Surface-active antibiotic production as a multifunctional adaptation for postfire microorganisms

Wildfires affect soils in multiple ways, leading to numerous challenges for colonizing microorganisms. Although it is thought that fire-adapted microorganisms lie at the forefront of postfire ecosystem recovery, the specific strategies that these organisms use to thrive in burned soils remain largely unknown. Through bioactivity screening of bacterial isolates from burned soils, we discovered that several Paraburkholderia spp. isolates produced a set of unusual rhamnolipid surfactants with a natural methyl ester modification. These rhamnolipid methyl esters (RLMEs) exhibited enhanced antimicrobial activity against other postfire microbial isolates, including pyrophilous Pyronema fungi and Amycolatopsis bacteria, compared to the typical rhamnolipids made by organisms such as Pseudomonas spp. RLMEs also showed enhanced surfactant properties and facilitated bacterial motility on agar surfaces. In vitro assays further demonstrated that RLMEs improved aqueous solubilization of polycyclic aromatic hydrocarbons, which are potential carbon sources found in char. Identification of the rhamnolipid biosynthesis genes in the postfire isolate, Paraburkholderia kirstenboschensis str. F3, led to the discovery of rhlM, whose gene product is responsible for the unique methylation of rhamnolipid substrates. RhlM is the first characterized bacterial representative of a large class of integral membrane methyltransferases that are widespread in bacteria. These results indicate multiple roles for RLMEs in the postfire lifestyle of Paraburkholderia isolates, including enhanced dispersal, solubilization of potential nutrients, and inhibition of competitors. Our findings shed new light on the chemical adaptations that bacteria employ to navigate, grow, and outcompete other soil community members in postfire environments.

Prescribed fire selects for a pyrophilous soil sub-community in a northern California mixed conifer forest

Prescribed fire is a critical strategy for mitigating the effects of catastrophic wildfires. While the above-ground response to fire has been well-documented, fewer studies have addressed the effect of prescribed fire on soil microorganisms. To understand how soil microbial communities respond to prescribed fire, we sampled four plots at a high temporal resolution (two burned, two controls), for 17 months, in a mixed conifer forest in northern California, USA. Using amplicon sequencing, we found that prescribed fire significantly altered both fungal and bacterial community structure. We found that most differentially abundant fungal taxa had a positive fold-change, while differentially abundant bacterial taxa generally had a negative fold-change. We tested the null hypothesis that these communities assembled due to neutral processes (i.e., drift and/or dispersal), finding that >90% of taxa fit this neutral prediction. However, a dynamic sub-community composed of burn-associated indicator taxa that were positively differentially abundant was enriched for non-neutral amplicon sequence variants, suggesting assembly via deterministic processes. In synthesizing these results, we identified 15 pyrophilous taxa with a significant and positive response to prescribed burns. Together, these results lay the foundation for building a process-driven understanding of microbial community assembly in the context of the classical disturbance regime of fire.

Diversity of genomic adaptations to the post-fire environment in Pezizales fungi points to crosstalk between charcoal tolerance and sexual development

Wildfires drastically impact the soil environment, altering the soil organic matter, forming pyrolyzed compounds, and markedly reducing the diversity of microorganisms. Pyrophilous fungi, especially the species from the orders Pezizales and Agaricales, are fire-responsive fungal colonizers of post-fire soil that have historically been found fruiting on burned soil and thus may encode mechanisms of processing these compounds in their genomes. Pyrophilous fungi are diverse. In this work, we explored this diversity and sequenced six new genomes of pyrophilous Pezizales fungi isolated after the 2013 Rim Fire near Yosemite Park in California, USA: Pyronema domesticum, Pyronema omphalodes, Tricharina praecox, Geopyxis carbonaria, Morchella snyderi, and Peziza echinospora. A comparative genomics analysis revealed the enrichment of gene families involved in responses to stress and the degradation of pyrolyzed organic matter. In addition, we found that both protein sequence lengths and G + C content in the third base of codons (GC3) in pyrophilous fungi fall between those in mesophilic/nonpyrophilous and thermophilic fungi. A comparative transcriptome analysis of P. domesticum under two conditions - growing on charcoal, and during sexual development - identified modules of genes that are co-expressed in the charcoal and light-induced sexual development conditions. In addition, environmental sensors such as transcription factors STE12, LreA, LreB, VosA, and EsdC were upregulated in the charcoal condition. Taken together, these results highlight genomic adaptations of pyrophilous fungi and indicate a potential connection between charcoal tolerance and fruiting body formation in P. domesticum.

Pyrolyzed Substrates Induce Aromatic Compound Metabolism in the Post-fire Fungus, Pyronema domesticum

Wildfires represent a fundamental and profound disturbance in many ecosystems, and their frequency and severity are increasing in many regions of the world. Fire affects soil by removing carbon in the form of CO2 and transforming remaining surface carbon into pyrolyzed organic matter (PyOM). Fires also generate substantial necromass at depths where the heat kills soil organisms but does not catalyze the formation of PyOM. Pyronema species strongly dominate soil fungal communities within weeks to months after fire. However, the carbon pool (i.e., necromass or PyOM) that fuels their rise in abundance is unknown. We used a Pyronema domesticum isolate from the catastrophic 2013 Rim Fire (CA, United States) to ask whether P. domesticum is capable of metabolizing PyOM. Pyronema domesticum grew readily on agar media where the sole carbon source was PyOM (specifically, pine wood PyOM produced at 750°C). Using RNAseq, we investigated the response of P. domesticum to PyOM and observed a comprehensive induction of genes involved in the metabolism and mineralization of aromatic compounds, typical of those found in PyOM. Lastly, we used 13C-labeled 750°C PyOM to demonstrate that P. domesticum is capable of mineralizing PyOM to CO2. Collectively, our results indicate a robust potential for P. domesticum to liberate carbon from PyOM in post-fire ecosystems and return it to the bioavailable carbon pool.

Exceptional Changes in Skeletal Anatomy under Domestication: The Case of Brachycephaly

"Brachycephaly" is generally considered a phenotype in which the facial part of the head is pronouncedly shortened. While brachycephaly is characteristic for some domestic varieties and breeds (e.g., Bulldog, Persian cat, Niata cattle, Anglo-Nubian goat, Middle White pig), this phenotype can also be considered pathological. Despite the superficially similar appearance of "brachycephaly" in such varieties and breeds, closer examination reveals that "brachycephaly" includes a variety of different cranial modifications with likely different genetic and developmental underpinnings and related with specific breed histories. We review the various definitions and characteristics associated with brachycephaly in different domesticated species. We discern different types of brachycephaly ("bulldog-type," "katantognathic," and "allometric" brachycephaly) and discuss morphological conditions related to brachycephaly, including diseases (e.g., brachycephalic airway obstructive syndrome). Further, we examine the complex underlying genetic and developmental processes and the culturally and developmentally related reasons why brachycephalic varieties may or may not be prevalent in certain domesticated species. Knowledge on patterns and mechanisms associated with brachycephaly is relevant for domestication research, veterinary and human medicine, as well as evolutionary biology, and highlights the profound influence of artificial selection by humans on animal morphology, evolution, and welfare.

"Cladus" and clade: a taxonomic odyssey

The fate of “clade,” both as concept and word, is reconstructed here beginning with its first appearance in 1866 as “Cladus,” in Haeckel’s Generelle Morphologie, continuing up to the present. Although central to phylogenetics, the concept of clade is paradoxical since it has been ambiguously understood or even misunderstood by its own promoters. Writings by Ernst Haeckel, Lucien Cuénot, and Julian Huxley, the three authors who discussed the notion of clade at length, are analyzed here in detail as a means of exploring this paradox. First conceived as a rank for a higher-level category, and later as a taxon, the clade is understood today in connection with Hennig’s definition of a monophyletic group rather than through Huxley’s successful but somehow ambiguous formalization. The inability of these authors to formulate a clear-cut exposition of the concept is considered here within three contexts: firstly, the burden of pre-Darwinian classifications based on similarity; secondly, the underestimation of Darwin’s description of a genealogical group; and thirdly, the predominance of thinking in process (vs thinking in pattern), which was the basis of evolutionary systematics in the mid-twentieth century.

Cooperation, Competition, and Specialized Metabolism in a Simplified Root Nodule Microbiome

Microbiomes associated with various plant structures often contain members with the potential to make specialized metabolites, e.g., molecules with antibacterial, antifungal, or siderophore activities. However, when and where microbes associated with plants produce specialized metabolites, and the potential role of these molecules in mediating intramicrobiome interactions, is not well understood. Root nodules of legume plants are organs devoted to hosting symbiotic bacteria that fix atmospheric nitrogen and have recently been shown to harbor a relatively simple accessory microbiome containing members with the ability to produce specialized metabolites in vitro On the basis of these observations, we sought to develop a model nodule microbiome system for evaluating specialized microbial metabolism in planta Starting with an inoculum derived from field-grown Medicago sativa nodules, serial passaging through gnotobiotic nodules yielded a simplified accessory community composed of four members: Brevibacillus brevis, Paenibacillus sp., Pantoea agglomerans, and Pseudomonas sp. Some members of this community exhibited clear cooperation in planta, while others were antagonistic and capable of disrupting cooperation between other partners. Using matrix-assisted laser desorption ionization-imaging mass spectrometry, we found that metabolites associated with individual taxa had unique distributions, indicating that some members of the nodule community were spatially segregated. Finally, we identified two families of molecules produced by B. brevisin planta as the antibacterial tyrocidines and a novel set of gramicidin-type molecules, which we term the britacidins. Collectively, these results indicate that in addition to nitrogen fixation, legume root nodules are likely also sites of active antimicrobial production.

Communicate and Fuse: How Filamentous Fungi Establish and Maintain an Interconnected Mycelial Network

Cell-to-cell communication and cell fusion are fundamental biological processes across the tree of life. Survival is often dependent upon being able to identify nearby individuals and respond appropriately. Communication between genetically different individuals allows for the identification of potential mating partners, symbionts, prey, or predators. In contrast, communication between genetically similar (or identical) individuals is important for mediating the development of multicellular organisms or for coordinating density-dependent behaviors (i.e., quorum sensing). This review describes the molecular and genetic mechanisms that mediate cell-to-cell communication and cell fusion between cells of Ascomycete filamentous fungi, with a focus on Neurospora crassa. Filamentous fungi exist as a multicellular, multinuclear network of hyphae, and communication-mediated cell fusion is an important aspect of colony development at each stage of the life cycle. Asexual spore germination occurs in a density-dependent manner. Germinated spores (germlings) avoid cells that are genetically different at specific loci, while chemotropically engaging with cells that share identity at these recognition loci. Germlings with genetic identity at recognition loci undergo cell fusion when in close proximity, a fitness attribute that contributes to more rapid colony establishment. Communication and cell fusion also occur between hyphae in a colony, which are important for reinforcing colony architecture and supporting the development of complex structures such as aerial hyphae and sexual reproductive structures. Over 70 genes have been identified in filamentous fungi (primarily N. crassa) that are involved in kind recognition, chemotropic interactions, and cell fusion. While the hypothetical signal(s) and receptor(s) remain to be described, a dynamic molecular signaling network that regulates cell-cell interactions has been revealed, including two conserved MAP-Kinase cascades, a conserved STRIPAK complex, transcription factors, a NOX complex involved in the generation of reactive oxygen species, cell-integrity sensors, actin, components of the secretory pathway, and several other proteins. Together these pathways facilitate the integration of extracellular signals, direct polarized growth, and initiate a transcriptional program that reinforces signaling and prepares cells for downstream processes, such as membrane merger, cell fusion and adaptation to heterokaryon formation.

Sagittal joint instability in the cranial cruciate ligament insufficient canine stifle. Caudal slippage of the femur and not cranial tibial subluxation

OBJECTIVE

This in vivo study qualitatively describes the sagittal motion pattern of the cranial cruciate ligament (CrCL) insufficient canine stifle in operated and unoperated joints with cranio-caudal laxity on palpation.

MATERIAL AND METHODS

Sagittal stifle kinematics were recorded in vivo in dogs (> 15 kg BW) with unilateral (n = 7) or bilateral (n = 6) complete CrCL rupture and positive cranial drawer test as well as two sound control dogs using uniplanar fluoroscopic kinematography with the dogs walking on a treadmill. Stifle stability and sagittal motion pattern of the femur and the tibia were determined by visual inspection of the fluoroscopic video sequences.

RESULTS

Control dogs showed no cranio-caudal instability, identical to the contralateral stifles of the dogs with unilateral rupture. All unoperated stifles with CrCL rupture (n = 6) showed caudal slippage of the femur at the beginning of the stance phase. Of the 13 operated stifles (TightRope: n = 1, tibial tuberosity advancement, TTA: n = 6, tibial plateau leveling osteotomy, TPLO: n = 5, cranial closing wedge osteotomy, CCWO: n = 1) nine were unstable, showing the same motion pattern as the unoperated stifles.

CONCLUSION

In the CrCL insufficient stifle with in vivo cranio-caudal instability caudal slippage of the distal femur at tow touch is the predominant motion pattern.

CLINICAL SIGNIFICANCE

The discrepancy between in vivo motion pattern and in vitro simulation of CrCL insufficiency in which cranial tibial subluxation is the predominant sagittal motion pattern warrants further studies.

Fibre type composition in the lumbar perivertebral muscles of primates: implications for the evolution of orthogrady in hominoids

The axial musculoskeletal system is important for the static and dynamic control of the body during both locomotor and non-locomotor behaviour. As a consequence, major evolutionary changes in the positional habits of a species are reflected by morpho-functional adaptations of the axial system. Because of the remarkable phenotypic plasticity of muscle tissue, a close relationship exists between muscle morphology and function. One way to explore major evolutionary transitions in muscle function is therefore by comparative analysis of fibre type composition. In this study, the three-dimensional distribution of slow and fast muscle fibres was analysed in the lumbar perivertebral muscles of two lemuriform (mouse lemur, brown lemur) and four hominoid primate species (white-handed gibbon, orangutan, bonobo, chimpanzee) in order to develop a plausible scenario for the evolution of the contractile properties of the axial muscles in hominoids and to discern possible changes in muscle physiology that were associated with the evolution of orthogrady. Similar to all previously studied quadrupedal mammals, the lemuriform primates in this study exhibited a morpho-functional dichotomy between deep slow contracting local stabilizer muscles and superficial fast contracting global mobilizers and stabilizers and thus retained the fibre distribution pattern typical for quadrupedal non-primates. In contrast, the hominoid primates showed no regionalization of the fibre types, similar to previous observations in Homo. We suggest that this homogeneous fibre composition is associated with the high functional versatility of the axial musculature that was brought about by the evolution of orthograde behaviours and reflects the broad range of mechanical demands acting on the trunk in orthograde hominoids. Because orthogrady is a derived character of euhominoids, the uniform fibre type distribution is hypothesized to coincide with the evolution of orthograde behaviours.

Arboreal locomotion in small new-world monkeys

The postural and locomotor activity and its relation to substrates was observed in 3 Saguinus oedipus, and 3 Saimiri sciureus for comparison, during a period of 10 h for each individual. The animals moved freely in cages of 3.40 m x 3.40 m x 2.40 m (height) on rather diverse substrates. Observations were made according to the focal-animal-method, with combined instantaneous and continuous sampling. They were protocoled in schematic form and video-recorded. In addition, 3 further Saguinus oedipus were subjected to an X-ray cinematographic study on a modified treadmill to unveil metric parameters of the locomotor pattern preferred on slender and compliant ("arboreal") substrates, the walk. Independent from the substrates, the postures of the two species differed in details, as do the preferred substrates. Horizontal, comfortable substrates are favored most. Walking ranked top in frequency, followed by jumping and galloping (in a strict sense). All other locomotor modes described for primates played a minor role or lacked entirely, like the trot. Average distance of leaps was only 0.60 m, landings were mainly on the same level as take-offs. In Saguinus, the movements of both limbs, including the shoulder blade, followed the pattern common to small mammals in general: At the end of the stance phase, humerus and tibia are nearly parallel to the substrate, while just before touchdown ulna and femur are in this position. The walk in both species was surprisingly fast (1 m/s), reaching the speed of much larger cursorial animals, like humans.

Three-dimensional reconstruction of the anal striated musculature in a human fetus

Conflicting opinions in the recent literature indicate that the morphological organization and function of the anorectal continence organ has, up to now, not been clearly understood. But a clear imagination of the spatial arrangements of this compound muscle system is of clinical relevance for the pediatric surgeon performing reconstructive surgery. We analyzed 18-microm sections of the pelvic region of 4 human fetuses in order to describe the individual components of this muscle complex. A series of 630 Azan-stained sections was the base for the computer-assisted 3D reconstruction of the levator ani and the external sphincter complex in a male human fetus (14th week p.c.). In this context, special attention was paid to the intermediate muscle layer of the puborectalis which develops ventrally from the funnel-shaped levator ani and joins the tripartite ring system of the sphincter muscle dorsally. Our findings lead to a clear imagination of the spatial arrangement of this intermediate layer and characterize the anorectal muscle complex as an integrated ensemble in which the puborectalis holds a key position.

Spatiotemporal surface EMG characteristics from rat triceps brachii muscle during treadmill locomotion indicate selective recruitment of functionally distinct muscle regions

Multichannel surface EMG recordings of a multiheaded skeletal muscle during cyclic locomotion combined with cineradiography were analysed in a chronic experiment. The resulting detailed two-dimensional activation pattern from the long and lateral triceps brachii heads of the rat during treadmill locomotion were combined with gait characteristics and fibre typing of the muscle. Shortly before ground contact of the forelimb, maximum muscle activity was found in the proximal part of the long head of the muscle. During the stance phase maximum activity was observed in the proximal part of the lateral head. The frequency dependent behaviour of cross-covariance functions over both muscle heads confirmed this selective shift in activation. In the lateral triceps brachii head of the investigated rats, exclusively type II fibres were found. In the long head the frequency of type I fibres was the highest in the deep muscle layers, proximally more than distally, whereas type II fibres were dominant in more superficial muscle layers. A combination of physiological and histological findings supports an anticipating mechanism whereby fine-tuning of the vertical foot down manoeuvre is mainly achieved by the (type I fibre dominated) proximal deep compartment of the biarticular long triceps brachii head and force generation is predominantly executed by the monoarticular lateral triceps brachii head.

Surface EMG-recordings using a miniaturised matrix electrode: a new technique for small animals

A new method for multichannel surface-EMG measurements in small animals is presented. The underlying scientific aim is the characterisation of the spreading and the co-ordination of skeletal muscle activation between different muscles or muscle parts, depending on various motor tasks. The myoelectrical signals were recorded monopolarly by a 16-channel matrix electrode on the muscle surface directly under the skin on the fascia of the investigated muscle, without damaging the muscle. Surface-EMG's were recorded for at least 5 days after surgery without electrical interferences. During defined motor tasks, the projection of the myoelectrical activation of the different parts of the M. triceps brachii of rats (Rattus norvegicus), pikas (Ochotona rufescens) and cuis (Galea musteloides) or the M. anconeus of toads (Bufo marinus) on the muscle surface was mapped. The locomotion of the investigated animals was monitored by a three-dimensional kinematic analysis (video and/or high-speed cineradiography). There was no perceptible influence from application of EMG matrix electrode. The miniaturised matrix electrode seemed practicable in gaining insight into changes in myoelectrical activation patterns (EMG mapping). This allows a characterisation of the intramuscular co-ordination processes corresponding to the actual morphofunctional state of the investigated animals.

Accelerated contractile function and improved fatigue resistance of calf muscles in newborn piglets with IUGR

Asymmetrical intrauterine growth restriction is denoted by disproportional reduction of muscle mass compared with body weight reduction. However, effects on contractile function or tissue development of skeletal muscles were not studied until now. Therefore, isometric force output of serial-stimulated hindlimb plantar flexors was measured in thiopental-anesthetized normal weight (NW) and intrauterine growth-restricted (IUGR) 1-day-old piglets under conditions of normal, reduced (aortic cross clamping), and reestablished (clamp release) blood supply (measured by colored microspheres technique). Furthermore, muscle fiber type distribution was determined after histochemical staining, specific muscle force of the plantar flexors [quotient from absolute force divided by muscle mass (N/g)] was calculated, and glycogen content and morphometric data of the investigated muscles were estimated. Regional blood flow of hindlimb muscles was similar in NW (6 +/- 2 ml. min(-1). 100 g(-1)) and IUGR piglets (8 +/- 1 ml. min(-1). 100 g(-1)). Isometric muscle contractions induced a marked increase in regional blood flow of 4.1-fold in NW and 5-fold in stimulated hindlimb muscles of IUGR piglets (baseline blood flow). Specific force of NW piglet muscles (5.2 +/- 0.2 N/g) was significantly lower than IUGR piglet muscles (6.1 +/- 0.6 N/g; P < 0.05). Isometric muscle contractions (NW: 32.7 +/- 4.7 N; IUGR: 21.7 +/- 4.0 N) resulted in a higher rate of force decrease in the calf muscles of NW animals compared with IUGR piglets (8 +/- 2 vs. 3 +/- 1%; P < 0. 01). Functional restoration of contractile performance after hindlimb recirculation was nearly complete in IUGR piglets (98 +/- 1%), whereas in NW piglets a deficit of 9 +/- 3% was found (P < 0. 01). Muscle fiber type estimation revealed an increased proportion of type I fibers in flexor digitalis superficialis and gastrocnemius medialis in IUGR piglets (P < 0.05). These data clearly indicate that contractile function is accelerated in newborn IUGR piglets.

Cineradiographic study of forelimb movements during quadrupedal walking in the brown lemur (Eulemur fulvus, Primates: Lemuridae)

Movements of forelimb joints and segments during walking in the brown lemur (Eulemur fulvus) were analyzed using cineradiography (150 frames/sec). Metric gait parameters, forelimb kinematics, and intralimb coordination are described. Calculation of contribution of segment displacements to stance propulsion shows that scapular retroversion in a fulcrum near the vertebral border causes more than 60% of propulsion. The contribution by the shoulder joint is 30%, elbow joint 5%, and wrist joint 1%. Correlation analysis was applied to reveal the interdependency between metric and kinematic parameters. Only the effective angular movement of the elbow joint during stance is speed-dependent. Movements of all other forelimb joints and segments are independent of speed and influence, mainly, linear gait parameters (stride length, stance length). Perhaps the most important result is the hitherto unknown and unexpected degree of scapular mobility. Scapular movements consist of ante-/retroversion, adduction/abduction, and scapular rotation about the longitudinal axis. Inside rotation of the scapula (60 degrees -70 degrees ), together with flexion in the shoulder joint, mediates abduction of the humerus, which is not achieved in the shoulder joint, and is therefore strikingly different from humeral abduction in man. Movements of the shoulder joint are restricted to flexion and extension. At touch down, the shoulder joint of the brown lemur is more extended compared to that of other small mammals. The relatively long humerus and forearm, characteristic for primates, are thus effectively converted into stride length. Observed asymmetries in metric and kinematic behavior of the left and right forelimb are caused by an unequal lateral bending of the spinal column.

Three-dimensional reconstruction of histological sections using modern product-design software

BACKGROUND

Computer-based, three-dimensional reconstruction of histological sections is necessary for solving a diversity of questions in morphology and anatomy. Programming software for this purpose is difficult and time-consuming. Therefore, we tested if commercially available product-design software is useful for reconstructing anatomical virtual models.

METHODS

We used Alias Wavefront Studio software on a Silicon Graphics workstation. Sections were projected with a newly developed microslide projector on a digitizing board and digitized interactively. Alternatively, pictures of sections were digitized on screen. The resulting set of contours was used as scaffold around which a surface was created manually.

RESULTS

Repeated creation of the same object and subsequent measurements gave results that will be sufficiently reliable for many purposes. The visual presentation of objects satisfy highest demands. Little time is needed to reconstruct simple objects, and the time used for complex virtual models is acceptable. Manipulation of objects on screen is nearly in real time and rendering speed is high.

CONCLUSIONS

Product-design software packages are a readily available and useful option for anatomists who want to do virtual reconstructions quickly without programming software themselves.

Professional practice model research collaboration. Issues in longitudinal, multisite designs

Implementing and evaluating professional practice models in dynamic healthcare settings is difficult and poses unique problems for nurse researchers and administrators. The authors describe issues associated with collaborating during the conduct of longitudinal studies.

Constructional morphology: the analysis of constraints in evolution dedicated to A. Seilacher in honour of his 60. birthday

Evolutionary change is opportunistic, but its course is strongly constrained in several fundamental ways. These constraints (historical/phylogenetic, functional/adaptive, constructional/morphogenetic) and their dynamic relationships are discussed here and shown to constitute the conceptual framework of Constructional Morphology. Notwithstanding recent published opinions which claim that the "discovery" of constraints renders Neodarwinian selection theory obsolete, we regard the insights of Constructional Morphology as being entirely consistent with this theory. As is shown here in the case of the Hyracoidea, formal analysis of the constraints which have framed the evolution of various characters extends our understanding of the evolution of a taxon.

Nuclear ribonucleoprotein complexes of amphibian liver. II. Changes in the protein moiety during development

Ribonucleoprotein complexes composed of small molecular weight nuclear RNA (4--9 S) and proteins were isolated from hepatic nuclei of Rana catesbeiana (bullfrog) and the protein moiety of this nuclear ribonucleoprotein complex compared during different stages of development. SDS-polyacrylamide gel analysis of premetamorphic tadpoles and adult frog nuclear ribonucleoprotein complexes revealed that while the protein profiles of these two particles were very similar polypeptides of 47,000, 70,000, and 11,000 molecular weight were present in significantly higher concentrations in the frog ribonucleoprotein complexes. Comparison of the chromatin proteins isolated from these two developmental stages demonstrated that these three polypeptides of frog ribonucleoprotein were not contaminants from chromatin. Since these three polypeptides could not be preferentially extracted from the frog ribonucleoprotein complex by 0.5 M KCl or 1 M urea, it was unlikely that these polypeptides were bound nonspecifically to the ribonucleoprotein particle. Polypeptide analysis of the nuclear ribonucleoprotein complexes isolated from tadpoles immersed in the thyroid hormone L-thyroxine revealed an increase in two polypeptides of 37,000 and 45,000 molecular weight during metamorphosis. The absence of reduced amount of these two polypeptides in either the premetamorphic tadpole or adult frog demonstrated that their presence in Rana catesbeiana nuclear ribonucleoprotein was transient during development and specifically associated with tadpole metamorphosis. We conclude from these experiments that the nuclear ribonucleoprotein complex is a dynamic structure during Rana catesbeiana development and that specific changes in its protein composition are associated with discrete stages of amphibian development.

Nuclear ribonucleoprotein complexes of amphibian liver. I. Characterization of the complex and its small molecular weight RNA moiety

Nuclear RNA-protein complexes containing small molecular weight RNAs were isolated from hepatic nuclei of Rana catesbeiana tadpoles and frogs according to a procedure normally used for the isolation of heterogeneous nuclear ribonucleoprotein complexes from other eukaryotic tissues. Preliminary characterization of the tadpole nuclear RNP indicated a particle size of 50--70 S in sucrose density gradients and a buoyant density of 1.40 gm/ml in CsCl gradients. When analyzed on SDS-polyacrylamide gels, this complex was observed to contain at least 40 polypeptides ranging in molecular weight from 15,000 to 200,000. Nuclear RNA-protein complexes were also isolated from adult frog hepatic nuclei by the same protocol and the RNA moiety which had been purified from the frog complex was compared with the nuclear RNA isolated from the tadpole particles. Electrophoretic analysis of the nuclear RNA-protein-associated RNA revealed minor qualitative and quantitive differences in the more than 25 discrete bands (4--9 S) associated with each particle. Base analysis of tadpole and frog nuclear RNA revealed a nucleotide composition of approximately 50% adenosine plus uridine nucleotides, with an unusually high content of cytosine residues (approximately 30%). Comparison of the two RNA samples demonstrated a large increase in the adenosine content of frog unclear RNA, and the presence of a minor base in frog nuclear RNA which was absent in the tadpole sample. These results indicated that changes in the RNA content of the amphibian nuclear RNP complex had occurred during bullfrog development.

Temperature-dependent intracellular distribution of thyroxine in amphibian liver

Rana catesbeiana tadpoles were injected with [(14)C]thyroxine, and the subcellular distribution of the labeled hormone was determined. At 25 degrees the amount of isotope found in the liver was maximal after 1-2 hr and then rapidly decreased to a relatively constant value. A large percentage of the hormone was found associated with the purified nuclei isolated 24 hr after injection of [(14)C]thyroxine. Injection of [(14)C]thyroxine into tadpoles maintained at 5 degrees resulted in a much slower but constant accumulation of isotope in the liver, with virtually no movement of thyroxine into the cell nucleus. Thyroxine was bound very tightly to the chromatin fraction of the nucleus, but extraction and chromatography revealed no chemical modification of the thyroxine itself. These results suggest the presence of two temperature-dependent processes: one concerned with the transport of thyroxine into the liver cell and a second concerned with the transport of the intracellular thyroxine into the cell nucleus. It is proposed that the latter process is involved in the low-temperature inhibition of thyroxine-induced metamorphosis.