Morphological variation of hypaxial musculature in salamanders (Lissamphibia: Caudata)

Despite the acknowledged importance of the locomotory and respiratory functions associated with hypaxial musculature in salamanders, variation in gross morphology of this musculature has not been documented or evaluated within a phylogenetic or ecological context. In this study, we characterize and quantify the morphological variation of lateral hypaxial muscles using phylogenetically and ecologically diverse salamander species from eight families: Ambystomatidae (Ambystoma tigrinum), Amphiumidae (Amphiuma tridactylum), Cryptobranchidae (Cryptobranchus alleganiensis), Dicamptodontidae (Dicamptodon sp.), Plethodontidae (Gyrinophilus porphyriticus), Proteidae (Necturus maculosus), Salamandridae (Pachytriton sp.), and Sirenidae (Siren lacertina). For the lateral hypaxial musculature, we document 1) the presence or absence of muscle layers, 2) the muscle fiber angles of layers at mid‐trunk, and 3) the relative dorsoventral positions and cross‐sectional areas of muscle layers. Combinations of two, three, or four layers are observed. However, all species retain at least two layers with opposing fiber angles. The number of layers and the presence or absence of layers vary within species (Necturus maculosus and Siren lacertina), within genera (e.g., Triturus), and within families. No phylogenetic pattern in the number of layers can be detected with a family‐level phylogeny. Fiber angle variation of hypaxial muscles is considerable: fiber angles of the M. obliquus externus range from 20–80°; M. obliquus internus, 14–34°; M. transversus abdominis, 58–80° (acute angles measured relative to the horizontal septum). Hypaxial musculature comprises 17–37% of total trunk cross‐sectional area. Aquatic salamanders show relatively larger total cross‐sectional hypaxial area than salamanders that are primarily terrestrial. J. Morphol. 241:153–164, 1999. © 1999 Wiley‐Liss, Inc.     

Architectural gear ratio and muscle fiber strain homogeneity in segmented musculature

In the segmented axial musculature of fishes and amphibians, the patterns of muscle fiber shortening depend on both the orientation of muscle fibers relative to the long axis of the body as well as the distance of fibers from the neutral axis of bending (vertebral column). In this study we use the relatively simple architecture of salamander hypaxial muscles to explore the separate and combined effects of these morphological features on muscle fiber strains during swimming. In Siren lacertina the external oblique (EO) muscle has more obliquely oriented muscle fibers and is located further from the neutral axis of bending than the internal oblique (IO) muscle. To examine the effect of muscle fiber angle on strain patterns during swimming, we used sonomicrometry to quantify architectural gear ratio (AGR=longitudinal strain/fiber strain) in these two hypaxial muscles. By comparing the muscle fiber strains and shortening velocities of the EO and IO during swimming, we test whether variation in mediolateral position of the muscle layers is counteracted by their differences in AGR. We find that despite substantial differences in mediolateral position, the EO and IO undergo similar fiber strains and shortening velocities for a given amount of axial bending. Our results show that variation in muscle fiber angle acts to counteract differences in mediolateral position, thereby minimizing variation in muscle fiber strain and shortening velocity during swimming. These results highlight the significance of both muscle architecture and muscle moment arms in determining the fiber strains required for a given movement.     

Locomotion pattern and trunk musculoskeletal architecture among Urodela

We comparatively examined the trunk musculature and prezygapophyseal angle of mid‐trunk vertebra in eight urodele species with different locomotive modes (aquatic Siren intermedia, Amphiuma tridactylum, Necturus maculosus and Andrias japonicus; semi‐aquatic Cynops pyrrhogaster, Cynops ensicauda; and terrestrial Hynobius nigrescens, Hynobius lichenatus and Ambystoma tigrinum). We found that the more terrestrial species were characterized by larger dorsal and abdominal muscle weight ratios compared with those of the more aquatic species, whereas muscle ratios of the lateral hypaxial musculature were larger in the more aquatic species. The lateral hypaxial muscles were thicker in the more aquatic species, whereas the M. rectus abdominis was more differentiated in the more terrestrial species. Our results suggest that larger lateral hypaxial muscles function for lateral bending during underwater locomotion in aquatic species. Larger dorsalis and abdominal muscles facilitate resistance against sagittal extension of the trunk, stabilization and support of the ventral contour line against gravity in terrestrial species. The more aquatic species possessed a more horizontal prezygapophyseal angle for more flexible lateral locomotion. In contrast, the more terrestrial species have an increasingly vertical prezygapophyseal angle to provide stronger column support against gravity. Thus, we conclude trunk structure in urodeles differs clearly according to their locomotive modes.     

Distinct modes of vertebrate hypaxial muscle formation contribute to the teleost body wall musculature

The formation of the body wall musculature in vertebrates is assumed to be initiated by direct ventral extension of the somites/myotomes. This contrasts to the formation of limb muscles and muscles involved in feeding or respiration/ventilation, which are founded by migratory muscle precursors (MMPs) distant to the somites. Here, we present evidence from morphology and expression of molecular markers proposing that the formation of the two muscle layers of the teleost body wall involves both of the above mechanisms: (1) MMPs from somites 5 and 6 found an independent muscle primordium–the so-called posterior hypaxial muscle (PHM)–which subsequently gives rise to the most anterior two segments of the medial obliquus inferioris (OI) muscle. (2) Direct epithelial extension of the hypaxial myotomes generates the OI segments from somite 7 caudalward and the entire lateral obliquus superioris (OS) muscle. The findings are discussed in relation to the evolution of hypaxial myogenic patterning including functional considerations. We hypothesise that the potential of the most anterior somites to generate migratory muscle precursors is a general vertebrate feature that has been differently utilised in the evolution in vertebrate groups.     

Expression of lbx1 involved in the hypaxial musculature formation of the mouse embryo

Somites are the source of hypaxial musculature including skeletal muscles of the limb, tongue, and trunk. To get insight into the function of mouse Lbx1 homeobox gene in early somitic mesoderm differentiation, in situ hybridization analyses were performed. At the 4-6 somite stage (8 dpc), Lbx1 was first expressed in the lateral portion of the epithelial somite and dermomyotomal epithelium. This was in contrast to the expression of myf-5 in the medial region of the somite. The lateral expression of Lbx1 in somitic mesoderm then occurred regionally along the anterior-posterior body axis. Later, at 10 dpc (stage 1 of limb bud development), Lbx1-positive migrating cells originated in the lateral dermomyotomal lips at occipital, forelimb, and hindlimb levels. They also expressed Pax-3 and c-met, known as markers of the migrating limb muscle precursor cells. In stage 4 hindlimb bud (11.5 dpc), the dorsal and ventral muscle precursor populations expressed Lbx1. In stage 8 forelimb buds (12.5 dpc), Lbx1 expression was reduced in the proximal muscle masses, where the high expression of myogenin accompanying muscle differentiation was detected. These results suggest that mouse Lbx1 might be involved in the commitment or determination of a muscle cell subpopulation during hypaxial musculature development. J. Exp. Zool. 286:270-279, 2000.     

Morphology and mechanics of myosepta in a swimming salamander (Siren lacertina)

In contrast to the complex, three-dimensional shape of myomeres in teleost fishes, the lateral hypaxial muscles of salamanders are nearly planar and their myosepta run in a roughly straight line from mid-lateral to mid-ventral. We used this relatively simple system as the basis for a mathematical model of segmented musculature. Model results highlight the importance of the mechanics of myosepta in determining the shortening characteristics of a muscle segment. We used sonomicrometry to measure the longitudinal deformation of myomeres and the dorsoventral deformation of myosepta in a swimming salamander (Siren lacertina). Sonomicrometry results show that the myosepta allow some dorsoventral lengthening, indicating an amplification of myomere shortening that is greater than that produced by muscle fiber angle alone (10% muscle fiber shortening produces 28.7% myomere shortening). Polarized light and DIC microscopy of isolated hypaxial myosepta revealed that the collagen fiber orientation in hypaxial myomeres is primarily mediolateral. The mediolateral collagen fiber orientation, combined with our finding that the hypaxial myosepta lengthen dorsoventrally during swimming, suggests that one possible function of hypaxial myosepta in S. lacertina is to increase the strain amplification of the muscle fibers by reducing the mediolateral bulging of the myomeres and redirecting the bulging toward the dorsoventral direction.     

The Evolution of the Functional Role of Trunk Muscles During Locomotion in Adult Amphibians

SYNOPSIS. The axial musculature of all vertebrates consistsof two principal masses, the epaxial and hypaxial muscles. Theprimitive function of both axial muscle masses is to generatelateral bending of the trunk during swimming, as is seen inmost fishes. Within amphibians we see multiple functional andmorphological elaborations of the axial musculature. These elaborationsappear to be associated not only with movement into terrestrialhabits (salamanders), but also with subsequent locomotor specializationsof two of the three major extant amphibian clades (frogs andcaecilians). Salamanders use both epaxial and hypaxial musclesto produce lateral bending during swimming and terrestrial,quadrupedal locomotion. However during terrestrial locomotionthe hypaxial muscles are thought to perform an added function,resisting long-axis torsion of the trunk. Relative to salamanders,frogs have elaborate epaxial muscles, which function to bothstabilize and extend the iliosacral and coccygeosacral joints.These actions are important in the effective use of the hindlimbsduring terrestrial saltation and swimming. In contrast, caecilianshave relatively elaborate hypaxial musculature that is linkedto a helix of connective tissue embedded in the skin. The helixand associated hypaxial muscles form a hydrostatic skeletonaround the viscera that is continuously used to maintain bodyposture and also contributes to forward force production duringburrowing.     

The arrangement and function of octopus arm musculature and connective tissue

The morphology of the musculature and connective tissues of the arms of Octopus bimaculoides was analyzed with light microscopy. We also studied O. briareus and O. digueti, which possess relatively more elongate and less elongate arms, respectively. The morphology of the arms was found to be remarkably uniform among species. The arms consist of a densely packed three-dimensional arrangement of muscle fibers and connective tissue fibers surrounding a central axial nerve cord. Three primary muscle fiber orientations were observed: 1) transverse muscle fibers oriented in planes perpendicular to the long axis of the arm; 2) longitudinal muscle fibers oriented parallel to the long axis; and 3) oblique muscle fibers arranged in helixes around the arm. The proportion of the arm cross section occupied by each of these muscle fiber groups (relative to the total cross sectional area of the musculature) remains constant along the length of the arm, even though the arm tapers from base to tip. A thin circular muscle layer wraps the arm musculature on the aboral side only. Much of this musculature has its origin and insertion on several robust connective tissue sheets including a layer surrounding the axial nerve cord and crossed-fiber connective tissue sheets located on the oral and the aboral sides of the arm. An additional thin layer of connective tissue wraps the arm musculature laterally and also serves as a site of origin and insertion of some of the muscle fibers. The fibers of the oral and aboral crossed-fiber connective tissue sheets are arranged oblique to the long axis of the arm with the same fiber angle as the oblique muscle layers that originate and insert on the sheets. The oblique muscle layers and the crossed-fiber connective tissue sheets thus form composite right- and left-handed helical fiber arrays. Analysis of arm morphology from the standpoint of biomechanics suggests that the transverse musculature is responsible for elongation of the arms, the longitudinal musculature is responsible for shortening, and the oblique muscle layers and associated connective tissues create torsion. Arm bending may involve unilateral contraction of longitudinal muscle bundles in combination with resistance to arm diameter increase due to contraction of the transverse musculature or passive stiffness of the arm tissues. The arms may also be bent by a combination of decrease in diameter due to contraction of the transverse musculature and maintenance of constant length on one side of the arm by unilateral activity of longitudinal muscle bundles. An increase in flexural stiffness of the arm may be achieved by cocontraction of the transverse and longitudinal muscle. Torsional stiffness may be increased by simultaneous contraction of both the right- and left-handed oblique muscle layers.     

Gravitropism of Primary Roots of Zea mays L. at Different Displacement Angles

Primary roots of Zea mays were oriented at various angles fromthe vertical ranging from 99° to 1° and their subsequentbending analysed from filmed records. The maximum rate of bendingand the time before bending commenced both varied two-fold,but showed no correlation with the initial angle of tip displacement.Roots orientated to small initial angles (< 40°) oftenovershot the vertical and proceeded to oscillate around thisorientation, whereas roots oriented to large initial angles(> 60°) often failed to achieve the vertical. Roots inthis latter group resumed bending after an indeterminate time,or did so immediately after a second displacement of their tip,showing that they were not intrinsically unable to bend. Theapparently spontaneous resumption of bending after a temporaryplagiogravitropic phase is suggested as being due to noise inthe graviperception system in the root cap. The tips of rootsgrowing vertically downwards showed oscillatory bending movementsup to 10° either side of vertical. This angle correspondsto the minimum angle of displacement which induces gravitropicbending. Only when roots were oriented 10-20° from verticaldid they begin unequivocally to show a gravitropism since atsuch angles the deflection of their tips exceeded that due totheir natural oscillation.Copyright 1993, 1999 Academic Press Gravitropism, roots, Zea mays     

Myoseptal architecture of sarcopterygian fishes and salamanders with special reference to Ambystoma mexicanum

During axial undulatory swimming in fishes and salamanders muscular forces are transmitted to the vertebral axis and to the tail. One of the major components of force transmission is the myoseptal system. The structure of this system is well known in actinopterygian fishes, but has never been addressed in sarcopterygian fishes or salamanders. In this study we describe the spatial arrangement and collagen fiber architecture of myosepta in Latimeria, two dipnoans, and three salamanders in order to gain insight into function and evolution of the myoseptal system in these groups. Salamander myosepta lack prominent cones, and consist of homogenously distributed collagen fibers of various orientations that never form distinct tendons. Fiber orientations are difficult to homologize with those of fish myosepta. The myosepta of Latimeria and dipnoans (Protopterus and Neoceratodus) illustrate that major changes in architecture occurred in the sarcopterygian clade (loss of horizontal septum), in the rhipidistian (dipnoans + tetrapods) clade (loss of epineural and epipleural tendon), and in tetrapods (loss of lateral tendons and myoseptal folding). When compared to fishes, the myosepta of wholly aquatic salamanders (Ambystoma mexicanum, Amphiuma tridactylum, Necturus maculosus) do not have the lateral tendons we suppose serve to transfer muscular forces posteriorly. We propose that alternative structures (most conspicuously present in Ambystoma) perform this function: posteriorly the relative amount of connective tissue increases considerably, and myosepta are disintegrated to horizontal lamellae of connective tissue. The structures thought to be involved in modulation of body stiffness in fishes during swimming are also absent in salamanders. Our data also have implications for the hypothesis that salamander hypaxial myosepta are designed to increase shortening amplification of the hypaxial muscle fibers. The posterior hypaxial myosepta of all three salamander species possess only mediolaterally directed collagen fibers, which would indeed amplify the shortening of the associated muscle.     

An Experimental Investigation of the Resistance of Model Root Systems to Uprooting

The architecture of a tree root system may influence its abilityto withstand uprooting by wind loading. To determine how theroot branching pattern may alter the anchorage efficiency ofa tree, artificial model root systems with different topologiesand branching angles were built. The root systems were embeddedat various depths in wet sand and the pull-out resistance measured.A model to predict the uprooting resistance from the data collectedwas designed, allowing predictions of anchorage strength withregards to architecture. The dominant factors influencing pull-outresistance were the depth and length of roots in the soil. Themost efficient type of branching pattern predicted by the programwas one with an increased number of roots deep in the soil.The optimum branching angle most likely to resist pull-out isa vertical angle of 90° between a lateral and the main axis.The predicted mechanically optimal radial angle between a lateralbranch and its daughter is between 0 and 20°. Values ofbranching angle are compared with those measured in real woodyroot systems of European larch and Sitka spruce. Root architecture; root anchorage; pull-out resistance; windthrow; Picea sitchensis ; Larix decidua     

Analysis of muscle activation patterns during transitions into and out of high knee flexion postures

Increased risk of medial tibiofemoral osteoarthritis (OA) is linked to occupations that require frequent transitions into and out of postures which require high knee flexion (>90°). Muscle forces are major contributors to joint loading, and an association between compressive forces due to muscle activations and the degeneration of joint cartilage has been suggested. The purpose of this study was to evaluate muscle activation patterns of muscles crossing the knee during transitions into and out of full-flexion kneeling and squatting, sitting in a low chair, and gait. Both net and co-activation were greater when transitioning out of high flexion postures, with maximum activation occurring at knee angles greater than 100°. Compared to gait, co-activation levels during high flexion transitions were up to approximately 3 times greater. Co-activation was significantly greater in the lateral muscle group compared to the medial group during transitions into and out of high flexion postures. These results suggest that compression due to activation of the medial musculature of the knee may not be the link between high knee flexion postures and increased medial knee OA observed in occupational settings. Further research on a larger subject group and workers with varying degrees of knee OA is necessary.     

Orientation of the N-terminal lobe of the myosin regulatory light chain in skeletal muscle fibers

The orientation of the N-terminal lobe of the myosin regulatory light chain (RLC) in demembranated fibers of rabbit psoas muscle was determined by polarized fluorescence. The native RLC was replaced by a smooth muscle RLC with a bifunctional rhodamine probe attached to its A, B, C, or D helix. Fiber fluorescence data were interpreted using the crystal structure of the head domain of chicken skeletal myosin in the nucleotide-free state. The peak angle between the lever axis of the myosin head and the fiber or actin filament axis was 100—110° in relaxation, isometric contraction, and rigor. In each state the hook helix was at an angle of ～40° to the lever/filament plane. The in situ orientation of the RLC D and E helices, and by implication of its N- and C-lobes, was similar in smooth and skeletal RLC isoforms. The angle between these two RLC lobes in rigor fibers was different from that in the crystal structure. These results extend previous crystallographic evidence for bending between the two lobes of the RLC to actin-attached myosin heads in muscle fibers, and suggest that such bending may have functional significance in contraction and regulation of vertebrate striated muscle.     

Abdominal muscle function in ventilation and locomotion in new world opossums and basal eutherians: Breathing and running with and without epipubic bones

All tetrapods have the same four basic abdominal hypaxial muscle layers that wrap around the abdomen between the pelvis, ribcage, and spine. However, the marsupials and our immediate mammalian ancestors have epipubic bones extending anteriorly into the ventral hypaxial layers with two additional muscles connecting them to the ventral midline and femur. Studies of two marsupials have shown that all of the abdominal hypaxials play a part bilaterally in resting ventilation and during locomotion there is an asymmetrical pattern of activity as the hypaxial muscles form a cross‐couplet linkage that uses the epipubic bone as a lever to provide long‐axis support of the body between diagonal limb couplets during each step. The cross‐couplet epipubic lever system defines the earliest mammals and is lost in placental mammals. To expand our understanding of the evolution of mammalian abdominal muscle function and loco‐ventilatory integration we tested the generality of the cross‐couplet system in marsupials and conducted the first formal studies of hypaxial abdominal motor patterns in generalized placental mammals focusing on a representative rodent and insectivore. These new data reveal 1) that continuous abdominal muscle tonus during resting ventilation and a 1:1 breath to step cycle during locomotion appear to be the basal condition for mammals, 2) that the loss of epipubic bones in eutherians is associated with a shift from the cross‐couplet dominated motor pattern of marsupials to a shoulder‐to‐pelvis system with unilateral activation of abdominal muscles during locomotion and 3) that hypaxial function in generalized eutherians is more similar to marsupials than cursorial mammals. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

Mediolateral somitic origin of ribs and dermis determined by quail-chick chimeras

Somites are transient mesodermal structures giving rise to all skeletal muscles of the body, the axial skeleton and the dermis of the back. Somites arise from successive segmentation of the presomitic mesoderm (PSM). They appear first as epithelial spheres that rapidly differentiate into a ventral mesenchyme, the sclerotome, and a dorsal epithelial dermomyotome. The sclerotome gives rise to vertebrae and ribs while the dermomyotome is the source of all skeletal muscles and the dorsal dermis. Quail-chick fate mapping and diI-labeling experiments have demonstrated that the epithelial somite can be further subdivided into a medial and a lateral moiety. These two subdomains are derived from different regions of the primitive streak and give rise to different sets of muscles. The lateral somitic cells migrate to form the musculature of the limbs and body wall, known as the hypaxial muscles, while the medial somite gives rise to the vertebrae and the associated epaxial muscles. The respective contribution of the medial and lateral somitic compartments to the other somitic derivatives, namely the dermis and the ribs has not been addressed and therefore remains unknown. We have created quail-chick chimeras of either the medial or lateral part of the PSM to examine the origin of the dorsal dermis and the ribs. We demonstrate that the whole dorsal dermis and the proximal ribs exclusively originates from the medial somitic compartment, whereas the distal ribs derive from the lateral compartment.     

From head to tail: the myoseptal system in basal actinopterygians

Experimental studies indicated that myomeres play several functional roles during swimming. Some of the functions in question are thought to change rostrocaudally, e.g., anterior myomeres are thought to generate forces, whereas posterior myomeres are thought to transmit forces. In order to determine whether these putative functions are reflected in myoseptal morphology we carried out an analysis of the myoseptal system that includes epaxial and hypaxial myosepta of all body regions for the first time. We combined clearing and staining, microdissections, polarized light microscopy, SEM technique, and length measurements of myoseptal parts to reveal the spatial arrangement, collagen fiber architecture, and rostrocaudal gradients of myosepta. We included representatives of the four basal actinopterygian clades to evaluate our findings in an evolutionary and in a functional context. Our comparison revealed a set of actinopterygian groundplan features. This includes a set of specifically arranged myoseptal tendons (epineural, epipleural, lateral, and myorhabdoid tendons) in all epaxial and postanal hypaxial myosepta. Only preanal hypaxial myosepta lack tendons and exclusively consist of mediolateral fibers. Laterally, myosepta generally align with the helically wound fibers of the dermis in order not to limit the body's maximum curvature. Medially, the relationship of myosepta to vertebrae clearly differs from a 1:1 relationship: a myoseptum attaches to the anterior margin of a vertebra, turns caudally, and traverses at least three vertebrae in an almost horizontal orientation in all body regions. By this arrangement, horizontal multiple layers of myosepta are formed along the trunk dorsal and ventral to the horizontal septum. Due to their reinforcement by epineural or epipleural tendons, these multiple layers are hypothesized to resist the radial expansion of underlying muscle fibers and thus contribute to modulation of body stiffness. Rostrocaudally, a dorsoventral symmetry of epaxial and hypaxial myosepta in terms of spatial arrangement and collagen fiber architecture is gradually developed towards the postanal region. Furthermore, the rostrocaudal extension of myosepta measured between anterior and posterior cones gradually increases. This myoseptal region is reinforced by longitudinal fibers of lateral tendons. Furthermore, the percentage of connective tissue in a cross section increases. These morphological data indicate that posterior myosepta are equipped for multisegmental force transmission towards the caudal fin. Anteriormost myosepta have reinforced and elongated dorsal posterior cones. They are adequately designed to transmit epaxial muscular forces to the neurocranium in order to cause its elevation during suction feeding.     

Changes in human skeletal muscle architecture and function induced by extended spaceflight

The aim of this study was to quantitatively describe the relationships between joint angles and muscle architecture (lengths (L_f) and angles (Θ_f) of fascicles) of human triceps surae [medial (MG) and lateral (LG) gastrocnemius and soleus (SOL) muscles] in vivo for three men-cosmonaut after long-duration spaceflight. Sagittal sonographs of MG, LG, SOL were taken at ankle was positioned at 15° (dorsiflexion), 0° (neutral position), +15°, and +30° (plantarflexion), with the knee at 90° at rest and after a long-duration spaceflight. At each position, longitudinal ultrasonic images of the MG and LG and SOL were obtained while the cosmonauts was relaxed from which fascicle lengths and angles with respect to the aponeuroses were determined. After space flight plantarflexor force declined significantly (26%; p < 0.001). The internal architecture of the GM, and LG, and SOL muscle was significantly altered. In the passive condition, L_f changed from 45, 53, and 39 mm (knee, 0°, ankle, −15°) to 26, 33, and 28 mm (knee, 90° ankle, 30°) for MG, LG, and SOL, respectively. Different lengths and angles of fascicles, and their changes by contraction, might be related to differences in force-producing capabilities of the muscles and elastic characteristics of tendons and aponeuroses. The three heads of the triceps surae muscle substantially differ in architecture, which probably reflects their functional roles. Differences in fiber length and pennation angle that were observed among the muscles and could be associated with differences in force production and in elastic properties of musculo-tendinous complex and aponeuroses.     

Branch bending affected floral bud development and nutrient accumulation in shoot terminals of ‘Fuji’ and ‘Gala’ apples

Branch bending has been practiced for decades in China to induce flower buds in ‘Fuji’ apple. However, the optimum bending angle is yet to be elucidated. The main objectives of this study were to compare the effect of branch bending angles (70°, 90° and 110°) on the flowering and nutrient accumulation of 1-year-old shoots of ‘Fuji’ and ‘Gala’ apples and to determine the optimum branch bending angle for each cultivar. In both cultivars, the production of spurs and terminal flower buds, and the total sugar concentration and the carbon-to-nitrogen (C/N) ratio in the shoot terminals increased, whereas the N concentration decreased with increasing bending angles. The nutrient concentration was significantly higher in spurs than in medium and long shoots. The distinction between the changing patterns of C and N concentrations in the bent shoots during the growing season in our study suggested the competition of these two nutrients caused by vegetative and reproductive growth at different growing times. In ‘Fuji’ apple, the proportion of flowering buds appeared to increase more rapidly with the increase of bending angle from 70° to 110° than that in ‘Gala’ apple, and particularly a higher proportion of spurs was observed on ‘Fuji’ branches bent at a larger angle. The increase in the total sugar concentration and the C/N ratio in the shoot terminals of the bent branches might be involved in inducing floral buds after bending. The optimum bending angle was about 90° for ‘Gala’ apple and 110° for ‘Fuji’ apple, respectively. Bending could help farmers to reduce the severity of biennial fruiting in ‘Fuji’ apple.     

Function of the medial red muscle during sustained swimming in common thresher sharks: Contrast and convergence with thunniform swimmers

Through convergent evolution tunas and lamnid sharks share thunniform swimming and a medial position of the red, aerobic swimming musculature. During continuous cruise swimming these muscles move uniformly out of phase with local body curvature and the surrounding white muscle tissue. This design results in thrust production primarily from the caudal fin rather than causing whole-body undulations. The common thresher shark (Family Alopiidae) is the only other fish known to share the same medial red muscle anatomy as the thunniform swimmers. However, the overall body shape and extremely heterocercal caudal fin of the common thresher is not shared with the thunniform swimmers, which have both fusiform bodies and high aspect-ratio, lunate caudal fins. Our study used sonomicrometry to measure the dynamics of red and white muscle movement in common thresher sharks swimming in the ocean to test whether the medial position of red muscle is associated with uncoupling of muscle shortening and local body bending as characteristic of thunniform swimmers. Common threshers (～ 60–100 kg) instrumented with sonomicrometric and electromyographic (EMG) leads swam alongside of the vessel with a tail-beat frequency of ～ 0.5 Hz. EMG signals confirmed that only the red muscle was active during sustained swimming. Despite the more medial position of the red muscle relative to the white muscle, its strain was approximately 1.5-times greater than that of the overlying white muscle, and there was a notable phase shift between strain trajectories in the red muscle and adjacent white muscle. These results suggest an uncoupling (shearing) of the red muscle from the adjacent white muscle. Although the magnitude of the phase shift between red and white muscle strain was relatively constant within individuals, it varied among sharks, ranging from near zero (red and white in phase) to almost 180° out of phase. This extent in variability has not been documented previously for thunniform swimmers with a medial red muscle position and may be a characteristic of the thresher's unique body and caudal fin morphology. Nonetheless, the uncoupling of red and white muscle strain remains a consistent character associated with fishes having a medially positioned red muscle.     

Evolution of high-performance swimming in sharks: transformations of the musculotendinous system from subcarangiform to thunniform swimmers

In contrast to all other sharks, lamnid sharks perform a specialized fast and continuous "thunniform" type of locomotion, more similar to that of tunas than to any other known shark or bony fish. Within sharks, it has evolved from a subcarangiform mode. Experimental data show that the two swimming modes in sharks differ remarkably in kinematic patterns as well as in muscle activation patterns, but the morphology of the underlying musculotendinous system (red muscles and myosepta) that drives continuous locomotion remains largely unknown. The goal of this study was to identify differences in the musculotendinous system of the two swimming types and to evaluate these differences in an evolutionary context. Three subcarangiform sharks (the velvet belly lantern shark, Etmopterus spinax, the smallspotted catshark, Scyliorhinus canicula, and the blackmouth catshark, Galeus melanostomus) from the two major clades (two galeans, one squalean) and one lamnid shark, the shortfin mako, Isurus oxyrhinchus, were compared with respect to 1) the 3D shape of myomeres and myosepta of different body positions; 2) the tendinous architecture (collagenous fiber pathways) of myosepta from different body positions; and 3) the association of red muscles with myoseptal tendons. Results show that the three subcarangiform sharks are morphologically similar but differ remarkably from the lamnid condition. Moreover, the "subcarangiform" morphology is similar to the condition known from teleostomes. Thus, major features of the "subcarangiform" condition in sharks have evolved early in gnathostome history: Myosepta have one main anterior-pointing cone and two posterior-pointing cones that project into the musculature. Within a single myoseptum cones are connected by longitudinally oriented tendons (the hypaxial and epaxial lateral and myorhabdoid tendons). Mediolaterally oriented tendons (epineural and epipleural tendons; mediolateral fibers) connect vertebral axis and skin. An individual lateral tendon spans only a short distance along the body (a fraction between 0.05 and 0.075 of total length, L, of the shark). This span is similar in all tendons along the body. Red muscles insert into the midregion of the lateral tendons. The shortfin mako differs substantially from this condition in several respects: Red muscles are internalized and separated from white muscles by a sheath of lubricative connective tissue. They insert into the anterior part of the hypaxial lateral tendon. Rostrocaudally, this tendon becomes very distinct and its span increases threefold (0.06L anteriorly to 0.19L posteriorly). Mediolateral fibers do not form distinct epineural/epipleural tendons in the mako. Since our morphological findings are in good accordance with experimental data it seems likely that the thunniform swimming mode has evolved along with the described morphological specializations.