Brassica rapa CURLY LEAF is a major H3K27 methyltransferase regulating flowering time.

MAIN CONCLUSION: In Brassica rapa, the epigenetic modifier BraA.CLF orchestrates flowering by modulating H3K27me3 levels at the floral integrator genes FT, SOC1, and SEP3, thereby influencing their expression. CURLY LEAF (CLF) is the catalytic subunit of the plant Polycomb Repressive Complex 2 that mediates the trimethylation of histone H3 lysine 27 (H3K27me3), an epigenetic modification that leads to gene silencing. While the function of CURLY LEAF (CLF) has been extensively studied in Arabidopsis thaliana, its role in Brassica crops is barely known. In this study, we focused on the Brassica rapa homolog of CLF and found that the loss-of-function mutant braA.clf-1 exhibits an accelerated flowering together with pleiotropic phenotypic alterations compared to wild-type plants. In addition, we carried out transcriptomic and H3K27me3 genome-wide analyses to identify the genes regulated by BraA.CLF. Interestingly, we observed that several floral regulatory genes, including the B. rapa homologs of FT, SOC1 and SEP3, show reduced H3K27me3 levels and increased transcript levels compared to wild-type plants, suggesting that they are direct targets of BraA.CLF and key players in regulating flowering time in this crop. In addition, the results obtained will enhance our understanding of the epigenetic mechanisms regulating key developmental traits and will aid to increase crop yield by engineering new Brassica varieties with different flowering time requirements.

Severely compromised supply of patch test allergens in Europe hampers adequate diagnosis of occupational and non-occupational contact allergy. A European Society of Contact Dermatitis (ESCD), European Academy of Allergy and Clinical Immunology (EAACI), European Academy of Dermatology and Venereology (EADV) task forces 'Contact Dermatitis' and 'Occupational Skin Disease' position paper.

Patch testing is the only clinically applicable diagnostic method for Type IV allergy. The availability of Type IV patch test (PT) allergens in Europe, however, is currently scarce. This severely compromises adequate diagnostics of contact allergy, leading to serious consequences for the affected patients. Against this background, the European Society of Contact Dermatitis (ESCD) has created a task force (TF) (i) to explore the current availability of PT substances in different member states, (ii) to highlight some of the unique characteristics of Type IV vs. other allergens and (iii) to suggest ways forward to promote and ensure availability of high-quality patch testing substances for the diagnosis of Type IV allergies throughout Europe. The suggestions of the TF on how to improve the availability of PT allergens are supported by the ESCD, the European Academy of Allergy and Clinical Immunology, and the European Academy of Dermatology and Venereology and intend to provide potential means to resolve the present medical crisis.

Differential diagnosis of contact dermatitis: A practical-approach review by the EADV Task Force on contact dermatitis.

The diagnosis of eczema ('dermatitis') is mostly clinical and depends on the clinical history and exploratory objective findings (primary lesions, patterns). Contact dermatitis remains as an important condition in the group of eczematous disorders, with important socioeconomic and occupational relevance. Although irritant and allergic contact dermatitis have a different pathogenesis, both are characterized by a rather typical morphology, are triggered by external factors and tend to occur primarily in the area of contact with the exogenous agent. In addition, allergic and irritant dermatitis may also co-exist. The importance of diagnosing contact dermatitis, especially when allergic in nature, is both due to the possibility of avoiding the trigger, and due to its role in aggravating other skin conditions. Nevertheless, the heterogeneity of clinical presentations in daily practice may pose an important challenge for the suspicion and correct diagnosis of contact dermatitis. Furthermore, other conditions, with different pathogenesis and treatment, may clinically simulate contact dermatitis. The Task Force aims to conduct a review of the unifying clinical features of contact dermatitis and characterize its main clinical phenotypes, and its simulators, in order to contribute to an early suspicion or recognition of contact dermatitis and enable a correct differential diagnosis.

Getting your DUCs in a row - standardising the representation of Digital Use Conditions.

Improving patient care and advancing scientific discovery requires responsible sharing of research data, healthcare records, biosamples, and biomedical resources that must also respect applicable use conditions. Defining a standard to structure and manage these use conditions is a complex and challenging task. This is exemplified by a near unlimited range of asset types, a high variability of applicable conditions, and differing applications at the individual or collective level. Furthermore, the specifics and granularity required are likely to vary depending on the ultimate contexts of use. All these factors confound alignment of institutional missions, funding objectives, regulatory and technical requirements to facilitate effective sharing. The presented work highlights the complexity and diversity of the problem, reviews the current state of the art, and emphasises the need for a flexible and adaptable approach. We propose Digital Use Conditions (DUC) as a framework that addresses these needs by leveraging existing standards, striking a balance between expressiveness versus ambiguity, and considering the breadth of applicable information with their context of use.

Common conditions of use elements. Atomic concepts for consistent and effective information governance.

Myriad policy, ethical and legal considerations underpin the sharing of biological resources, implying the need for standardised and yet flexible ways to digitally represent diverse 'use conditions'. We report a core lexicon of terms that are atomic, non-directional 'concepts of use', called Common Conditions of use Elements. This work engaged biobanks and registries relevant to the European Joint Programme for Rare Diseases and aimed to produce a lexicon that would have generalised utility. Seventy-six concepts were initially identified from diverse real-world settings, and via iterative rounds of deliberation and user-testing these were optimised and condensed down to 20 items. To validate utility, support software and training information was provided to biobanks and registries who were asked to create Sharing Policy Profiles. This succeeded and involved adding standardised directionality and scope annotations to the employed terms. The addition of free-text parameters was also explored. The approach is now being adopted by several real-world projects, enabling this standard to evolve progressively into a universal basis for representing and managing conditions of use.

Dermonecrosis caused by a spitting cobra snakebite results from toxin potentiation and is prevented by the repurposed drug varespladib.

Snakebite envenoming is a neglected tropical disease that causes substantial mortality and morbidity globally. The venom of African spitting cobras often causes permanent injury via tissue-destructive dermonecrosis at the bite site, which is ineffectively treated by current antivenoms. To address this therapeutic gap, we identified the etiological venom toxins in Naja nigricollis venom responsible for causing local dermonecrosis. While cytotoxic three-finger toxins were primarily responsible for causing spitting cobra cytotoxicity in cultured keratinocytes, their potentiation by phospholipases A2 toxins was essential to cause dermonecrosis in vivo. This evidence of probable toxin synergism suggests that a single toxin-family inhibiting drug could prevent local envenoming. We show that local injection with the repurposed phospholipase A2-inhibiting drug varespladib significantly prevents local tissue damage caused by several spitting cobra venoms in murine models of envenoming. Our findings therefore provide a therapeutic strategy that may effectively prevent life-changing morbidity caused by snakebite in rural Africa.

Genomic and global gene expression profiling in pediatric and young adult acute leukemia with PICALM::MLLT10 Fusion.

PICALM: MLLT10 fusion is a rare but recurrent genetic driver in acute leukemias. To better understand the genomic landscape of PICALM::MLLT10 (PM) positive acute leukemia, we performed genomic profiling and gene expression profiling in twenty PM-positive patients, including AML (n = 10), T-ALL/LLy (n = 8), Mixed-phenotype acute leukemia (MPAL), T/B (n = 1) and acute undifferentiated leukemia (AUL) (n = 1). Besides confirming the known activation of HOXA, differential gene expression analysis compared to hematopoietic stem cells demonstrated the enrichment of genes associated with cell proliferation-related pathways and relatively high expression of XPO1 in PM-AML and PM-T-ALL/LLy. Our study also suggested PHF6 disruption as a key cooperating event in PICALM::MLLT10-positive leukemias. In addition, we demonstrated differences in gene expression profiles as well as remarkably different spectra of co-occurring mutations between PM-AML and PM-T-ALL/LLy. Alterations affecting TP53 and NF1, hallmarks of PM-AML, are strongly associated with disease progression and relapse, whereas EZH2 alterations are highly enriched in PM-T-ALL/LLy. This comprehensive genomic and transcriptomic profiling provides insights into the pathogenesis and development of PICALM::MLLT10 positive acute leukemia.

FAIR Header Reference genome: a TRUSTworthy standard.

The lack of interoperable data standards among reference genome data-sharing platforms inhibits cross-platform analysis while increasing the risk of data provenance loss. Here, we describe the FAIR bioHeaders Reference genome (FHR), a metadata standard guided by the principles of Findability, Accessibility, Interoperability and Reuse (FAIR) in addition to the principles of Transparency, Responsibility, User focus, Sustainability and Technology. The objective of FHR is to provide an extensive set of data serialisation methods and minimum data field requirements while still maintaining extensibility, flexibility and expressivity in an increasingly decentralised genomic data ecosystem. The effort needed to implement FHR is low; FHR's design philosophy ensures easy implementation while retaining the benefits gained from recording both machine and human-readable provenance.

Diversity and Molecular Evolution of Antimicrobial Peptides in Caecilian Amphibians.

Antimicrobial peptides (AMPs) are key molecules in the innate immune defence of vertebrates with rapid action, broad antimicrobial spectrum, and ability to evade pathogen resistance mechanisms. To date, amphibians are the major group of vertebrates from which most AMPs have been characterised, but most studies have focused on the bioactive skin secretions of anurans (frogs and toads). In this study, we have analysed the complete genomes and/or transcriptomes of eight species of caecilian amphibians (order Gymnophiona) and characterised the diversity, molecular evolution, and antimicrobial potential of the AMP repertoire of this order of amphibians. We have identified 477 candidate AMPs within the studied caecilian genome and transcriptome datasets. These candidates are grouped into 29 AMP families, with four corresponding to peptides primarily exhibiting antimicrobial activity and 25 potentially serving as AMPs in a secondary function, either in their entirety or after cleavage. In silico prediction methods were used to identify 62 of those AMPs as peptides with promising antimicrobial activity potential. Signatures of directional selection were detected for five candidate AMPs, which may indicate adaptation to the different selective pressures imposed by evolutionary arms races with specific pathogens. These findings provide encouraging support for the expectation that caecilians, being one of the least-studied groups of vertebrates, and with ~300 million years of separate evolution, are an underexplored resource of great pharmaceutical potential that could help to contest antibiotic resistance and contribute to biomedical advance.

African polyvalent antivenom can maintain pharmacological stability and ability to neutralise murine venom lethality for decades post-expiry: evidence for increasing antivenom shelf life to aid in alleviating chronic shortages.

INTRODUCTION: Antivenom is a lifesaving medicine for treating snakebite envenoming, yet there has been a crisis in antivenom supply for many decades. Despite this, substantial quantities of antivenom stocks expire before use. This study has investigated whether expired antivenoms retain preclinical quality and efficacy, with the rationale that they could be used in emergency situations when in-date antivenom is unavailable. METHODS: Using WHO guidelines and industry test requirements, we examined the in vitro stability and murine in vivo efficacy of eight batches of the sub-Saharan African antivenom, South African Institute for Medical Research polyvalent, that had expired at various times over a period of 30 years. RESULTS: We demonstrate modest declines in immunochemical stability, with antivenoms older than 25 years having high levels of turbidity. In vitro preclinical analysis demonstrated all expired antivenoms retained immunological recognition of venom antigens and the ability to inhibit key toxin families. All expired antivenoms retained comparable in vivo preclinical efficacy in preventing the lethal effects of envenoming in mice versus three regionally and medically important venoms. CONCLUSIONS: This study provides strong rationale for stakeholders, including manufacturers, regulators and health authorities, to explore the use of expired antivenom more broadly, to aid in alleviating critical shortages in antivenom supply in the short term and the extension of antivenom shelf life in the longer term.

A Retrospective Longitudinal Study of 460 Patients with ABCA4-Associated Retinal Disease.

PURPOSE: To investigate the distribution of genotypes and natural history of ABCA4-associated retinal disease in a large cohort of patients seen at a single institution. DESIGN: Retrospective, single-institution cohort review. PARTICIPANTS: Patients seen at the University of Iowa between November 1986 and August 2022 clinically suspected to have disease caused by sequence variations in ABCA4. METHODS: DNA samples from participants were subjected to a tiered testing strategy progressing from allele-specific screening to whole genome sequencing. Charts were reviewed, and clinical data were tabulated. The pathogenic severity of the most common alleles was estimated by studying groups of patients who shared 1 allele. Groups of patients with shared genotypes were reviewed for evidence of modifying factor effects. MAIN OUTCOME MEASURES: Age at first uncorrectable vision loss, best-corrected visual acuity, and the area of the I2e isopter of the Goldmann visual field. RESULTS: A total of 460 patients from 390 families demonstrated convincing clinical features of ABCA4-associated retinal disease. Complete genotypes were identified in 399 patients, and partial genotypes were identified in 61. The median age at first vision loss was 16 years (range, 4-76 years). Two hundred sixty-five families (68%) harbored a unique genotype, and no more than 10 patients shared any single genotype. Review of the patients with shared genotypes revealed evidence of modifying factors that in several cases resulted in a > 15-year difference in age at first vision loss. Two hundred forty-one different alleles were identified among the members of this cohort, and 161 of these (67%) were found in only a single individual. CONCLUSIONS: ABCA4-associated retinal disease ranges from a very severe photoreceptor disease with an onset before 5 years of age to a late-onset retinal pigment epithelium-based condition resembling pattern dystrophy. Modifying factors frequently impact the ABCA4 disease phenotype to a degree that is similar in magnitude to the detectable ABCA4 alleles themselves. It is likely that most patients in any cohort will harbor a unique genotype. The latter observations taken together suggest that patients' clinical findings in most cases will be more useful for predicting their clinical course than their genotype. FINANCIAL DISCLOSURE(S): Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

EURO-NMD registry: federated FAIR infrastructure, innovative technologies and concepts of a patient-centred registry for rare neuromuscular disorders.

BACKGROUND: The EURO-NMD Registry collects data from all neuromuscular patients seen at EURO-NMD's expert centres. In-kind contributions from three patient organisations have ensured that the registry is patient-centred, meaningful, and impactful. The consenting process covers other uses, such as research, cohort finding and trial readiness. RESULTS: The registry has three-layered datasets, with European Commission-mandated data elements (EU-CDEs), a set of cross-neuromuscular data elements (NMD-CDEs) and a dataset of disease-specific data elements that function modularly (DS-DEs). The registry captures clinical, neuromuscular imaging, neuromuscular histopathology, biological and genetic data and patient-reported outcomes in a computer-interpretable format using selected ontologies and classifications. The EURO-NMD registry is connected to the EURO-NMD Registry Hub through an interoperability layer. The Hub provides an entry point to other neuromuscular registries that follow the FAIR data stewardship principles and enable GDPR-compliant information exchange. Four national or disease-specific patient registries are interoperable with the EURO-NMD Registry, allowing for federated analysis across these different resources. CONCLUSIONS: Collectively, the Registry Hub brings together data that are currently siloed and fragmented to improve healthcare and advance research for neuromuscular diseases.

Hydrogen energy futures - foraging or farming?

Exploration for commercially viable natural hydrogen accumulations within the Earth's crust, here compared to 'foraging' for wild food, holds promise. However, a potentially more effective strategy lies in the in situ artificial generation of hydrogen in natural underground reservoirs, akin to 'farming'. Both biotic and abiotic processes can be employed, converting introduced or indigenous components, gases, and nutrients into hydrogen. Through studying natural hydrogen-generating reactions, we can discern pathways for optimized engineering. Some reactions may be inherently slow, allowing for a 'seed and leave' methodology, where sites are infused with gases, nutrients, and specific bacterial strains, then left to gradually produce hydrogen. However, other reactions could offer quicker outcomes to harvest hydrogen. A crucial element of this strategy is our innovative concept of 'X' components-ranging from trace minerals to bioengineered microbes. These designed components enhance biotic and/or abiotic reactions and prove vital in accelerating hydrogen production. Drawing parallels with our ancestors' transition from hunter-gathering to agriculture, we propose a similar paradigm shift in the pursuit of hydrogen energy. As we transition towards a hydrogen-centric energy landscape, the amalgamation of geochemistry, advanced biology, and engineering emerges as a beacon, signalling a pathway towards a sustainable and transformative energy future.

Comparative morphology of oral glands in snakes of the family Homalopsidae reveals substantial variation and additional independent origins of salt glands within Serpentes.

Using diffusible iodine-based contrast-enhanced computed tomography (diceCT), we examined the morphology of the oral glands of 12 species of the family Homalopsidae. Snakes of this family exhibit substantial interspecific morphological variation in their oral glands. Particular variables are the venom glands, ranging from large (e.g., Subsessor bocourti) to small (e.g., Erpeton tentaculatum). The supra- and infralabial glands are more uniform in morphology, being the second most developed in almost all the sampled species. Premaxillary glands distinct from the supralabial glands were observed in five species (Myron richardsonii, Bitia hydroides, Cantoria violacea, Fordonia leucobalia, and Gerarda prevostiana), in addition to Cerberus rynchops, the only species in which this condition was previously documented associated with the excretion of salt. In the three species of the saltwater group of homalopsids (C. violacea, F. leucobalia, and G. prevostiana), the premaxillary glands also extend posteriorly, occupying a large area above the supralabial gland, a condition not observed in any other species of snake studied thus far. Character evolution analyses indicate that premaxillary glands differentiated from the supralabial gland and evolved independently three or four times in the family, always in lineages that invaded marine habitats. Our results suggest that the differentiated premaxillary glands are likely salt glands, as is the case in C. rynchops. If corroborated, this increases to six or seven the number of independent evolutionary origins of salt glands in snakes that have undergone an evolutionary transition to marine life.

DATA RESOURCES AND ANALYSES FAIR Header Reference genome: A TRUSTworthy standard.

The lack of interoperable data standards among reference genome data-sharing platforms inhibits cross-platform analysis while increasing the risk of data provenance loss. Here, we describe the FAIR-bioHeaders Reference genome (FHR), a metadata standard guided by the principles of Findability, Accessibility, Interoperability, and Reuse (FAIR) in addition to the principles of Transparency, Responsibility, User focus, Sustainability, and Technology (TRUST). The objective of FHR is to provide an extensive set of data serialisation methods and minimum data field requirements while still maintaining extensibility, flexibility, and expressivity in an increasingly decentralised genomic data ecosystem. The effort needed to implement FHR is low; FHR's design philosophy ensures easy implementation while retaining the benefits gained from recording both machine and human-readable provenance.

Comparing morphological and secretory aspects of cephalic glands among the New World coral snakes brings novel insights on their biological roles.

Oral and other cephalic glands have been surveyed by several studies with distinct purposes. Despite the wide diversity and medical relevance of the New World coral snakes, studies focusing on understanding the biological roles of the glands within this group are still scarce. Specifically, the venom glands of some coral snakes were previously investigated but all other cephalic glands remain uncharacterized. In this sense, performing morphological and molecular analysis of these glands may help better understand their biological role. Here, we studied the morphology of the venom, infralabial, rictal, and harderian glands of thirteen species of Micrurus and Micruroides euryxanthus. We also performed a molecular characterization of these glands from selected species of Micrurus using transcriptomic and proteomic approaches. We described substantial morphological variation in the cephalic glands of New World coral snakes and structural evidence for protein-secreting cells in the inferior rictal glands. Our molecular analysis revealed that the venom glands, as expected, are majorly devoted to toxin production, however, the infralabial and inferior rictal glands also expressed some toxin genes at low to medium levels, despite the marked morphological differences. On the other hand, the harderian glands were dominated by the expression of lipocalins, but do not produce toxins. Our integrative analysis, including the prediction of biological processes and pathways, helped decipher some important traits of cephalic glands and better understand their biology.

Resistance Is Not Futile: Widespread Convergent Evolution of Resistance to Alpha-Neurotoxic Snake Venoms in Caecilians (Amphibia: Gymnophiona).

Predatory innovations impose reciprocal selection pressures upon prey. The evolution of snake venom alpha-neurotoxins has triggered the corresponding evolution of resistance in the post-synaptic nicotinic acetylcholine receptors of prey in a complex chemical arms race. All other things being equal, animals like caecilians (an Order of legless amphibians) are quite vulnerable to predation by fossorial elapid snakes and their powerful alpha-neurotoxic venoms; thus, they are under strong selective pressure. Here, we sequenced the nicotinic acetylcholine receptor alpha-1 subunit of 37 caecilian species, representing all currently known families of caecilians from across the Americas, Africa, and Asia, including species endemic to the Seychelles. Three types of resistance were identified: (1) steric hindrance from N-glycosylated asparagines; (2) secondary structural changes due to the replacement of proline by another amino acid; and (3) electrostatic charge repulsion of the positively charged neurotoxins, through the introduction of a positively charged amino acid into the toxin-binding site. We demonstrated that resistance to alpha-neurotoxins convergently evolved at least fifteen times across the caecilian tree (three times in Africa, seven times in the Americas, and five times in Asia). Additionally, as several species were shown to possess multiple resistance modifications acting synergistically, caecilians must have undergone at least 20 separate events involving the origin of toxin resistance. On the other hand, resistance in non-caecilian amphibians was found to be limited to five origins. Together, the mutations underlying resistance in caecilians constitute a robust signature of positive selection which strongly correlates with elapid presence through both space (sympatry with caecilian-eating elapids) and time (Cenozoic radiation of elapids). Our study demonstrates the extent of convergent evolution that can be expected when a single widespread predatory adaptation triggers parallel evolutionary arms races at a global scale.

Geochemical Integrity of Wellbore Cements during Geological Hydrogen Storage.

Increasing greenhouse gas emissions have put pressure on global economies to adopt strategies for climate-change mitigation. Large-scale geological hydrogen storage in salt caverns and porous rocks has the potential to achieve sustainable energy storage, contributing to the development of a low-carbon economy. During geological storage, hydrogen is injected and extracted through cemented and cased wells. In this context, well integrity and leakage risk must be assessed through in-depth investigations of the hydrogen-cement-rock physical and geochemical processes. There are significant scientific knowledge gaps pertaining to hydrogen-cement interactions, where chemical reactions among hydrogen, in situ reservoir fluids, and cement could degrade the well cement and put the integrity of the storage system at risk. Results from laboratory batch reaction experiments concerning the influence of hydrogen on cement samples under simulated reservoir conditions of North Sea fields, including temperature, pressure, and salinity, provided valuable insights into the integrity of cement for geological hydrogen storage. This work shows that, under the experimental conditions, hydrogen does not induce geochemical or structural alterations to the tested wellbore cements, a promising finding for secure hydrogen subsurface storage.

Long-term daily stream temperature record for Scotland reveals spatio-temporal patterns in warming of rivers in the past and further warming in the future.

Stream temperature is directly and indirectly affected by climate change. To be able to project future changes in stream temperature, historic trends and factors influencing these trends need to be understood. There is a demand for daily data to analyse historical trends and future changes in stream temperature. However, long-term daily stream temperature data are rare and observations of coarse temporal resolution (e.g. once-a-month) do not allow for robust trend analyses. Here, we present a methodology to reconstruct a national long-term daily stream temperature record (1960-2080) from 40 years of once-a-month observations (for 45 Scottish catchments). This involved implementing climatic and hydrological variables in generalized additive models. These models were then used in combination with regional climate projections (UKCP18 Strand 3 - RCP8.5) to predict future spatio-temporal temperature patterns. The results indicate that for the Scottish dataset (i) in addition to air temperature, the dominant environmental controls on stream temperature are unique combinations for each catchment; (ii) a general increase of up to 0.06 °C/year in historic stream temperature over all catchments resulted mainly from increases in spring and summer stream temperatures; (iii) future spatial patterns in stream temperature are more homogenous and differ therefore from the past where temperatures in N Scotland were relatively lower; (iv) future changes of up to +4.0 °C in annual stream temperature are strongest in those catchments which show lower stream temperature in the past (NW and W Scotland). These results are important in the context of water quality and stream temperature management. The methodology can be applied to smaller scale sites or to other national/global datasets enabling the analysis of historic trends and future changes at a high temporal resolution.

Caecilian Genomes Reveal the Molecular Basis of Adaptation and Convergent Evolution of Limblessness in Snakes and Caecilians.

We present genome sequences for the caecilians Geotrypetes seraphini (3.8 Gb) and Microcaecilia unicolor (4.7 Gb), representatives of a limbless, mostly soil-dwelling amphibian clade with reduced eyes, and unique putatively chemosensory tentacles. More than 69% of both genomes are composed of repeats, with retrotransposons being the most abundant. We identify 1,150 orthogroups that are unique to caecilians and enriched for functions in olfaction and detection of chemical signals. There are 379 orthogroups with signatures of positive selection on caecilian lineages with roles in organ development and morphogenesis, sensory perception, and immunity amongst others. We discover that caecilian genomes are missing the zone of polarizing activity regulatorysequence (ZRS) enhancer of Sonic Hedgehog which is also mutated in snakes. In vivo deletions have shown ZRS is required for limb development in mice, thus, revealing a shared molecular target implicated in the independent evolution of limblessness in snakes and caecilians.