Evolution Of Developmental Mechanisms Research Articles

The early embryonic transcriptome of a Hawaiian Drosophila picture-wing fly shows evidence of altered gene expression and novel gene evolution.

A massive adaptive radiation on the Hawaiian archipelago has produced approximately one-quarter of the fly species in the family Drosophilidae. The Hawaiian Drosophila clade has long been recognized as a model system for the study of both the ecology of island endemics and the evolution of developmental mechanisms, but relatively few genomic and transcriptomic datasets are available for this group. We present here a differential expression analysis of the transcriptional profiles of two highly conserved embryonic stages in the Hawaiian picture-wing fly Drosophila grimshawi. When we compared our results to previously published datasets across the family Drosophilidae, we identified cases of both gains and losses of gene representation in D. grimshawi, including an apparent delay in Hox gene activation. We also found a high expression of unannotated genes. Most transcripts of unannotated genes with open reading frames do not have identified homologs in non-Hawaiian Drosophila species, although the vast majority have sequence matches in genomes of other Hawaiian picture-wing flies. Some of these unannotated genes may have arisen from noncoding sequence in the ancestor of Hawaiian flies or during the evolution of the clade. Our results suggest that both the modified use of ancestral genes and the evolution of new ones may occur in rapid radiations.

Evolution and patterning of the ovule in seed plants.

The ovule and its developmental successor, the seed, together represent a highly characteristic feature of seed plants that has strongly enhanced the reproductive and dispersal potential of this diverse group of taxa. Ovules encompass multiple tissues that perform various roles within a highly constrained space, requiring a complex cascade of genes that generate localized cell proliferation and programmed cell death during different developmental stages. Many heritable morphological differences among lineages reflect relative displacement of these tissues, but others, such as the second (outer) integuments of angiosperms and Gnetales, represent novel and apparently profound and independent innovations. Recent studies, mostly on model taxa, have considerably enhanced our understanding of gene expression in the ovule. However, understanding its evolutionary history requires a comparative and phylogenetic approach that is problematic when comparing extant angiosperms not only with phylogenetically distant extant gymnosperms but also with taxa known only from fossils. This paper reviews ovule characters across a phylogenetically broad range of seed plants in a dynamic developmental context. It discusses both well-established and recent theories of ovule and seed evolution and highlights potential gaps in comparative data that will usefully enhance our understanding of evolutionary transitions and developmental mechanisms.

Anatomical correlates of face patches in macaque inferotemporal cortex

Primate brains typically have regions within the ventral visual stream that are selectively responsive to faces. In macaques, these face patches are located in similar parts of inferotemporal cortex across individuals although correspondence with particular anatomical features has not been reported previously. Here, using high-resolution functional and anatomical imaging, we show that small "bumps," or buried gyri, along the lower bank of the superior temporal sulcus are predictive of the location of face-selective regions. Recordings from implanted multielectrode arrays verified that these bumps contain face-selective neurons. These bumps were present in monkeys raised without seeing faces and that lack face patches, indicating that these anatomical landmarks are predictive of, but not sufficient for, the presence of face selectivity. These bumps are found across primate species that span taxonomy lines, indicating common evolutionary developmental mechanisms. The bumps emerge during fetal development in macaques, indicating that they arise from general developmental mechanisms that result in the regularity of cortical folding of the entire brain.

Heterochronic Developmental Shifts Underlying Squamate Cerebellar Diversity Unveil the Key Features of Amniote Cerebellogenesis.

Despite a remarkable conservation of architecture and function, the cerebellum of vertebrates shows extensive variation in morphology, size, and foliation pattern. These features make this brain subdivision a powerful model to investigate the evolutionary developmental mechanisms underlying neuroanatomical complexity both within and between anamniote and amniote species. Here, we fill a major evolutionary gap by characterizing the developing cerebellum in two non-avian reptile species—bearded dragon lizard and African house snake—representative of extreme cerebellar morphologies and neuronal arrangement patterns found in squamates. Our data suggest that developmental strategies regarded as exclusive hallmark of birds and mammals, including transit amplification in an external granule layer (EGL) and Sonic hedgehog expression by underlying Purkinje cells (PCs), contribute to squamate cerebellogenesis independently from foliation pattern. Furthermore, direct comparison of our models suggests the key importance of spatiotemporal patterning and dynamic interaction between granule cells and PCs in defining cortical organization. Especially, the observed heterochronic shifts in early cerebellogenesis events, including upper rhombic lip progenitor activity and EGL maintenance, are strongly expected to affect the dynamics of molecular interaction between neuronal cell types in snakes. Altogether, these findings help clarifying some of the morphogenetic and molecular underpinnings of amniote cerebellar corticogenesis, but also suggest new potential molecular mechanisms underlying cerebellar complexity in squamates. Furthermore, squamate models analyzed here are revealed as key animal models to further understand mechanisms of brain organization.

Evolution and development of three highly specialized floral structures of bee-pollinated Phalaenopsis species

BackgroundVariation in shape and size of many floral organs is related to pollinators. Evolution of such organs is driven by duplication and modification of MADS-box and MYB transcription factors. We applied a combination of micro-morphological (SEM and micro 3D-CT scanning) and molecular techniques (transcriptome and RT-PCR analysis) to understand the evolution and development of the callus, stelidia and mentum, three highly specialized floral structures of orchids involved in pollination. Early stage and mature tissues were collected from flowers of the bee-pollinated Phalaenopsis equestris and Phalaenopsis pulcherrima, two species that differ in floral morphology: P. equestris has a large callus but short stelidia and no mentum, whereas P. pulcherrima has a small callus, but long stelidia and a pronounced mentum.ResultsOur results show the stelidia develop from early primordial stages, whereas the callus and mentum develop later. In combination, the micro 3D-CT scan analysis and gene expression analyses show that the callus is of mixed petaloid-staminodial origin, the stelidia of staminodial origin, and the mentum of mixed sepaloid-petaloid-staminodial origin. SEP clade 1 copies are expressed in the larger callus of P. equestris, whereas AP3 clade 1 and AGL6 clade 1 copies are expressed in the pronounced mentum and long stelidia of P. pulcherrima. AP3 clade 4, PI-, AGL6 clade 2 and PCF clade 1 copies might have a balancing role in callus and gynostemium development. There appears to be a trade-off between DIV clade 2 expression with SEP clade 1 expression in the callus, on the one hand, and with AP3 clade 1 and AGL6 clade 1 expression in the stelidia and mentum on the other.ConclusionsWe detected differential growth and expression of MADS box AP3/PI-like, AGL6-like and SEP-like, and MYB DIV-like gene copies in the callus, stelidia and mentum of two species of Phalaenopsis, of which these floral structures are very differently shaped and sized. Our study provides a first glimpse of the evolutionary developmental mechanisms driving adaptation of Phalaenopsis flowers to different pollinators by providing combined micro-morphological and molecular evidence for a possible sepaloid–petaloid–staminodial origin of the orchid mentum.

Developmental constraints on fin diversity.

The fish fin is a breathtaking repository full of evolutionary diversity, novelty, and convergence. Over 500 million years, the adaptation to novel habitats has provided landscapes of fin diversity. Although comparative anatomy of evolutionarily divergent patterns over centuries has highlighted the fundamental architectures and evolutionary trends of fins, including convergent evolution, the developmental constraints on fin evolution, which bias the evolutionary trajectories of fin morphology, largely remain elusive. Here, we review the evolutionary history, developmental mechanisms, and evolutionary underpinnings of paired fins, illuminating possible developmental constraints on fin evolution. Our compilation of anatomical and genetic knowledge of fin development sheds light on the canalized and the unpredictable aspects of fin shape in evolution. Leveraged by an arsenal of genomic and genetic tools within the working arena of spectacular fin diversity, evolutionary developmental biology embarks on the establishment of conceptual framework for developmental constraints, previously enigmatic properties of evolution.

Transgenesis in Hydra to characterize gene function and visualize cell behavior.

The freshwater polyp Hydra is a cnidarian used as a model organism in a number of fields, including the study of the origin and evolution of developmental mechanisms, aging, symbiosis and host-microbe interactions. Here, we describe a procedure for the establishment of stable transgenic Hydra lines by embryo microinjection. The three-stage protocol comprises (i) the design and preparation of a transgenic construct, (ii) the microinjection of the vector into early embryos of Hydra vulgaris, and (iii) the selection and enrichment of mosaic animals in order to develop uniformly transgenic clonal lines. The preparation of a transgenic construct requires ~2 weeks, and transgenic lines can be obtained within 3 months. The method allows constitutive or inducible gain- and loss-of-function approaches, as well as in vivo tracing of individual cells. Hydra polyps carrying transgenic cells reveal functional properties of the ancestral circuitry controlling animal development.

A Morphological Integration Perspective on the Evolution of Dimorphism among Sexes and Social Insect Castes.

Many species have evolved alternate phenotypes, thus enabling individuals to conditionally produce phenotypes that are favorable for reproductive success. Examples of this phenomenon include sexual dimorphism, alternative reproductive strategies, and social insect castes. While the evolutionary functions and developmental mechanisms of dimorphic phenotypes have been studied extensively, little attention has focused on the evolutionary covariance between each phenotype. We extend the conceptual framework and methods of morphological integration to hypothesize that dimorphic traits tend to be less integrated between sexes or social castes. In the case of social insects, we describe results from our recent study of an ant genus in which workers have major and minor worker castes that perform different behavioral repertoires in and around the nest. In the case of birds, we describe a new analysis of a family of songbirds that exhibits plumage coloration that can differ greatly between males and females, with apparently independent changes in each sex. Ant head shape, which is highly specialized in each worker caste, was weakly integrated between worker castes, whereas thorax shape, which is more monomorphic, was tightly integrated. Similarly, in birds, we found a negative association between dimorphism and the degree of integration between sexes. We also found that integration decreased in fairy wrens (Malurus) for many feather patches that evolved greater dichromatism. Together, this suggests that the process of evolving increased dimorphism results in a decrease in integration between sexes and social castes. We speculate that once a mechanism for dimorphism evolves, that mechanism can create independent variation in one sex or caste upon which selection may act.

Evolutionary developmental mechanisms underlying the origin and diversification of the fruits

The origin and diversity of fruits are the results of the long-term adaptation of plants to terrestrial ecosystems, which is closely related to plant reproduction and adaptation to various living environments through seed dispersal. This evolutionary phenomenon and process have resulted in the origin and great prosperity of angiosperms. Dissecting the mechanisms underlying fruit origin, diversity and adaption is one of the core scientific issues in evolutionary biology. In this review, we summarized the main progress in evolutionary developmental genetics of fruit morphology (size and shape), morphological novelty of fruit accessory structures, and fruit types. The research directions and topics of future work in this field are also proposed.

Modular cis-regulatory logic of yellow gene expression in silkmoth larvae.

Colour patterns in butterflies and moths are crucial traits for adaptation. Previous investigations have highlighted genes responsible for pigmentation (ie yellow and ebony). However, the mechanisms by which these genes are regulated in lepidopteran insects remain poorly understood. To elucidate this, molecular studies involving dipterans have largely analysed the cis‐regulatory regions of pigmentation genes and have revealed cis‐regulatory modularity. Here, we used well‐developed transgenic techniques in Bombyx mori and demonstrated that cis‐regulatory modularity controls tissue‐specific expression of the yellow gene. We first identified which body parts are regulated by the yellow gene via black pigmentation. We then isolated three discrete regulatory elements driving tissue‐specific gene expression in three regions of B. mori larvae. Finally, we found that there is no apparent sequence conservation of cis‐regulatory regions between B. mori and Drosophila melanogaster, and no expression driven by the regulatory regions of one species when introduced into the other species. Therefore, the trans‐regulatory landscapes of the yellow gene differ significantly between the two taxa. The results of this study confirm that lepidopteran species use cis‐regulatory modules to control gene expression related to pigmentation, and represent a powerful cadre of transgenic tools for studying evolutionary developmental mechanisms.

Homology, neocortex, and the evolution of developmental mechanisms.

The six-layered neocortex of the mammalian pallium has no clear homolog in birds or non-avian reptiles. Recent research indicates that although these extant amniotes possess a variety of divergent and nonhomologous pallial structures, they share a conserved set of neuronal cell types and circuitries. These findings suggest a principle of brain evolution: that natural selection preferentially preserves the integrity of information-processing pathways, whereas other levels of biological organization, such as the three-dimensional architectures of neuronal assemblies, are less constrained. We review the similarities of pallial neuronal cell types in amniotes, delineate candidate gene regulatory networks for their cellular identities, and propose a model of developmental evolution for the divergence of amniote pallial structures.

Evolution of caudal fin ray development and caudal fin hypural diastema complex in spotted gar, teleosts, and other neopterygian fishes.

The caudal fin of actinopterygians transitioned from a heterocercal dorsoventrally asymmetrical fin to a homocercal externally symmetrical fin in teleosts through poorly understood evolutionary developmental mechanisms. We studied the caudal skeleton of major living actinopterygian lineages, including polypteriformes, acipenseriformes, Holostei (gars and bowfin), and teleosts, compared with reports of extinct neopterygians and basal teleosteans. We focused on the hypural diastema complex, which includes (1) a gap between hypurals 2 and 3, that (2) separates two plates of connective tissue at (3) the branching of caudal vasculature; these features had been considered as a shared, derived trait of teleosts, a synapomorphy. These studies revealed that gars and teleosts share all three features of the hypural diastema complex. Absence of a complex with these features from bowfin, fossil Holostei, and stem Teleostei argues in favor of repetitive, independent emergence in several neopterygian and basal Teleostei lineages, or less likely, many independent losses. We further observed that, in gars and teleosts, the earliest developing lepidotrichia align with the horizontal adult body axis, thus participating in external symmetry. These results suggest that the hypural diastema complex in teleosts and gars represents a homoplasy among neopterygians and that it emerged repeatedly by parallel evolution due to shared inherited underlying genetic and developmental programs (latent homology). Because the hypural diastema complex exists in gars with heterocercal tails, this complex is independent of homocercality. Developmental Dynamics 247:832-853, 2018. © 2018 Wiley Periodicals, Inc.

The evo-devo of plant speciation.

Speciation research bridges the realms of macro- and microevolution. Evolutionary developmental biology (evo-devo) has classically dealt with macroevolutionary questions through a comparative approach to distantly related organisms, but the field later broadened in focus to address recent speciation and microevolution. Here we review available evidence of the power of evo-devo approaches to understand speciation in plants at multiple scales. At a macroevolutionary scale, evidence is accumulating for evolutionary developmental mechanisms giving rise to key innovations promoting speciation. At the macro microevolution transition, we review instances of evo-devo change underlying both the origin of reproductive barriers and phenotypic changes distinguishing closely related species. At the microevolutionary scale, the study of developmental variation within species provides insight into the processes that generate the raw material for evolution and speciation. We conclude by advocating a strong interaction between developmental biology and evolutionary biology at multiple scales to gain a deeper understanding of plant speciation.

Sensory evolution in blind cavefish is driven by early embryonic events during gastrulation and neurulation.

Natural variations in sensory systems constitute adaptive responses to the environment. Here, we compared sensory placode development in the blind cave-adapted morph and the eyed river-dwelling morph of Astyanax mexicanus Focusing on the lens and olfactory placodes, we found a trade-off between these two sensory components in the two morphs: from neural plate stage onwards, cavefish have larger olfactory placodes and smaller lens placodes. In a search for developmental mechanisms underlying cavefish sensory evolution, we analyzed the roles of Shh, Fgf8 and Bmp4 signaling, which are known to be fundamental in patterning the vertebrate head and are subtly modulated in space and time during cavefish embryogenesis. Modulating these signaling systems at the end of gastrulation shifted the balance toward a larger olfactory derivative. Olfactory tests to assess potential behavioral outcomes of such developmental evolution revealed that Astyanax cavefish are able to respond to a 105-fold lower concentration of amino acids than their surface-dwelling counterparts. We suggest that similar evolutionary developmental mechanisms may be used throughout vertebrates to drive adaptive sensory specializations according to lifestyle and habitat.

Evolution, comparative biology and ontogeny of vertebrate heart regeneration.

There are 64,000 living species of vertebrates on our planet and all of them have a heart. Comparative analyses devoted to understanding the regenerative potential of the myocardium have been performed in a dozen vertebrate species with the aim of developing regenerative therapies for human heart disease. Based on this relatively small selection of animal models, important insights into the evolutionary conservation of regenerative mechanisms have been gained. In this review, we survey cardiac regeneration studies in diverse species to provide an evolutionary context for the lack of regenerative capacity in the adult mammalian heart. Our analyses highlight the importance of cardiac adaptations that have occurred over hundreds of millions of years during the transition from aquatic to terrestrial life, as well as during the transition from the womb to an oxygen-rich environment at birth. We also discuss the evolution and ontogeny of cardiac morphological, physiological and metabolic adaptations in the context of heart regeneration. Taken together, our findings suggest that cardiac regenerative potential correlates with a low-metabolic state, the inability to regulate body temperature, low heart pressure, hypoxia, immature cardiomyocyte structure and an immature immune system. A more complete understanding of the evolutionary context and developmental mechanisms governing cardiac regenerative capacity would provide stronger scientific foundations for the translation of cardiac regeneration therapies into the clinic.

On the evolution of developmental mechanisms: Divergent polarities in leaf growth as a case study

ABSTRACTMost model plants used to study leaf growth share a common developmental mechanism, namely basipetal growth polarity, wherein the distal end differentiates first with progressively more proliferative cells toward the base. Therefore, this base-to-tip growth pattern has served as a paradigm to explain leaf growth and also formed the basis for several computational models. However, our recent report in The Plant Cell on the investigation of leaf growth in 75 eudicot species covering a wide range of eudicot families showed that leaves grow with divergent polarities in the proximo-distal axis or without any obvious polarity. This divergence in growth polarity is linked to the expression divergence of a conserved microRNA-transcription factor module. This study raises several questions on the evolutionary origin of leaf growth pattern, such as ‘when and why in evolution did the divergent growth polarities arise?’ and ‘what were the molecular changes that led to this divergence?’. Here, we discuss a few of these questions in some detail.

Life habits, hox genes, and affinities of a 311 million-year-old holometabolan larva.

BackgroundHolometabolous insects are the most diverse, speciose and ubiquitous group of multicellular organisms in terrestrial and freshwater ecosystems. The enormous evolutionary and ecological success of Holometabola has been attributed to their unique postembryonic life phases in which nonreproductive and wingless larvae differ significantly in morphology and life habits from their reproductive and mostly winged adults, separated by a resting stage, the pupa. Little is known of the evolutionary developmental mechanisms that produced the holometabolous larval condition and their Paleozoic origin based on fossils and phylogeny.ResultsWe provide a detailed anatomic description of a 311 million-year-old specimen, the oldest known holometabolous larva, from the Mazon Creek deposits of Illinois, U.S.A. The head is ovoidal, downwardly oriented, broadly attached to the anterior thorax, and bears possible simple eyes and antennae with insertions encircled by molting sutures; other sutures are present but often indistinct. Mouthparts are generalized, consisting of five recognizable segments: a clypeo-labral complex, mandibles, possible hypopharynx, a maxilla bearing indistinct palp-like appendages, and labium. Distinctive mandibles are robust, triangular, and dicondylic. The thorax is delineated into three, nonoverlapping regions of distinctive surface texture, each with legs of seven elements, the terminal-most bearing paired claws. The abdomen has ten segments deployed in register with overlapping tergites; the penultimate segment bears a paired, cercus-like structure. The anterior eight segments bear clawless leglets more diminutive than the thoracic legs in length and cross-sectional diameter, and inserted more ventrolaterally than ventrally on the abdominal sidewall.ConclusionsSrokalarva berthei occurred in an evolutionary developmental context likely responsible for the early macroevolutionary success of holometabolous insects. Srokalarva berthei bore head and prothoracic structures, leglet series on successive abdominal segments – in addition to comparable features on a second taxon eight million-years-younger – that indicates Hox-gene regulation of segmental and appendage patterning among earliest Holometabola. Srokalarva berthei body features suggest a caterpillar-like body plan and head structures indicating herbivory consistent with known, contemporaneous insect feeding damage on seed plants. Taxonomic resolution places Srokalarva berthei as an extinct lineage, apparently possessing features closer to neuropteroid than other holometabolous lineages.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0428-8) contains supplementary material, which is available to authorized users.

Discovery of germline-related genes in Cephalochordate amphioxus: A genome wide survey using genome annotation and transcriptome data

The generation of germline cells is a critical process in the reproduction of multicellular organisms. Studies in animal models have identified a common repertoire of genes that play essential roles in primordial germ cell (PGC) formation. However, comparative studies also indicate that the timing and regulation of this core genetic program vary considerably in different animals, raising the intriguing questions regarding the evolution of PGC developmental mechanisms in metazoans. Cephalochordates (commonly called amphioxus or lancelets) represent one of the invertebrate chordate groups and can provide important information about the evolution of developmental mechanisms in the chordate lineage. In this study, we used genome and transcriptome data to identify germline-related genes in two distantly related cephalochordate species, Branchiostoma floridae and Asymmetron lucayanum. Branchiostoma and Asymmetron diverged more than 120 MYA, and the most conspicuous difference between them is their gonadal morphology. We used important germline developmental genes in several model animals to search the amphioxus genome and transcriptome dataset for conserved homologs. We also annotated the assembled transcriptome data using Gene Ontology (GO) terms to facilitate the discovery of putative genes associated with germ cell development and reproductive functions in amphioxus. We further confirmed the expression of 14 genes in developing oocytes or mature eggs using whole mount in situ hybridization, suggesting their potential functions in amphioxus germ cell development. The results of this global survey provide a useful resource for testing potential functions of candidate germline-related genes in cephalochordates and for investigating differences in gonad developmental mechanisms between Branchiostoma and Asymmetron species.

Evolutionary variations in the expression of dorso-ventral patterning genes and the conservation of pioneer neurons in Tribolium castaneum

Insects are ideally suited for gaining insight into the evolutionary developmental mechanisms that have led to adaptive changes of the nervous system since the specific structure of the nervous system can be directly linked to the neural stem cell (neuroblast) lineages, which in turn can be traced back to the last common ancestor of insects. The recent comparative analysis of the Drosophila melanogaster and Tribolium castaneum neuroblast maps revealed substantial differences in the expression profiles of neuroblasts. Here we show that despite the overall conservation of the dorso-ventral expression domains of muscle segment homeobox, intermediate neuroblasts defective and ventral nervous system defective, the expression of these genes relative to the neuroblasts in the respective domains has changed considerably during insect evolution. Furthermore, functional studies show evolutionary changes in the requirement of ventral nervous system defective in the formation of neuroblast 1-1 and the correct differentiation of its presumptive progeny, the pioneer neurons aCC and pCC. The inclusion of the expression data of the dorso-ventral genes into the recently established T. castaneum neuroblast map further increases the differences in the neuroblast expression profiles between D. melanogaster and T. castaneum. Despite these molecular variations, the Even-skipped positive pioneer neurons show an invariant arrangement, except for an additional Even-skipped positive cluster that we discovered in T. castaneum. Given the importance of these pioneer neurons in establishing the intersegmental nerves and the longitudinal tracts, which are part of the conserved axonal scaffold of arthropods, we discuss internal buffering mechanisms that might ensure that neuroblast lineages invariantly generate pioneer neurons over a wide range of molecular variations.

Two waves of anisotropic growth generate enlarged follicles in the spiny mouse.

BackgroundMammals exhibit a remarkable variety of phenotypes and comparative studies using novel model species are needed to uncover the evolutionary developmental mechanisms generating this diversity. Here, we undertake a developmental biology and numerical modeling approach to investigate the development of skin appendages in the spiny mouse, Acomys dimidiatus.ResultsWe demonstrate that Acomys spines, possibly involved in display and protection, are enlarged awl hairs with a concave morphology. The Acomys spines originate from enlarged placodes that are characterized by a rapid downwards growth which results in voluminous follicles. The dermal condensation (dermal papilla) at the core of the follicle is very large and exhibits a curved geometry. Given its off-centered position, the dermal papilla generates two waves of anisotropic proliferation, first of the posterior matrix, then of the anterior inner root sheath (IRS). Higher in the follicle, the posterior and anterior cortex cross-section areas substantially decrease due to cortex cell elongation and accumulation of keratin intermediate filaments. Milder keratinization in the medulla gives rise to a foamy material that eventually collapses under the combined compression of the anterior IRS and elongation of the cortex cells. Simulations, using linear elasticity theory and the finite-element method, indicate that these processes are sufficient to replicate the time evolution of the Acomys spine layers and the final shape of the emerging spine shaft.ConclusionsOur analyses reveal how hair follicle morphogenesis has been altered during the evolution of the Acomys lineage, resulting in a shift from ancestral awl follicles to enlarged asymmetrical spines. This study contributes to a better understanding of the evolutionary developmental mechanisms that generated the great diversity of skin appendage phenotypes observed in mammals.