, and dentin. During this segregation, Runx2 assumed its position at the top of the bone GRN, and Sox9 was excluded from bone, retaining its ancient role in cartilage.

Developmental and evolutionary significance of the mandibular arch and prechordal/premandibular cranium in vertebrates: revising the heterotopy scenario of gnathostome jaw evolution

The cephalic neural crest produces streams of migrating cells that populate pharyngeal arches and a more rostral, premandibular domain, to give rise to an extensive ectomesenchyme in the embryonic vertebrate head. The crest cells forming the trigeminal stream are the major source of the craniofacial skeleton; however, there is no clear distinction between the mandibular arch and the premandibular domain in this ectomesenchyme. The question regarding the evolution of the gnathostome jaw is, in part, a question about the differentiation of the mandibular arch, the rostralmost component of the pharynx, and in part a question about the developmental fate of the premandibular domain. We address the developmental definition of the mandibular arch in connection with the developmental origin of the trabeculae, paired cartilaginous elements generally believed to develop in the premandibular domain, and also of enigmatic cartilaginous elements called polar cartilages. Based on comparative embryology, we propose that the mandibular arch ectomesenchyme in gnathostomes can be defined as a Dlx1-positive domain, and that the polar cartilages, which develop from the Dlx1-negative premandibular ectomesenchyme, would represent merely posterior parts of the trabeculae. We also show, in the lamprey embryo, early migration of mandibular arch mesenchyme into the premandibular domain, and propose an updated version of the heterotopy theory on the origin of the jaw.

SoxE gene duplication and development of the lamprey branchial skeleton: Insights into development and evolution of the neural crest

SoxE genes are multifunctional transcriptional regulators that play key roles in specification and differentiation of neural crest. Three members (Sox8, Sox9, Sox10) are expressed in the neural crest and are thought to modulate the expression and activity of each other. In addition to regulating the expression of other early neural crest marker genes, SoxE genes are required for development of cartilage. Here we investigated the role of SoxE genes in development of the neural crest-derived branchial skeleton in the sea lamprey. Using a morpholino knockdown approach, we show that all three SoxE genes described in lamprey are required for branchial basket development. Our results suggest that SoxE1 and SoxE2 are required for specification of the chondrogenic neural crest. SoxE3 plays a morphogenetic role in patterning of the branchial basket and may be required for the development of mucocartilage, a tissue unique to larval lampreys. While the lamprey branchial basket develops primarily from an elastin-like major extracellular matrix protein that is specific to lampreys, fibrillar collagen is also expressed in developing branchial cartilage and may be regulated by the lamprey SoxE genes. Our data suggest that the regulation of Type II collagen by Sox9 might have been co-opted by the neural crest in development of the branchial skeleton following the divergence of agnathan and gnathostome vertebrates. Finally, our results also have implications for understanding the independent evolution of duplicated SoxE genes among agnathan and gnathostome vertebrates.

A new mechanistic scenario for the origin and evolution of vertebrate cartilage

The appearance of cellular cartilage was a defining event in vertebrate evolution because it made possible the physical expansion of the vertebrate "new head". Despite its central role in vertebrate evolution, the origin of cellular cartilage has been difficult to understand. This is largely due to a lack of informative evolutionary intermediates linking vertebrate cellular cartilage to the acellular cartilage of invertebrate chordates. The basal jawless vertebrate, lamprey, has long been considered key to understanding the evolution of vertebrate cartilage. However, histological analyses of the lamprey head skeleton suggest it is composed of modern cellular cartilage and a putatively unrelated connective tissue called mucocartilage, with no obvious transitional tissue. Here we take a molecular approach to better understand the evolutionary relationships between lamprey cellular cartilage, gnathostome cellular cartilage, and lamprey mucocartilage. We find that despite overt histological similarity, lamprey and gnathostome cellular cartilage utilize divergent gene regulatory networks (GRNs). While the gnathostome cellular cartilage GRN broadly incorporates Runx, Barx, and Alx transcription factors, lamprey cellular cartilage does not express Runx or Barx, and only deploys Alx genes in certain regions. Furthermore, we find that lamprey mucocartilage, despite its distinctive mesenchymal morphology, deploys every component of the gnathostome cartilage GRN, albeit in different domains. Based on these findings, and previous work, we propose a stepwise model for the evolution of vertebrate cellular cartilage in which the appearance of a generic neural crest-derived skeletal tissue was followed by a phase of skeletal tissue diversification in early agnathans. In the gnathostome lineage, a single type of rigid cellular cartilage became dominant, replacing other skeletal tissues and evolving via gene cooption to become the definitive cellular cartilage of modern jawed vertebrates.

Evidence for the prepattern/cooption model of vertebrate jaw evolution

The appearance of jaws was a turning point in vertebrate evolution because it allowed primitive vertebrates to capture and process large, motile prey. The vertebrate jaw consists of separate dorsal and ventral skeletal elements connected by a joint. How this structure evolved from the unjointed gill bar of a jawless ancestor is an unresolved question in vertebrate evolution. To understand the developmental bases of this evolutionary transition, we examined the expression of 12 genes involved in vertebrate pharyngeal patterning in the modern jawless fish lamprey. We find nested expression of Dlx genes, as well as combinatorial expression of Msx, Hand and Gsc genes along the dorso-ventral (DV) axis of the lamprey pharynx, indicating gnathostome-type pharyngeal patterning evolved before the appearance of the jaw. In addition, we find that Bapx and Gdf5/6/7, key regulators of joint formation in gnathostomes, are not expressed in the lamprey first arch, whereas Barx, which is absent from the intermediate first arch in gnathostomes, marks this domain in lamprey. Taken together, these data support a new scenario for jaw evolution in which incorporation of Bapx and Gdf5/6/7 into a preexisting DV patterning program drove the evolution of the jaw by altering the identity of intermediate first-arch chondrocytes. We present this "Pre-pattern/Cooption" model as an alternative to current models linking the evolution of the jaw to the de novo appearance of sophisticated pharyngeal DV patterning.