Polyphenism predicts actuarial senescence and lifespan in tiger salamanders

Actuarial senescence (called 'senescence' hereafter) often shows broad variation at the intraspecific level. Phenotypic plasticity likely plays a central role in among-individual heterogeneity in senescence rate (i.e. the rate of increase in mortality with age), although our knowledge on this subject is still very fragmentary. Polyphenism-the unique sub-type of phenotypic plasticity where several discrete phenotypes are produced by the same genotype-may provide excellent study systems to investigate if and how plasticity affects the rate of senescence in nature. In this study, we investigated whether facultative paedomorphosis influences the rate of senescence in a salamander, Ambystoma mavortium nebulosum. Facultative paedomorphosis, a unique form of polyphenism found in dozens of urodele species worldwide, leads to the production of two discrete, environmentally induced phenotypes: metamorphic and paedomorphic individuals. We leveraged an extensive set of capture-recapture data (8948 individuals, 24 years of monitoring) that were analysed using multistate capture-recapture models and Bayesian age-dependent survival models. Multistate models revealed that paedomorphosis was the most common developmental pathway used by salamanders in our study system. Bayesian age-dependent survival models then showed that paedomorphs have accelerated senescence in both sexes and shorter adult lifespan (in females only) compared to metamorphs. In paedomorphs, senescence rate and adult lifespan also varied among ponds and individuals. Females with good body condition and high lifetime reproductive success had slower senescence and longer lifespan. Late-breeding females also lived longer but showed a senescence rate similar to that of early-breeding females. Moreover, males with good condition had longer lifespan than males with poor body condition, although they had similar senescence rates. In addition, late-breeding males lived longer but, unexpectedly, had higher senescence than early-breeding males. Overall, our work provides one of the few empirical cases suggesting that environmentally cued polyphenism could affect the senescence of a vertebrate in nature, thus providing insights on the ecological and evolutionary consequences of developmental plasticity on ageing.

What is missing from the sturgeon jaw: Developmental morphology of the upper jaw in Acipenser

Sturgeons belong to the family Acipenseridae, the most species-rich extant family of Acipenseriformes, a basal actinopterygian group of key importance in assessing the early radiations of the actinopterygians. At the same time, acipenseriforms display unique specializations in the morphology of the snout and jaws which make them a valuable model for studying evolutionary novelties. However, despite a long history of research, the homologies of the snout and the mandibular arch of acipenseriforms remain uncertain preventing further studies on the evolutionary origin of their unique snout and jaw structure, and in particular, of the upper jaw symphysis, the key apomorphy of the group and the preoral snout. In the present study, a detailed description of the upper jaw morphology and development in sturgeons is provided in order to address its composition in terms of the common actinopterygian archetype. Based on the obtained results, the upper jaw of acipenseriforms is assumed to have lost the autopalatine portion, which most likely is represented by the separate cartilages supporting the tentacles. Also, the conventional interpretation of the sturgeon's maxilla as dermopalatine is rejected on the grounds of this bone structure and development. Paedomorphosis is proposed to be the most likely mechanism explaining the evolutionary origin of the upper jaw symphysis and supposed modifications of the snout in sturgeons.

Ontogeny drives shifts in skin bacterial communities in facultatively paedomorphic salamanders

Microbiomes are major determinants of host growth, development and survival. In amphibians, host-associated bacteria in the skin can inhibit pathogen infection, but many processes can influence the structure and composition of the community. Here we quantified the shifts in skin-associated bacteria across developmental stages in the striped newt (Notophthalmus perstriatus), a threatened salamander species with a complex life history and vulnerable to infection by the amphibian chytrid fungus Batrachochytrium dendrobatidis and ranavirus. Our analyses show that pre-metamorphic larval and paedomorphic stages share similar bacterial compositions, and that the changes in the microbiome coincided with physiological restructuring during metamorphosis. Newts undergoing metamorphosis exhibited microbiome compositions that were intermediate between paedomorphic and post-metamorphic stages, further supporting the idea that metamorphosis is a major driver of host-associated microbes in amphibians. We did not find support for infection-related disruption of the microbiome, though infection replicates were small for each respective life stage.

Embryonic development of the skull in a parthenogenetic lizard, the mourning gecko (Lepidodactylus lugubris)

Gekkotans are one of the major clades of squamate reptiles. As one of the earliest-diverging lineages, they are crucial in studying deep-level squamate phylogeny and evolution. Developmental studies can shed light on the origin of many important morphological characters, yet our knowledge of cranial development in gekkotans is very incomplete. Here, we describe the embryonic development of the skull in a parthenogenetic gekkonid, the mourning gecko (Lepidodactylus lugubris), studied using non-acidic double staining and histological sectioning. Our analysis indicates that the pterygoid is the first ossifying bone in the skull, as in almost all other studied squamates, followed closely by the surangular and prearticular. The next to appear are the dentary, frontal, parietal and squamosal. The tooth-bearing upper jaw bones, the premaxilla and maxilla, develop relatively late. In contrast to previous reports, the premaxilla starts ossifying from two distinct centres, reminiscent of the condition observed in diplodactylids and eublepharids. Only a single ossification centre of the postorbitofrontal is observed. Some of the endochondral bones of the braincase (prootic, opisthotic, supraoccipital) and the dermal parasphenoid are the last bones to appear. The skull roof is relatively poorly ossified near the time of hatching, with a large frontoparietal fontanelle still present. Many bones begin ossifying relatively later in L. lugubris than in the phyllodactylid Tarentola annularis, which suggests that its ossification sequence is heterochronic with respect to T. annularis.

The role of environmental variation in mediating fitness trade-offs for an amphibian polyphenism

Fitness trade-offs are a foundation of ecological and evolutionary theory because trade-offs can explain life history variation, phenotypic plasticity, and the existence of polyphenisms. Using a 32-year mark-recapture dataset on lifetime fitness for 1093 adult Arizona tiger salamanders (Ambystoma mavortium nebulosum) from a high elevation, polyphenic population, we evaluated the extent to which two life history morphs (aquatic paedomorphs vs. terrestrial metamorphs) exhibited fitness trade-offs in breeding and body condition with respect to environmental variation (e.g. climate) and internal state-based variables (e.g. age). Both morphs displayed a similar response to higher probabilities of breeding during years of high spring precipitation (i.e. not indicative of a morph-specific fitness trade-off). There were likely no climate-induced fitness trade-offs on breeding state for the two life history morphs because precipitation and water availability are vital to amphibian reproduction. Body condition displayed a contrasting response for the two morphs that was indicative of a climate-induced fitness trade-off. While metamorphs exhibited a positive relationship with summer snowpack conditions, paedomorphs were unaffected. Fitness trade-offs from summer snowpack are likely due to extended hydroperiods in temporary ponds, where metamorphs gain a fitness advantage during the summer growing season by exploiting resources that are unavailable to paeodomorphs. However, paedomorphs appear to have the overwintering fitness advantage because they consistently had higher body condition than metamorphs at the start of the summer growing season. Our results reveal that climate and habitat type (metamorphs as predominately terrestrial, paedomorphs as fully aquatic) interact to confer different advantages for each morph. These results advance our current understanding of fitness trade-offs in this well-studied polyphenic amphibian by integrating climate-based mechanisms. Our conclusions prompt future studies to explore how climatic variation can maintain polyphenisms and promote life history diversity, as well as the implications of climate change for polyphenisms.

Pedomorphosis in the ancestry of marsupial mammals

Within mammals, different reproductive strategies (e.g., egg laying, live birth of extremely underdeveloped young, and live birth of well-developed young) have been linked to divergent evolutionary histories. How and when developmental variation across mammals arose is unclear. While egg laying is unquestionably considered the ancestral state for all mammals, many long-standing biases treat the extreme underdeveloped state of marsupial young as the ancestral state for therian mammals (clade including both marsupials and placentals), with the well-developed young of placentals often considered the derived mode of development. Here, we quantify mammalian cranial morphological development and estimate ancestral patterns of cranial shape development using geometric morphometric analysis of the largest comparative ontogenetic dataset of mammals to date (165 specimens, 22 species). We identify a conserved region of cranial morphospace for fetal specimens, after which cranial morphology diversified through ontogeny in a cone-shaped pattern. This cone-shaped pattern of development distinctively reflected the upper half of the developmental hourglass model. Moreover, cranial morphological variation was found to be significantly associated with the level of development (position on the altricial-precocial spectrum) exhibited at birth. Estimation of ancestral state allometry (size-related shape change) reconstructs marsupials as pedomorphic relative to the ancestral therian mammal. In contrast, the estimated allometries for the ancestral placental and ancestral therian were indistinguishable. Thus, from our results, we hypothesize that placental mammal cranial development most closely reflects that of the ancestral therian mammal, while marsupial cranial development represents a more derived mode of mammalian development, in stark contrast to many interpretations of mammalian evolution.

Time to synchronize our clocks: Connecting developmental mechanisms and evolutionary consequences of heterochrony

Heterochrony, defined as a change in the timing of developmental events altering the course of evolution, was first recognized by Ernst Haeckel in 1866. Haeckel's original definition was meant to explain the observed parallels between ontogeny and phylogeny, but the interpretation of his work became a source of controversy over time. Heterochrony took its modern meaning following the now classical work in the 1970-80s by Steven J. Gould, Pere Alberch, and co-workers. Predicted and described heterochronic scenarios emphasize the many ways in which developmental changes can influence evolution. However, while important examples of heterochrony detected with comparative morphological methods have multiplied, the more mechanistic understanding of this phenomenon lagged conspicuously behind. Considering the rapid progress in imaging and molecular tools available now for developmental biologists, this review aims to stress the need to take heterochrony research to the next level. It is time to synchronize the different levels of heterochrony research into a single analysis flow: from studies on organismal-level morphology to cells to molecules and genes, using complementary techniques. To illustrate how to achieve a more comprehensive understanding of phyletic morphological diversification associated with heterochrony, we discuss several recent case studies at various phylogenetic scales that combine morphological, cellular, and molecular analyses. Such a synergistic approach offers to more fully integrate phylogenetic and ontogenetic dimensions of the fascinating evolutionary phenomenon of heterochrony.

Repeated ecological and life cycle transitions make salamanders an ideal model for evolution and development

Observations on the ontogeny and diversity of salamanders provided some of the earliest evidence that shifts in developmental trajectories have made a substantial contribution to the evolution of animal forms. Since the dawn of evo-devo there have been major advances in understanding developmental mechanisms, phylogenetic relationships, evolutionary models, and an appreciation for the impact of ecology on patterns of development (eco-evo-devo). Molecular phylogenetic analyses have converged on strong support for the majority of branches in the Salamander Tree of Life, which includes 764 described species. Ancestral reconstructions reveal repeated transitions between life cycle modes and ecologies. The salamander fossil record is scant, but key Mesozoic species support the antiquity of life cycle transitions in some families. Colonization of diverse habitats has promoted phenotypic diversification and sometimes convergence when similar environments have been independently invaded. However, unrelated lineages may follow different developmental pathways to arrive at convergent phenotypes. This article summarizes ecological and endocrine-based causes of life cycle transitions in salamanders, as well as consequences to body size, genome size, and skeletal structure. Salamanders offer a rich source of comparisons for understanding how the evolution of developmental patterns has led to phenotypic diversification following shifts to new adaptive zones.

Evolutionary lability in floral ontogeny affects pollination biology in Trimezieae

PREMISE

There is little direct evidence linking floral development and pollination biology in plants. We characterize both aspects in plain and ornamented flowers of Trimezieae (Iridaceae) to investigate how changes in floral ontogeny may affect their interactions with pollinators through time.

METHODS

We examined floral ontogeny in 11 species and documented pollination biology in five species displaying a wide range of floral morphologies. We coded and reconstructed ancestral states of flower types over the tribal phylogeny to estimate the frequency of transition between different floral types.

RESULTS

All Trimezieae flowers are similar in early floral development, but ornamented flowers have additional ontogenetic steps compared with plain flowers, indicating heterochrony. Ornamented flowers have a hinge pollination mechanism (newly described here) and attract more pollinator guilds, while plain flowers offer less variety of resources for a shorter time. Although the ornamented condition is plesiomorphic in this clade, shifts to plain flowers have occurred frequently and abruptly during the past 5 million years, with some subsequent reversals.

CONCLUSIONS

Heterochrony has resulted in labile morphological changes during flower evolution in Trimezieae. Counterintuitively, species with plain flowers, which are endemic to the campo rupestre, are derived within the tribe and show a higher specialization than the ornamented species, with the former being visited by pollen-collecting bees only.

Insights into skull evolution in fossorial snakes, as revealed by the cranial morphology of Atractaspis irregularis (Serpentes: Colubroidea)

Comparative osteological analyses of extant organisms provide key insight into major evolutionary transitions and phylogenetic hypotheses. This is especially true for snakes, given their unique morphology relative to other squamates and the persistent controversy regarding their evolutionary origins. However, the osteology of several major snake groups remains undescribed, thus hindering efforts to accurately reconstruct the phylogeny of snakes. One such group is the Atractaspididae, a family of fossorial colubroids. We herein present the first detailed description of the atractaspidid skull, based on fully segmented micro-computed tomography (micro-CT) scans of Atractaspis irregularis. The skull of Atractaspis presents a highly unique morphology influenced by both fossoriality and paedomorphosis. This paedomorphosis is especially evident in the jaws, palate, and suspensorium, the major elements associated with macrostomy (large-gaped feeding in snakes). Comparison to scolecophidians-a group of blind, fossorial, miniaturized snakes-in turn sheds light on current hypotheses of snake phylogeny. Features of both the naso-frontal joint and the morphofunctional system related to macrostomy refute the traditional notion that scolecophidians are fundamentally different from alethinophidians (all other extant snakes). Instead, these features support the controversial hypothesis of scolecophidians as "regressed alethinophidians," in contrast to their traditional placement as the earliest-diverging snake lineage. We propose that Atractaspis and scolecophidians fall along a morphological continuum, characterized by differing degrees of paedomorphosis. Altogether, a combination of heterochrony and miniaturization provides a mechanism for the derivation of the scolecophidian skull from an ancestral fossorial alethinophidian morphotype, exemplified by the nonminiaturized and less extreme paedomorph Atractaspis.

Embryonic skull development in the gecko, Tarentola annularis (Squamata: Gekkota: Phyllodactylidae)

Tarentola annularis is a climbing gecko with a wide distribution in Africa north of the equator. In the present paper, we describe the development of the osteocranium of this lizard, from the first appearance of the cranial elements up to the point of hatching. This is based on a combination of histology and cleared and stained specimens. This is the first comprehensive account of gekkotan pre-hatching skull development based on a comprehensive series of embryos, rather than a few selected stages. Given that Gekkota is now widely regarded as representing the sister group to other squamates, this account helps to fill a significant gap in the literature. Moreover, as many authors have considered features of the gekkotan skull and skeleton to be indicative of paedomorphosis, it is important to know whether this hypothesis is supported by delays in the onset of cranial ossification. In fact, we found the sequence of cranial bone ossification to be broadly comparable to that of other squamates studied to date, with no significant lags in development.

Larger Genomes Linked to Slower Development and Loss of Late-Developing Traits

Genome size varies widely among organisms and is known to affect vertebrate development, morphology, and physiology. In amphibians, genome size is hypothesized to contribute to loss of late-forming structures, although this hypothesis has mainly been discussed in salamanders. Here we estimated genome size for 22 anuran species and combined this novel data set with existing genome size data for an additional 234 anuran species to determine whether larger genome size is associated with loss of a late-forming anuran sensory structure, the tympanic middle ear. We established that genome size is negatively correlated with development rate across 90 anuran species and found that genome size evolution is correlated with evolutionary loss of the middle ear bone (columella) among 241 species (224 eared and 17 earless). We further tested whether the development of the tympanic middle ear could be constrained by large cell sizes and small body sizes during key stages of tympanic middle ear development (metamorphosis). Together, our evidence suggests that larger genomes, slower development rate, and smaller body sizes at metamorphosis may contribute to the loss of the anuran tympanic middle ear. We conclude that increases in anuran genome size, although less drastic than those in salamanders, may affect development of late-forming traits.

Cranial ontogeny of Thamnophis radix (Serpentes: Colubroidea) with a re-evaluation of current paradigms of snake skull evolution

Accurate knowledge of skeletal ontogeny in extant organisms is crucial in understanding important morpho-functional systems and in enabling inferences of the ontogenetic stage of fossil specimens. However, detailed knowledge of skeletal ontogeny is lacking for most squamates, including snakes. Very few studies have discussed postnatal development in snakes, with none incorporating data from all three major ontogenetic stages-embryonic, juvenile and adult. Here, we provide the first analysis encompassing these three ontogenetic stages for any squamate, using the first complete micro-computed tomography (micro-CT)-based segmentations of any non-adult snake, based on fresh specimens of Thamnophis radix. The most significant ontogenetic changes involve the feeding apparatus, with major elongation of the tooth-bearing elements and jaw suspensorium causing a posterior shift in the jaw articulation. This shift enables macrostomy (large-gaped feeding in snakes) and occurs in T. radix via a different developmental trajectory than in most other macrostomatans, indicating that the evolution of macrostomy is more complex than previously thought. The braincase of T. radix is also evolutionarily unique among derived snakes in lacking a crista circumfenestralis, a phenomenon considered herein to represent paedomorphic retention of the embryonic condition. We thus present numerous important challenges to current paradigms regarding snake cranial evolution.

Postmetamorphic ontogenetic allometry and the evolution of skull shape in Nest-building frogs Leptodactylus (Anura: Leptodactylidae)

Allometry constitutes an important source of morphological variation. However, its influence in head development in anurans has been poorly explored. By using geometric morphometrics followed by statistical and comparative methods we analyzed patterns of allometric change during cranial postmetamorphic ontogeny in species of Nest-building frogs Leptodactylus (Leptodactylidae). We found that the anuran skull is not a static structure, and allometry plays an important role in defining its shape in this group. Similar to other groups with biphasic life-cycle, and following a general trend in vertebrates, ontogenetic changes mostly involve rearrangement in rostral, otoccipital, and suspensorium regions. Ontogenetic transformations are paralleled by shape changes associated with evolutionary change in size, such that the skulls of species of different intrageneric groups are scaled to each other, and small and large species show patterns of paedomorphic/peramorphic features, respectively. Allometric trajectories producing those phenotypes are highly evolvable though, with shape change direction and magnitude varying widely among clades, and irrespective of changes in absolute body size. These results reinforce the importance of large-scale comparisons of growth patterns to understand the plasticity, evolution, and polarity of morphological changes in different clades.

Thyroid hormone coordinates developmental trajectories but does not underlie developmental truncation in danionins

BACKGROUND

Differences in postembryonic developmental trajectories can profoundly alter adult phenotypes and life histories. Thyroid hormone (TH) regulates metamorphosis in many vertebrate taxa with multiphasic ecologies, and alterations to TH metabolism underlie notable cases of paedomorphosis in amphibians. We tested the requirement for TH in multiple postembryonic developmental processes in zebrafish, which has a monophasic ecology, and asked if TH production was compromised in paedomorphic Danionella.

RESULTS

We showed that TH regulates allometric growth in juvenile zebrafish, and inhibits relative head growth. The lateral line system showed differential requirements for TH: the hormone promotes canal neuromast formation and inhibits neuromast proliferation in the head, but causes expansion of the neuromast population in the trunk. While Danionella morphology resembled that of larval zebrafish, the two Danionella species analyzed were not similar to hypothyroid zebrafish in their shape or neuromast distribution, and both possessed functional thyroid follicles.

CONCLUSIONS

Although zebrafish do not undergo a discrete ecological transformation, we found that multiple tissues undergo transitions in developmental trajectories that are dependent on TH, suggesting the TH axis and its downstream pathways as likely targets for adaptation. Nonetheless, we found no evidence that evolutionary paedomorphosis in Danionella is the result of compromised TH production.

Terrestriality constrains salamander limb diversification: Implications for the evolution of pentadactyly

Patterns of phenotypic evolution can abruptly shift as species move between adaptive zones. Extant salamanders display three distinct life cycle strategies that range from aquatic to terrestrial (biphasic), to fully aquatic (paedomorphic) and to fully terrestrial (direct development). Life cycle variation is associated with changes in body form such as loss of digits, limb reduction or body elongation. However, the relationships among these traits and life cycle strategy remain unresolved. Here, we use a Bayesian modelling approach to test whether life cycle transitions by salamanders have influenced rates, optima and integration of primary locomotory structures (limbs and trunk). We show that paedomorphic salamanders have elevated rates of limb evolution with optima shifted towards smaller size and fewer digits compared to all other salamanders. Rate of hindlimb digit evolution is shown to decrease in a gradient as life cycles become more terrestrial. Paedomorphs have a higher correlation between hindlimb digit loss and increases in vertebral number, as well as reduced correlations between limb lengths. Our results support the idea that terrestrial plantigrade locomotion constrains limb evolution and, when lifted, leads to higher rates of trait diversification and shifts in optima and integration. The basic tetrapod body form of most salamanders and the independent losses of terrestrial life stages provide an important framework for understanding the evolutionary and developmental mechanisms behind major shifts in ecological zones as seen among early tetrapods during their transition from water to land.

Rapid reshaping: the evolution of morphological changes in an introduced beach daisy

Thousands of species have been introduced to new ranges worldwide. These introductions provide opportunities for researchers to study evolutionary changes in form and function in response to new environmental conditions. However, almost all previous studies of morphological change in introduced species have compared introduced populations to populations from across the species' native range, so variation within native ranges probably confounds estimates of evolutionary change. In this study, we used microsatellites to locate the source population for the beach daisy Arctotheca populifolia that had been introduced to eastern Australia. We then compared four introduced populations from Australia with their original South African source population in a common-environment experiment. Despite being separated for less than 100 years, source and introduced populations of A. populifolia display substantial heritable morphological differences. Contrary to the evolution of increased competitive ability hypothesis, introduced plants were shorter than source plants, and introduced and source plants did not differ in total biomass. Contrary to predictions based on higher rainfall in the introduced range, introduced plants had smaller, thicker leaves than source plants. Finally, while source plants develop lobed adult leaves, introduced plants retain their spathulate juvenile leaf shape into adulthood. These changes indicate that rapid evolution in introduced species happens, but not always in the direction predicted by theory.

Rapid phenotypic evolution following shifts in life cycle complexity

Life cycle strategies have evolved extensively throughout the history of metazoans. The expression of disparate life stages within a single ontogeny can present conflicts to trait evolution, and therefore may have played a major role in shaping metazoan forms. However, few studies have examined the consequences of adding or subtracting life stages on patterns of trait evolution. By analysing trait evolution in a clade of closely related salamander lineages we show that shifts in the number of life cycle stages are associated with rapid phenotypic evolution. Specifically, salamanders with an aquatic-only (paedomorphic) life cycle have frequently added vertebrae to their trunk skeleton compared with closely related lineages with a complex aquatic-to-terrestrial (biphasic) life cycle. The rate of vertebral column evolution is also substantially lower in biphasic lineages, which may reflect the functional compromise of a complex cycle. This study demonstrates that the consequences of life cycle evolution can be detected at very fine scales of divergence. Rapid evolutionary responses can result from shifts in selective regimes following changes in life cycle complexity.

Rediscovering the Axolotl as a Model for Thyroid Hormone Dependent Development

The Mexican axolotl (Ambystoma mexicanum) is an important model organism in biomedical research. Much current attention is focused on the axolotl's amazing ability to regenerate tissues and whole organs after injury. However, not forgotten is the axolotl's equally amazing ability to thwart aspects of tissue maturation and retain juvenile morphology into the adult phase of life. Unlike close tiger salamander relatives that undergo a thyroid hormone regulated metamorphosis, the axolotl does not typically undergo a metamorphosis. Instead, the axolotl exhibits a paedomorphic mode of development that enables a completely aquatic life cycle. The evolution of paedomorphosis allowed axolotls to exploit relatively permanent habitats in Mexico, and preadapted axolotls for domestication and laboratory study. In this perspective, we first introduce the axolotl and the various meanings of paedomorphosis, and then stress the need to move beyond endocrinology-guided approaches to understand the axolotl's hypothyroid state. With the recent completion of the axolotl genome assembly and established methods to manipulate gene functions, the axolotl is poised to provide new insights about paedomorphosis and the role of thyroid hormone in development and evolution.

Opposite T3 Response of ACTG1-FOS Subnetwork Differentiate Tailfin Fate in Xenopus Tadpole and Post-hatching Axolotl

Amphibian post-embryonic development and Thyroid Hormones (TH) signaling are deeply and intimately connected. In anuran amphibians, TH induce the spectacular and complex process known as metamorphosis. In paedomorphic salamanders, at similar development time, raising levels of TH fail to induce proper metamorphosis, as many "larval" tissues (e.g., gills, tailfin) are maintained. Why does the same evolutionary conserved signaling pathway leads to alternative phenotypes? We used a combination of developmental endocrinology, functional genomics and network biology to compare the transcriptional response of tailfin to TH, in the post-hatching paedormorphic Axolotl salamander and Xenopus tadpoles. We also provide a technological framework that efficiently reduces large lists of regulated genes down to a few genes of interest, which is well-suited to dissect endocrine regulations. We first show that Axolotl tailfin undergoes a strong and robust TH-dependent transcriptional response at post embryonic transition, despite the lack of visible anatomical changes. We next show that Fos and Actg1, which structure a single and dense subnetwork of cellular sensors and regulators, display opposite regulation between the two species. We finally show that TH treatments and natural variations of TH levels follow similar transcriptional dynamics. We suggest that, at the molecular level, tailfin fate correlates with the alternative transcriptional states of an fos-actg1 sub-network, which also includes transcription factors and regulators of cell fate. We propose that this subnetwork is one of the molecular switches governing the initiation of distinct TH responses, with transcriptional programs conducting alternative tailfin fate (maintenance vs. resorption) 2 weeks post-hatching.

Dental ontogeny in extinct synapsids reveals a complex evolutionary history of the mammalian tooth attachment system

The mammalian dentition is uniquely characterized by a combination of precise occlusion, permanent adult teeth and a unique tooth attachment system. Unlike the ankylosed teeth in most reptiles, mammal teeth are supported by a ligamentous tissue that suspends each tooth in its socket, providing flexible and compliant tooth attachment that prolongs the life of each tooth and maintains occlusal relationships. Here we investigate dental ontogeny through histological examination of a wide range of extinct synapsid lineages to assess whether the ligamentous tooth attachment system is unique to mammals and to determine how it evolved. This study shows for the first time that the ligamentous tooth attachment system is not unique to crown mammals within Synapsida, having arisen in several non-mammalian therapsid clades as a result of neoteny and progenesis in dental ontogeny. Mammalian tooth attachment is here re-interpreted as a paedomorphic condition relative to the ancestral synapsid form of tooth attachment.

Integrating restriction site-associated DNA sequencing (RAD-seq) with morphological cladistic analysis clarifies evolutionary relationships among major species groups of bee orchids

BACKGROUND AND AIMS

Bee orchids (Ophrys) have become the most popular model system for studying reproduction via insect-mediated pseudo-copulation and for exploring the consequent, putatively adaptive, evolutionary radiations. However, despite intensive past research, both the phylogenetic structure and species diversity within the genus remain highly contentious. Here, we integrate next-generation sequencing and morphological cladistic techniques to clarify the phylogeny of the genus.

METHODS

At least two accessions of each of the ten species groups previously circumscribed from large-scale cloned nuclear ribosomal internal transcibed spacer (nrITS) sequencing were subjected to restriction site-associated sequencing (RAD-seq). The resulting matrix of 4159 single nucleotide polymorphisms (SNPs) for 34 accessions was used to construct an unrooted network and a rooted maximum likelihood phylogeny. A parallel morphological cladistic matrix of 43 characters generated both polymorphic and non-polymorphic sets of parsimony trees before being mapped across the RAD-seq topology.

KEY RESULTS

RAD-seq data strongly support the monophyly of nine out of ten groups previously circumscribed using nrITS and resolve three major clades; in contrast, supposed microspecies are barely distinguishable. Strong incongruence separated the RAD-seq trees from both the morphological trees and traditional classifications; mapping of the morphological characters across the RAD-seq topology rendered them far more homoplastic.

CONCLUSIONS

The comparatively high level of morphological homoplasy reflects extensive convergence, whereas the derived placement of the fusca group is attributed to paedomorphic simplification. The phenotype of the most recent common ancestor of the extant lineages is inferred, but it post-dates the majority of the character-state changes that typify the genus. RAD-seq may represent the high-water mark of the contribution of molecular phylogenetics to understanding evolution within Ophrys; further progress will require large-scale population-level studies that integrate phenotypic and genotypic data in a cogent conceptual framework.

Humidity-dependent wound sealing in succulent leaves of Delosperma cooperi - An adaptation to seasonal drought stress

During evolution, plants evolved various reactions to wounding. Fast wound sealing and subsequent healing represent a selective advantage of particular importance for plants growing in arid habitats. An effective self-sealing function by internal deformation has been found in the succulent leaves of Delosperma cooperi. After a transversal incision, the entire leaf bends until the wound is closed. Our results indicate that the underlying sealing principle is a combination of hydraulic shrinking and swelling as the main driving forces and growth-induced mechanical pre-stresses in the tissues. Hydraulic effects were measured in terms of the relative bending angle over 55 minutes under various humidity conditions. The higher the relative air humidity, the lower the bending angle. Negative bending angles were found when a droplet of liquid water was applied to the wound. The statistical analysis revealed highly significant differences of the single main effects such as "humidity conditions in the wound region" and "time after wounding" and their interaction effect. The centripetal arrangement of five tissue layers with various thicknesses and significantly different mechanical properties might play an additional role with regard to mechanically driven effects. Injury disturbs the mechanical equilibrium, with pre-stresses leading to internal deformation until a new equilibrium is reached. In the context of self-sealing by internal deformation, the highly flexible wide-band tracheids, which form a net of vascular bundles, are regarded as paedomorphic tracheids, which are specialised to prevent cell collapse under drought stress and allow for building growth-induced mechanical pre-stresses.

Specialized specialists and the narrow niche fallacy: a tale of scale-feeding fishes

Although rare within the context of 30 000 species of extant fishes, scale-feeding as an ecological strategy has evolved repeatedly across the teleost tree of life. Scale-feeding (lepidophagous) fishes are diverse in terms of their ecology, behaviour, and specialized morphologies for grazing on scales and mucus of sympatric species. Despite this diversity, the underlying ontogenetic changes in functional and biomechanical properties of associated feeding morphologies in lepidophagous fishes are less understood. We examined the ontogeny of feeding mechanics in two evolutionary lineages of scale-feeding fishes: Roeboides, a characin, and Catoprion, a piranha. We compare these two scale-feeding taxa with their nearest, non-lepidophagous taxa to identify traits held in common among scale-feeding fishes. We use a combination of micro-computed tomography scanning and iodine staining to measure biomechanical predictors of feeding behaviour such as tooth shape, jaw lever mechanics and jaw musculature. We recover a stark contrast between the feeding morphology of scale-feeding and non-scale-feeding taxa, with lepidophagous fishes displaying some paedomorphic characters through to adulthood. Few traits are shared between lepidophagous characins and piranhas, except for their highly-modified, stout dentition. Given such variability in development, morphology and behaviour, ecological diversity within lepidophagous fishes has been underestimated.

Floral morphology and anatomy of Ophiocaryon, a paedomorphic genus of Sabiaceae

BACKGROUND AND AIMS

Ophiocaryon is a lesser known genus in Sabiaceae. This study examines flowers of six Ophiocaryon species in comparison with Meliosmaalba, to identify taxonomically informative characters for understanding relationships within the family Sabiaceae, to imply previously unknown pollination mechanisms of Ophiocaryon, and to contribute to the placement of Sabiaceae within the early-diverging eudicots.

METHODS

Floral morphology and anatomy of six Ophiocaryon species and M. alba were studied and described using scanning electron microscopy, clearing techniques and resin sectioning.

KEY RESULTS

Novel characters of Ophiocaryon were identified, e.g. conical cells on petals, different kinds of orbicules in anthers, stomata on nectary appendage tips and ovary, two distinct surface patterns on stamens and ovary, tanniferous cell layers in the ovary wall, and acorn-shaped unitegmic ovules with very short integuments. Comparison of floral characters between Ophiocaryon and Meliosma found that the calyx, corolla, androecium and gynoecium of Ophiocaryon resemble an undeveloped state of the latter taxon, reflecting a paedomorphic regression of the flower of Ophiocaryon. The flower morphology and anatomy of Ophiocaryon was compared with its putative sister species M. alba, but no clear shared derived characters could be detected. Moreover, the findings of scent, presence of conical cells on petals and a nectary suggest flowers are pollinated by small insects with a secondary pollen presentation on the cupula of fertile stamens.

CONCLUSIONS

We found that Ophiocaryon may be derived from ancestors that were similar to extant Meliosma in their flower structure and pollination mechanism. However, the lack of shared derived characters between Ophiocaryon and its phylogenetic sister group M. alba is puzzling and requires further investigations on the diversity of the latter species.

Development of the muscles associated with the mandibular and hyoid arches in the Siberian sturgeon, Acipenser baerii (Acipenseriformes: Acipenseridae)

The skeleton of the jaws and neurocranium of sturgeons (Acipenseridae) are connected only through the hyoid arch. This arrangement allows considerable protrusion and retraction of the jaws and is highly specialized among ray-finned fishes (Actinopterygii). To better understand the unique morphology and the evolution of the jaw apparatus in Acipenseridae, we investigated the development of the muscles of the mandibular and hyoid arches of the Siberian sturgeon, Acipenser baerii. We used a combination of antibody staining and formalin-induced fluorescence of tissues imaged with confocal microscopy and subsequent three-dimensional reconstruction. These data were analyzed to address the identity of previously controversial and newly discovered muscle portions. Our results indicate that the anlagen of the muscles in A. baerii develop similarly to those of other actinopterygians, although they differ by not differentiating into distinct muscles. This is exemplified by the subpartitioning of the m. adductor mandibulae as well as the massive m. protractor hyomandibulae, for which we found a previously undescribed portion in each. The importance of paedomorphosis for the evolution of Acipenseriformes has been discussed before and our results indicate that the muscles of the mandibular and the hyoid may be another example for heterochronic evolution.

Evidence for complex life cycle constraints on salamander body form diversification

Metazoans display a tremendous diversity of developmental patterns, including complex life cycles composed of morphologically disparate stages. In this regard, the evolution of life cycle complexity promotes phenotypic diversity. However, correlations between life cycle stages can constrain the evolution of some structures and functions. Despite the potential macroevolutionary consequences, few studies have tested the impacts of life cycle evolution on broad-scale patterns of trait diversification. Here we show that larval and adult salamanders with a simple, aquatic-only (paedomorphic) life cycle had an increased rate of vertebral column and body form diversification compared to lineages with a complex, aquatic-terrestrial (biphasic) life cycle. These differences in life cycle complexity explain the variations in vertebral number and adult body form better than larval ecology. In addition, we found that lineages with a simple terrestrial-only (direct developing) life cycle also had a higher rate of adult body form evolution than biphasic lineages, but still 10-fold lower than aquatic-only lineages. Our analyses demonstrate that prominent shifts in phenotypic evolution can follow long-term transitions in life cycle complexity, which may reflect underlying stage-dependent constraints.

COMPARATIVE ONTOGENY OF A WILD CUCURBIT AND ITS DERIVED CULTIVAR

Most previous studies of evolutionary modification of form in plants have focused primarily on individual organs or flowers. Few have investigated the role of evolutionary changes in timing or position at the level of whole plant ontogeny. This study compares ontogenies of the primary shoots of two subspecies of Cucurbita argyrosperma, one a cultivar and the other its wild progenitor. Differences in flowering times between these subspecies suggested that the cultivar may have evolved from the wild subspecies via heterochronic processes leading to paedomorphosis. Analyses showed that both subspecies are similar in vegetative architecture and rates of leaf production. Earlier flowering in the cultivar, both in terms of position and absolute time, appears to have arisen through progenesis. Initial observations of leaf blade morphology led to the hypothesis that paedomorphosis and gigantism also may have been involved in the evolution of leaf blade shape in the cultivar: all leaves of the cultivar are larger and visually similar in shape to early leaves of the wild subspecies. However, quantitative analysis revealed that leaves of the cultivar are neither geometrically, nor solely allometrically larger versions of early leaves of the progenitor. Leaf shape in the cultivar exhibits novel features as well as effects of allometry shared with the progenitor, hence a simple hypothesis of paedomorphic evolution of leaf shape is not supported.

Newt life after fish introduction: extirpation of paedomorphosis in a mountain fish lake and newt use of satellite pools

Fish introduction is one of the main causes of amphibian decline worldwide. It affects particularly rare aquatic phenotypes such as paedomorphs, which retain gills during the adult stage. In this context, we determined whether small wetlands, such as pools surrounding fished and fishless lakes, could sustain paedomorphic and metamorphic newts. To this end, we surveyed lakes known historically to sustain Alpine newts Ichthyosaura alpestris as well as 35 nearby pools. On the basis of the published records, the only known population exhibiting paedomorphosis in the Swiss Alps was found to be extirpated by salmonid introductions. However, the metamorphs persisted in peripheral pools, paedomorphosis was discovered at a new locality, and overwintering larvae were still present in one of the lakes. These results show the importance of conserving varied aquatic habitats such as pools in mountainous environments where the main resources can become unsuitable for amphibians because of fish introductions. Pools may also function as reservoirs in maintaining newt populations until programs to remove fish from lakes can be carried out. It is not known if paedomorphs could reappear after fish removal. However, the combined resilience of amphibians after fish removal and the genetic basis for paedomorphosis highlighted in other taxa by previous studies suggest that there is the potential to maintain this intraspecific case of diversity even after its disappearance.

A Tale of Two Axolotls

The Mexican axolotl (Ambystoma mexicanum) is an icon of culture, a revered aquarium pet, and a highly valued animal model in biomedical research. Unfortunately, Mexican axolotls are critically endangered in their natural Xochimilco habitat in Mexico City. If axolotls go extinct, current efforts to conserve the Xochimilico ecosystem will be undermined, as will efforts to genetically manage the laboratory populations that are needed to sustain research efforts around the world. A concerted global effort is needed to protect and manage this irreplaceable species in natural and laboratory environments.

Development, metamorphosis, morphology, and diversity: The evolution of chordate muscles and the origin of vertebrates

Recent findings that urochordates are the closest sister-group of vertebrates have dramatically changed our understanding of chordate evolution and vertebrate origins. To continue to deepen our understanding of chordate evolution and diversity, in particular the morphological and taxonomical diversity of the vertebrate clade, one must explore the origin, development, and comparative anatomy of not only hard tissues, but also soft tissues such as muscles. Building on a recent overview of the discovery of a cardiopharyngeal field in urochordates and the profound implications for reconstructing the origin and early evolution of vertebrates, in this study we focus on the broader comparative and developmental anatomy of chordate cephalic muscles and their relation to life history, and to developmental, morphological and taxonomical diversity. We combine our recent findings on cephalochordates, urochordates, and vertebrates with a literature review and suggest that developmental changes related to metamorphosis and/or heterochrony (e.g., peramorphosis) played a crucial role in the early evolution of chordates and vertebrates. Recent studies reviewed here supported de Beer's "law of diversity" that peramorphic animals (e.g., ascidians, lampreys) are taxonomically and morphologically less diverse than nonperamorphic animals (e.g., gnathostomes), probably because their "too specialized" development and adult anatomy constrain further developmental and evolutionary innovations. Developmental Dynamics 244:1046-1057, 2015. © 2014 Wiley Periodicals, Inc.

Expression of sexual ornaments in a polymorphic species: phenotypic variation in response to environmental risk

Secondary sexual traits may evolve under the antagonistic context of sexual and natural selection. In some polymorphic species, these traits are only expressed during the breeding period and are differently expressed in alternative phenotypes. However, it is unknown whether such phenotypes exhibit phenotypic plasticity of seasonal ornamentations in response to environmental pressures such as in the presence of fish (predation risk). This is an important question to understand the evolution of polyphenisms. We used facultative paedomorphosis in newts as a model system because it involves the coexistence of paedomorphs that retain gills in the adult stage with metamorphs that have undergone metamorphosis, but also because newts exhibit seasonal sexual traits. Our aim was therefore to determine the influence of fish on the development of seasonal ornamentation in the two phenotypes of the palmate newt (Lissotriton helveticus). During the entire newt breeding period, we assessed the importance of phenotype and fish presence with an information-theoretic approach. Our results showed that paedomorphs presented much less developed ornamentation than metamorphs and those ornamentations varied over time. Fish inhibited the development of sexual traits but differently between phenotypes: in contrast to metamorphs, paedomorphs lack the phenotypic plasticity of sexual traits to environmental risk. This study points out that internal and external parameters act in complex ways in the expression of seasonal sexual ornamentations and that similar environmental pressure can induce a contrasted evolution in alternative phenotypes.

Growth habit of the late Paleozoic rhizomorphic tree-lycopsid family Diaphorodendraceae: phylogenetic, evolutionary, and paleoecological significance

PREMISE OF THE STUDY

Rhizomorphic lycopsids evolved the tree habit independently of all other land plants. Newly discovered specimens allow radical revision of our understanding of the growth architectures of the extinct Paleozoic sister-genera Synchysidendron and Diaphorodendron.

METHODS

Detailed descriptions of six remarkable adpression specimens from the Pennsylvanian of the USA and three casts from the late Mississippian of Scotland are used to revise and reanalyze a previously published morphological cladistic matrix and to reinterpret their remarkable growth forms.

KEY RESULTS

Contrary to previous assertions, Synchysidendron resembled Diaphorodendron in having a distinct and relatively complex growth habit that emphasized serially homologous, closely spaced, deciduous lateral branches at the expense of reduced monocarpic crown branches. Lateral branches originated through several strongly anisotomous dichotomies before producing during extended periods large numbers of Achlamydocarpon strobili. The comparatively large diameter of abscission scars remaining on the main trunk and the emergence of branches above the horizontal plane suggest that the lateral branch systems were robust. Lateral branches were borne in two opposite rows on the main trunk and continued upward into an isotomously branched, determinate crown; their striking distichous arrangement caused preferred orientation of fallen trunks on bedding planes.

CONCLUSIONS

This discovery identifies the plagiotropic growth habit, dominated by serial lateral branches, as ubiquitous in the Diaphorodendraceae and also as unequivocally primitive within Isoetales s.l., a conclusion supported by both the revised morphological cladistic analysis and relative first appearances of taxa in the fossil record. Previously assumed complete homology between crown branching in Lepidodendraceae and that of all earlier-divergent genera requires reassessment. Saltational phenotypic transitions via modification of key developmental switches remains the most credible explanation for architectural evolution in the group. The resulting architecture allowed Diaphorodendraceae to co-dominate disturbed, clastic, equatorial wetlands from the Asbian to the Early Permian.

Thyroid hormone responsive QTL and the evolution of paedomorphic salamanders

The transformation of ancestral phenotypes into novel traits is poorly understood for many examples of evolutionary novelty. Ancestrally, salamanders have a biphasic life cycle with an aquatic larval stage, a brief and pronounced metamorphosis, followed by a terrestrial adult stage. Repeatedly during evolution, metamorphic timing has been delayed to exploit growth-permissive environments, resulting in paedomorphic salamanders that retain larval traits as adults. We used thyroid hormone (TH) to rescue metamorphic phenotypes in paedomorphic salamanders and then identified quantitative trait loci (QTL) for life history traits that are associated with amphibian life cycle evolution: metamorphic timing and adult body size. We demonstrate that paedomorphic tiger salamanders (Ambystoma tigrinum complex) carry alleles at three moderate effect QTL (met1-3) that vary in responsiveness to TH and additively affect metamorphic timing. Salamanders that delay metamorphosis attain significantly larger body sizes as adults and met2 explains a significant portion of this variation. Thus, substitution of alleles at TH-responsive loci suggests an adaptive pleiotropic basis for two key life-history traits in amphibians: body size and metamorphic timing. Our study demonstrates a likely pathway for the evolution of novel paedomorphic species from metamorphic ancestors via selection of TH-response alleles that delay metamorphic timing and increase adult body size.