Timing the developmental origins of mammalian limb diversity

Adult caudal fin shape is imprinted in the embryonic fin fold

Appendage shape is formed during development (and re-formed during regeneration) according to spatial and temporal cues that orchestrate local cellular morphogenesis. The caudal fin is the primary appendage used for propulsion in most fish species, and exhibits a range of distinct morphologies adapted for different swimming strategies, however the molecular mechanisms responsible for generating these diverse shapes remain mostly unknown. In zebrafish, caudal fins display a forked shape, with longer supportive bony rays at the periphery and shortest rays at the center. Here, we show that a premature, transient pulse of sonic hedgehog a (shha) overexpression during late embryonic development results in excess proliferation and growth of the central rays, causing the adult caudal fin to grow into a triangular, truncate shape. Both global and regional ectopic shha overexpression are sufficient to alter fin shape, and forked shape may be rescued by subsequent treatment with an antagonist of the canonical Shh pathway. The induced truncate fins show a decreased fin ray number and fail to form the hypural diastema that normally separates the dorsal and ventral fin lobes. While forked fins regenerate their original forked morphology, truncate fins regenerate truncate, suggesting that positional memory of the fin rays can be permanently altered by a transient treatment during embryogenesis. Ray finned fish have evolved a wide spectrum of caudal fin morphologies, ranging from truncate to forked, and the current work offers insights into the developmental mechanisms that may underlie this shape diversity.

Resolving the mechanisms underlying epithelial-to-mesenchymal transition of the lateral plate mesoderm

Formation of the vertebrate limb buds begins with a localized epithelial-to-mesenchymal transition (EMT) of the somatic lateral plate mesoderm (LPM). While the processes that drive proliferation and outgrowth of the limb mesenchyme are well established, the fundamental mechanisms that precede this process and initiate EMT are less understood. In this review, we outline putative drivers of EMT of the LPM, drawing from analyses across a range of vertebrates and developmental models. We detail the expression patterns of key EMT transcriptional regulators in the somatic LPM of the presumptive limb fields, and their potential role in producing a mesenchymal cell fate. These include a putative cooperative role between the EMT inducers PRRX1 and TWIST1, supported by evidence in zebrafish and chicken models but unconfirmed data from mice. As such, additional functional data are required to definitively determine the mechanisms that initiate and drive EMT of the somatic LPM, a critical transition preceding formation of the limb bud mesenchyme.

Distinct patterning responses of wing and leg neuromuscular systems to different preaxial polydactylies

The tetrapod limb has long served as a paradigm to study vertebrate pattern formation and evolutionary diversification. The distal part of the limb, the so-called autopod, is of particular interest in this regard, given the numerous modifications in both its morphology and behavioral motor output. While the underlying alterations in skeletal form have received considerable attention, much less is known about the accompanying changes in the neuromuscular system. However, modifications in the skeleton need to be properly integrated with both muscle and nerve patterns, to result in a fully functional limb. This task is further complicated by the distinct embryonic origins of the three main tissue types involved-skeleton, muscles and nerves-and, accordingly, how they are patterned and connected with one another during development. To evaluate the degree of regulative crosstalk in this complex limb patterning process, here we analyze the developing limb neuromuscular system of Silkie breed chicken. These animals display a preaxial polydactyly, due to a polymorphism in the limb regulatory region of the Sonic Hedgehog gene. Using lightsheet microscopy and 3D-reconstructions, we investigate the neuromuscular patterns of extra digits in Silkie wings and legs, and compare our results to Retinoic Acid-induced polydactylies. Contrary to previous findings, Silkie autopod muscle patterns do not adjust to alterations in the underlying skeletal topology, while nerves show partial responsiveness. We discuss the implications of tissue-specific sensitivities to global limb patterning cues for our understanding of the evolution of novel forms and functions in the distal tetrapod limb.

Mammalian forelimb evolution is driven by uneven proximal-to-distal morphological diversity

Vertebrate limb morphology often reflects the environment due to variation in locomotor requirements. However, proximal and distal limb segments may evolve differently from one another, reflecting an anatomical gradient of functional specialization that has been suggested to be impacted by the timing of development. Here, we explore whether the temporal sequence of bone condensation predicts variation in the capacity of evolution to generate morphological diversity in proximal and distal forelimb segments across more than 600 species of mammals. Distal elements not only exhibit greater shape diversity, but also show stronger within-element integration and, on average, faster evolutionary responses than intermediate and upper limb segments. Results are consistent with the hypothesis that late developing distal bones display greater morphological variation than more proximal limb elements. However, the higher integration observed within the autopod deviates from such developmental predictions, suggesting that functional specialization plays an important role in driving within-element covariation. Proximal and distal limb segments also show different macroevolutionary patterns, albeit not showing a perfect proximo-distal gradient. The high disparity of the mammalian autopod, reported here, is consistent with the higher potential of development to generate variation in more distal limb structures, as well as functional specialization of the distal elements.

Development and function explain the modular evolution of phalanges in gecko lizards

Selective regimes favouring the evolution of functional specialization probably affect covariation among phenotypic traits. Phalanges of most tetrapods develop from a conserved module that constrains their relative proportions. In geckos, however, biomechanical specializations associated with adhesive toepads involve morphological variation in the autopodium and might reorganize such modular structures. We tested two hypotheses to explain the modular architecture of hand bones in geckos, one based on developmental interactions and another incorporating functional associations related to locomotion, and compared the empirical support for each hypothetical module between padded and padless lineages. We found strong evidence for developmental modules in most species, which probably reflects embryological constraints during phalangeal formation. Although padded geckos exhibit a functional specialization involving the hyperextension of the distal phalanges that is absent in padless species, the padless species are the ones that show a distal functional module with high integration. Some ancestrally padless geckos apparently deviate from developmental predictions and present a relatively weak developmental module of phalanges and a strongly integrated distal module, which may reflect selective regimes involving incipient frictional adhesion in digit morphology. Modularity of digit elements seems dynamic along the evolutionary history of geckos, being associated with the presence/absence of adhesive toepads.

Ossification pattern of the unusual pisiform in two-toed (Choloepus) and three-toed sloths (Bradypus)

Two-toed (Choloepus sp.) and three-toed (Bradypus sp.) sloths possess short, rounded pisiforms that are rare among mammals and differ from other members of Xenarthra like the giant anteater (Myrmecophaga tridactyla) which retain elongated, rod-like pisiforms in common with most mammals. Using photographs, radiographs, and μCT, we assessed ossification patterns in the pisiform and the paralogous tarsal, the calcaneus, for two-toed sloths, three-toed sloths, and giant anteaters to determine the process by which pisiform reduction occurs in sloths and compare it to other previously studied examples of pisiform reduction in humans and orangutans. Both extant sloth genera achieve pisiform reduction through the loss of a secondary ossification center and the likely disruption of the associated growth plate based on an unusually porous subchondral surface. This represents a third unique mechanism of pisiform reduction among mammals, along with primary ossification center loss in humans and retention of two ossification centers with likely reduced growth periods in orangutans. Given the remarkable similarities between two-toed and three-toed sloth pisiform ossification patterns and the presence of pisiform reduction in fossil sloths, extant sloth pisiform morphology does not appear to represent a recent convergent adaptation to suspensory locomotion, but instead is likely to be an ancestral trait of Folivora that emerged early in the radiation of extant and fossil sloths.

Non-model systems in mammalian forelimb evo-devo

Mammal forelimbs are highly diverse, ranging from the elongated wing of a bat to the stout limb of the mole. The mammal forelimb has been a long-standing system for the study of early developmental patterning, proportional variation, shape change, and the reduction of elements. However, most of this work has been performed in mice, which neglects the wide variation present across mammal forelimbs. This review emphasizes the critical role of non-model systems in limb evo-devo and highlights new emerging models and their potential. We discuss the role of gene networks in limb evolution, and touch on functional analyses that lay the groundwork for further developmental studies. Mammal limb evo-devo is a rich field, and here we aim to synthesize the findings of key recent works and the questions to which they lead.

Regulation of vertebrate forelimb development and wing reduction in the flightless emu

The vertebrate limb is a dynamic structure which has evolved into many diverse forms to facilitate complex behavioral adaptations. The principle molecular and cellular processes that underlie development of the vertebrate limb are well characterized. However, how these processes are altered to drive differential limb development between vertebrates is less well understood. Several vertebrate models are being utilized to determine the developmental basis of differential limb morphogenesis, though these typically focus on later patterning of the established limb bud and may not represent the complete developmental trajectory. Particularly, heterochronic limb development can occur prior to limb outgrowth and patterning but receives little attention. This review summarizes the genetic regulation of vertebrate forelimb diversity, with particular focus on wing reduction in the flightless emu as a model for examining limb heterochrony. These studies highlight that wing reduction is complex, with heterochronic cellular and genetic events influencing the major stages of limb development. Together, these studies provide a broader picture of how different limb morphologies may be established during development.

Evidence of five digits in embryonic horses and developmental stabilization of tetrapod digit number

Previous work comparing the developmental mechanisms involved in digit reduction in horses with other mammals reported that horses have only a 'single digit', with two flanking metapodials identified as remnants of digit II and IV. Here we show that early Equus embryos go through a stage with five digit condensations, and that the flanking splint metapodials result from fusions of the two anterior digits I and II and the two posterior digits IV and V, in a striking parallel between ontogeny and phylogeny. Given that even this most extreme case of digit reduction exhibits primary pentadactyly, we re-examined the initial stages of digit condensation of all digit-reduced tetrapods where data are available and found that in all cases, five or four digits initiate (four with digit I missing). The persistent pentadactyl initiation in the horse and other digit-reduced modern taxa underscores a durable developmental stability at the initiation of digits. The digit evodevo model may help illuminate the biological circumstances under which organ systems become highly stabilized versus highly plastic.

Gene expression across mammalian organ development

The evolution of gene expression in mammalian organ development remains largely uncharacterized. Here we report the transcriptomes of seven organs (cerebrum, cerebellum, heart, kidney, liver, ovary and testis) across developmental time points from early organogenesis to adulthood for human, rhesus macaque, mouse, rat, rabbit, opossum and chicken. Comparisons of gene expression patterns identified correspondences of developmental stages across species, and differences in the timing of key events during the development of the gonads. We found that the breadth of gene expression and the extent of purifying selection gradually decrease during development, whereas the amount of positive selection and expression of new genes increase. We identified differences in the temporal trajectories of expression of individual genes across species, with brain tissues showing the smallest percentage of trajectory changes, and the liver and testis showing the largest. Our work provides a resource of developmental transcriptomes of seven organs across seven species, and comparative analyses that characterize the development and evolution of mammalian organs.

Pigeon foot feathering reveals conserved limb identity networks

The tetrapod limb is a stunning example of evolutionary diversity, with dramatic variation not only among distantly related species, but also between the serially homologous forelimbs (FLs) and hindlimbs (HLs) within species. Despite this variation, highly conserved genetic and developmental programs underlie limb development and identity in all tetrapods, raising the question of how limb diversification is generated from a conserved toolkit. In some breeds of domestic pigeon, shifts in the expression of two conserved limb identity transcription factors, PITX1 and TBX5, are associated with the formation of feathered HLs with partial FL identity. To determine how modulation of PITX1 and TBX5 expression affects downstream gene expression, we compared the transcriptomes of embryonic limb buds from pigeons with scaled and feathered HLs. We identified a set of differentially expressed genes enriched for genes encoding transcription factors, extracellular matrix proteins, and components of developmental signaling pathways with important roles in limb development. A subset of the genes that distinguish scaled and feathered HLs are also differentially expressed between FL and scaled HL buds in pigeons, pinpointing a set of gene expression changes downstream of PITX1 and TBX5 in the partial transformation from HL to FL identity. We extended our analyses by comparing pigeon limb bud transcriptomes to chicken, anole lizard, and mammalian datasets to identify deeply conserved PITX1- and TBX5-responsive components of the limb identity program. Our analyses reveal a suite of predominantly low-level gene expression changes that are conserved across amniotes to regulate the identity of morphologically distinct limbs.

Unexpected evolutionary patterns of dental ontogenetic traits in cetartiodactyl mammals

Studying ontogeny in both extant and extinct species can unravel the mechanisms underlying mammal diversification and specialization. Among mammalian clades, Cetartiodactyla encompass species with a wide range of adaptations, and ontogenetic evidence could clarify longstanding debates on the origins of modern specialized families. Here, we study the evolution of dental eruption patterns in early diverging cetartiodactyls to assess the ecological and biological significance of this character and shed new light on phylogenetic issues. After investigation of the ontogenetic dental series of 63 extinct genera, our parsimony reconstructions of eruption state evolution suggest that the eruption of molars before permanent premolars represents a plesiomorphic condition within Cetartiodactyla. This result substantially differs from a previous study based on modern species only. As a result, the presence of this pattern in most ruminants might represent an ancestral condition contributing to their specialized herbivory, rather than an original adaptation. In contrast, the late eruption of molars in hippopotamoids is more likely related to biological aspects, such as increases in body mass and slower pace of life. Our study mainly shows that eruption sequences reliably characterize higher level cetartiodactyl taxa and could represent a new source of phylogenetic characters, especially to disentangle the origin of hippopotamoids and cetaceans.