The locomotory system of pearlfish Carapus acus: What morphological features are characteristic for highly flexible fishes?

The body curvature displayed by fishes differs remarkably between species. Some nonmuscular features (e.g., number of vertebrae) are known to influence axial flexibility, but we have poor knowledge of the influence of the musculotendinous system (myosepta and muscles). Whereas this system has been described in stiff‐bodied fishes, we have little data on flexible fishes. In this study, we present new data on the musculotendinous system of a highly flexible fish and compare them to existing data on rigid fishes. We use microdissections with polarized light microscopy to study the three‐dimensional anatomy of myoseptal tendons, histology and immunohistology to study the insertion of muscle fiber types into tendons, and μ‐CT scans to study skeletal anatomy. Results are compared with published data from stiff‐bodied fishes. We identify four important morphological differences between stiff‐bodied fishes and Carapus acus: (1) Carapus bears short tendons in the horizontal septum, whereas rigid fishes have elongated tendons. (2) Carapus bears short lateral tendons in its myosepta, whereas stiff‐bodied fishes bear elongated tendons. Because of its short myoseptal tendons, Carapus retains high axial flexibility. In contrast, elongated tendons restrict axial flexibility in rigid fishes but are able to transmit anteriorly generated muscle forces through long tendons down to the tail. (3) Carapus bears distinct epineural and epipleural tendons in its myosepta, whereas these tendons are weak or absent in rigid fishes. As these tendons firmly connect vertebral axis and skin in Carapus, we consider them to constrain lateral displacement of the vertebral axis during extreme body flexures. (4) Ossifications of myoseptal tendons are only present in C. acus and other more flexible fishes but are absent in rigid fishes. The functional reasons for this remain unexplained. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

Development and metamorphic loss of the musculature in larvae of the nudibranch Phestilla sibogae: A functional ontogeny

Developmental programmes for many marine invertebrates include the assembly of muscular systems appropriate to the functions of swimming and feeding in pelagic larvae. Upon metamorphosis, that musculature is often radically re-organized to meet very different demands of post-larval life. To investigate the development and fate of musculature in the nudibranch Phestilla sibogae, embryos, larvae and metamorphosing stages were fixed, labelled with phalloidin and examined with confocal microscopy. The resultant images revealed the sequential development of both large retractor muscles and numerous finer muscles that allow the larva to manipulate the velum, foot and operculum. Observations of living specimens at the same stages as those fixed for microscopy revealed the actions of the muscles as they developed. During metamorphosis, muscles with shell attachments disintegrate as the larva transforms into a shell-less juvenile. Notably, the massive velar, pedal and opercular retractor muscles disappear during metamorphosis in a sequence that corresponds to their loss of function. Other muscles, however, that appear to be important to the embryo and free-swimming larva persist into juvenile life. The comprehensive and detailed observations of the musculature presented here provide a solid foundation for comparisons with other species with different phylogenies and life histories.     

Remodelling during the development of nematocysts in a siphonophore

Remodelling of a smoothly tapering internal tubule to form the enlarged shaft of microbasic mastigophores of the siphonophore Rosacea takes place very late in development. It occurs after deployment of the nematocyst in its final position, and after the pleats of the internal tubule have been completely obliterated in the shaft region by the growth of spine material. The optical and mechanical properties of the internal tubule at the earlier (bedspring) stage give no hint that a shaft will develop. The possible ways in which remodelling might occur inside the capsule and remote from the cytoplasm are discussed.     

An antigen present in the Drosophila central nervous system only during embryonic and metamorphic stages

We report here about an antigen that is expressed in the central nervous system (CNS) of Drosophila only during the embryonic and metamorphic stages. In Drosophila, axonogenesis and synaptogenesis occur twice during the development: first in the embryonic and second in the metamorphic stages. We generated monoclonal antibodies (MAbs) in order to obtain molecular probes for analyzing axonogenesis or synaptogenesis in the CNS on the assumption that good candidates for molecules responsible for such phenomena must be present in the neuropil during those stages exclusively. As a result, we found MAb 66B2 whose intense immunoreactivity in the neuropil of the CNS was observed exclusively in the embryo and pupa, and not in the larva and adult. Immunoblot analyses showed that MAb 66B2 binds specifically to a protein with an apparent molecular weight of 350 K and neutral pl in the prepupal CNS. A significant amount of the antigen was isolated in forms that were soluble without detergent. Results of immunohistochemistry with MAb 66B2 in a primary culture of embryos showed that some live cells in the ganglion-like cluster were stained, and that neuronal cell bodies and neurites emanating from there were negative. These results strongly suggest that the 66B2 antigen observed in the CNS is an extracellular matrix component secreted from nonneuronal cells. These developmental changes in the 66B2 immuno-reactivity in the CNS presumably reflect dynamic changes of an extracellular matrix in the CNS that are accompanied by axonogenesis or synaptogenesis. © 1992 John Wiley & Sons, Inc.     

Morphological changes during metamorphosis in cod (Gadus morhua L.), with particular reference to the development of the stomach and pyloric caeca

Metamorphosis in cod, Gadus morhua L., was investigated with respect to morphological changes in the alimentary tract (mouth, stomach and pyloric caeca) and external characters (finfold and fin development). Morphometric measurements and histological investigations were made on both larvae and juveniles and all fish lengths are given as standard length. Median unpaired fins started to develop when the fish were 9–10 mm in length. The larval median finfold was resorbed when fish reached a size of 11–15 mm. The vertebrae were formed in fish of 11–20 mm. The supraterminal mouth, seen in larvae <12 mm, changed gradually to a sub-terminal position in juveniles larger than 65 mm. The development of the stomach and pyloric caeca started at 15 mm and the juvenile form was seen in fish of 40 mm. The ability to store and grind food particles in the stomach developed gradually as fish increased in size from 20 to 40 mm. Compared to many other teleost species, cod juveniles are relatively large when the stomach and pyloric caeca become fully developed. These findings may have important implications for both start-feeding of cod larvae and the weaning of juveniles on to artificial diets.     

Warmer winters reduce frog fecundity and shift breeding phenology,which consequently alters larval development and metamorphic timing

One widely documented phenological response to climate change is the earlier occurrence of spring‐breeding events. While such climate change‐driven shifts in phenology are common, their consequences for individuals and populations have rarely been investigated. I addressed this gap in our knowledge by using a multi‐year observational study of six wood frog (Rana sylvatica) populations near the southern edge of their range. I tested first if winter temperature or precipitation affected the date of breeding and female fecundity, and second if timing of breeding affected subsequent larval development rate, mass at metamorphosis, date of metamorphosis, and survival. Warmer winters were associated with earlier breeding but reduced female fecundity. Winter precipitation did not affect breeding date, but was positively associated with female fecundity. There was no association between earlier breeding and larval survival or mass at metamorphosis, but earlier breeding was associated with delayed larval development. The delay in larval development was explained through a counterintuitive correlation between breeding date and temperature during larval development. Warmer winters led to earlier breeding, which in turn was associated with cooler post‐breeding temperatures that slowed larval development. The delay in larval development did not fully compensate for the earlier breeding, such that for every 2 days earlier that breeding took place, the average date of metamorphosis was 1 day earlier. Other studies have found that earlier metamorphosis is associated with increased postmetamorphic growth and survival, suggesting that earlier breeding has beneficial effects on wood frog populations.     

Candidate gene analysis of metamorphic timing in ambystomatid salamanders

Although much is known about the ecological significance of metamorphosis and metamorphic timing, few studies have examined the underlying genetic architecture of these traits, and no study has attempted to associate phenotypic variation to molecular variation in specific genes. Here we report on a candidate gene approach (CGA) to test specific loci for a statistical contribution to variation in metamorphic timing. Three segregating populations (SP1, SP2 and SP3) were constructed utilizing three species of paedomorphic Mexican ambystomatid salamander, including the axolotl, Ambystoma mexicanum. We used these replicated species to test the hypothesis that inheritance of alternate genotypes at two thyroid hormone receptor loci (TRalpha, TRbeta) affects metamorphic timing in ambystomatid salamanders. A significant TRalpha*SP effect indicated that variation in metamorphic timing may be influenced by TRalpha genotype, however, the effect was not a simple one, as both the magnitude and direction of the phenotypic effect depended upon the genetic background. These are the first data to implicate a specific gene in contributing to variation in metamorphic timing. In general, candidate gene approaches can be extended to any number of loci and to any organism where simple genetic crosses can be performed to create segregating populations. The approach is thus of particular value in ecological studies where target genes have been identified but the study organism is not one of the few well-characterized model systems that dominate genetic research.     

Correlated evolution of phenotypic plasticity in metamorphic timing

Phenotypic plasticity has long been a focus of research, but the mechanisms of its evolution remain controversial. Many amphibian species exhibit a similar plastic response in metamorphic timing in response to multiple environmental factors; therefore, more than one environmental factor has likely influenced the evolution of plasticity. However, it is unclear whether the plastic responses to different factors have evolved independently. In this study, we examined the relationship between the plastic responses to two experimental factors (water level and food type) in larvae of the salamander Hynobius retardatus, using a cause-specific Cox proportional hazards model on the time to completion of metamorphosis. Larvae from ephemeral ponds metamorphosed earlier than those from permanent ponds when kept at a low water level or fed conspecific larvae instead of larval Chironomidae. This acceleration of metamorphosis depended only on the permanency of the larvae's pond of origin, but not on the conspecific larval density (an indicator of the frequency of cannibalism) in the ponds. The two plastic responses were significantly correlated, indicating that they may evolve correlatively. Once plasticity evolved as an adaptation to habitat desiccation, it might have relatively easily become a response to other ecological factors, such as food type via the pre-existing developmental pathway.     

Characterization of metamorphic changes in anuran larval epidermis using lectins as probes

The skin of an adult frog of Xenopus laevis was characterized by the reactivity of 20 lectins. The lectins were classified into six groups in their binding to the epidermal cells: Lycopersicon esculentum lectin (LEL)-type which was positive for all epidermal cells; Pisum sativum agglutinin (PSA)-type for stratum germinativum; succinylated wheat germ agglutinin (sWGA)-type for strata spinosum, granulosum and corneum; Dolichos biflorus agglutinin (DBA)-type for strata germinativum and spinosum; peanut agglutinin (PNA)-type for stratum spinosum; and Ulex europaeus agglutinin (UEA-I)-type for strata granulosum and corneum. PSA and sWGA were utilized as markers of mitotically active germinative cells and the differentiated cells of the epidermis, respectively, to describe the metamorphic conversion of larval epidermal cells to adult type. PSA stained all epidermal cells of tadpoles before metamorphic climax. At the end of metamorphosis, PSA-positive cells were restricted to cells in the basal layer of body epidermis while all the tail epidermis remained PSA-positive. The other cell marker, sWGA, only stained apical cells in tadpole epidermis. During the metamorphic climax, sWGA-positive cells appeared in the cells beneath the stratum corneum of the body region, but not in the tail region. The present study demonstrates that PSA and sWGA are useful to investigate metamorphic changes in tadpole epidermal cells.     

Cloning of a cDNA for Xenopus prolactin receptor and its metamorphic expression profile

A pituitary hormone, prolactin (PRL) shows various effects on cellular metabolism in amphibians, such as stimulation of larval tissue growth and inhibition of metamorphic changes. All these effects are mediated by its cell surface receptor. However, lack of information on PRL receptor (PRL-R) gene expression has made the physiological importance of the PRL/PRL-R system obscure in amphibian metamorphosis. Hence, a Xenopus PRL-R cDNA was cloned, its structure was characterized, and specific binding of PRL to Xenopus PRL-R expressed in COS-7 cells was confirmed. In adult tissues, high level expression was found in the lung, heart, brain, thymus and skin, and low level in the oviduct, kidney and spinal cord. The developmental expression pattern showed that PRL-R messenger ribonucleic acid (mRNA) was expressed in the brain and tail from premetamorphosis and the level increased toward late metamorphosis, suggesting that PRL may inhibit the metamorphic changes in those organs. The level of brain PRL-R mRNA reached a peak just at the start of the metamorphic climax stages and then decreased, whereas in the tail, mRNA expression peaked at late metamorphosis. In the kidney, mRNA expression increased and reached a maximum level at the end of metamorphosis. The results obtained were discussed in relation to metamorphosis.     

Variation in body size and metamorphic traits of Iberian spadefoot toads over a short geographic distance

Determinants of geographic variation in body size are often poorly understood, especially in organisms with complex life cycles. We examined patterns of adult body size and metamorphic traits variation in Iberian spadefoot toad ( Pelobates cultripes ) populations, which exhibit an extreme reduction in adult body size, 71.6% reduction in body mass, within just about 30 km at south-western Spain. We hypothesized that size at and time to metamorphosis would be predictive of the spatial pattern observed in adult body size. Larvae from eight populations were raised in a common garden experiment at two different larval densities that allow to differentiate whether population divergence was genetically based or was simply a reflection of environmental variation and, in addition, whether this population divergence was modulated by differing crowding larval environments. Larger adult size populations had higher larval growth rates, attaining larger sizes at metamorphosis, and exhibited higher survival than smaller-sized populations at both densities, although accentuated at a low larval density. These population differences appeared to be consistent once embryo size variation was controlled for, suggesting that this phenotypic divergence is not due to maternal effects. Our results suggest considerable genetic differentiation in metamorphic traits that parallels and may be a causal determinant of geographic variation in adult body size.     

果蝇变态过程中的细胞凋亡和细胞自噬

程序化细胞死亡(programmed cell death,PCD)分为I型PCD细胞凋亡(apoptosis)和II型PCD细胞自噬(autophagy)。果蝇等完全变态昆虫有2种类型的器官:即细胞内分裂器官(如脂肪体、表皮、唾液腺、中肠、马氏管等)和有丝分裂器官(复眼、翅膀、足、神经系统等)。在昆虫变态过程中,细胞内分裂器官进行器官重建,幼虫器官大量发生细胞凋亡和细胞自噬到最后完全消亡,同时成虫器官由干细胞从新生成;而有丝分裂器官则由幼虫器官直接发育为成虫器官。在果蝇等昆虫的变态过程中,细胞凋亡和细胞自噬在幼虫器官的死亡和成虫器官的生成中发挥了非常重要的作用。文章简要介绍细胞凋亡和细胞自噬在果蝇变态过程中的生理功能和分子调控机制。     

Fine structure and development of Triaenophorus nodulosus (Cestoda) during metamorphosis: a review

The results of long‐term investigations of tissue differentiation during metamorphosis in the pseudophyllidean cestode Triaenophorus nodulosus are discussed. The process of tissue reorganization during transformation of the free‐swimming coracidium of T. nodulosus to a parasitic procercoid are studied and the development of all specialized systems (tegumental, muscular, excretory, nervous and glandular) are described and compared. As a consequence, the general cytological mechanism of differentiation has been identified in T. nodulosus. The cytodifferentiation processes are characterized by autophagy of the initial specialized cell population and its replacement by a second population with other morphological and functional potentials. The change in specialized cell populations during morphogenesis is caused by a change in the functional requirements of organs. The primitive nature of the differentiation processes in T. nodulosus at the initial stages of tissue formation is confirmed. It is supposed that destruction of the primary cell populations is the result of apoptosis or genetically programmed suicide.     

Re‐epithelialization of large wound in paedomorphic and metamorphic axolotls

Axolotls (Ambystoma mexicanum ) may heal their skin wounds scar‐free in both paedomorphs and metamorphs. In previous studies on small punch skin wounds, rapid re‐epithelialisation was noted in these two axolotl morphs. However, large wound size in mammals may affect wound healing. In this study, large circumferential full thickness excision wounds on the hind limbs were created on juvenile paedomorphic and metamorphic axolotls. The results showed re‐epithelialisation was more quickly initiated in paedomorphs than in metamorphs after wounding. The migrating rate of epidermis on the wound bed was faster in paedomorphs than in metamorphs and thus completion of re‐epithelialisation was faster in paedomorphs than in metamorphs. Within these re‐epithelialisation periods, neither basement membrane nor dermis was reformed. Epidermal cell proliferation was detected by EdU‐labelling technique. In the normal unwounded skin, epidermal proliferation rate was higher in paedomorphs than in metamorphs. After wounding, the epidermal proliferation rate was significantly lower in the migrating front on the wound bed than in the normal skin in paedomorphs. The EdU‐labelling rate between normal skin and migration front was not different in metamorphs. Lacking of more proliferating epidermal cells on the wound bed indicated that the new epidermis here derived rather from migrating epidermal cells than from cell proliferation in situ. In conclusion, re‐epithelialisation in the large wound might be fully completed in both morphs despite it was initiated earlier and with faster rate in paedomorphs than in metamorphs. The new epidermis on the wound bed derived mainly from cell migration than by cell proliferation in the re‐epithelialisation period. J. Morphol. 278:228–235, 2017. © 2016 Wiley Periodicals,Inc.     

Fitness consequences of larval traits persist across the metamorphic boundary

Metamorphosis is thought to provide an adaptive decoupling between traits specialized for each life-history stage in species with complex life cycles. However, an increasing number of studies are finding that larval traits can carry-over to influence postmetamorphic performance, suggesting that these life-history stages may not be free to evolve independently of each other. We used a phenotypic selection framework to compare the relative and interactive effects of larval size, time to hatching, and time to settlement on postmetamorphic survival and growth in a marine invertebrate, Styela plicata. Time to hatching was the only larval trait found to be under directional selection, individuals that took more time to hatch into larvae survived better after metamorphosis but grew more slowly. Nonlinear selection was found to act on multivariate trait combinations, once again acting in opposite directions for selection acting via survival and growth. Individuals with above average values of larval traits were most likely to survive, but surviving individuals with intermediate larval traits grew to the largest size. These results demonstrate that larval traits can have multiple, complex fitness consequences that persist across the metamorphic boundary; and thus postmetamorphic selection pressures may constrain the evolution of larval traits.     

Cellular and intracellular pathways mediating the metamorphic stimulus in hydrozoan planulae

Summary Both the natural metamorphic stimulus (an unidentified bacterial product) and an artificial trigger of metamorphosis (Cs⁺) cause large calcium transients in planula cells of the hydrozoanMitrocomella polydiademata. When these transients are inhibited with calcium channel blockers, metamorphosis is also inhibited. All cells of theMitrocomella planula contain a calcium-specific photoprotein. The cells where the calcium transients occur during natural- and Cs⁺-induced metamorphosis have been visualized in normal and entoderm free planulae that lack ganglion cells, using a compound microscope coupled to an image intensifier and video camera. During bacteria- and Cs⁺-induced metamorphosis, groups of contiguous cells, occupying from about 10% to the entire visible surface of the planula, simultaneously exhibit calcium transients. When the cells that initiate a transient comprise only part of the planula surface, the calcium transient frequently propagates and can eventually involve every cell on the visible planula surface. There is no special site on the planula surface where calcium transients are more apt to be initiated. There is no indication that propagation of a flash in one direction is more likely than in another. The velocity of propagation is virtually the same in all directions. The only feature of the spatial distribution of bacteria- and Cs⁺-induced calcium transients that appears to be necessary for the induction of metamorphosis is that at least one transient must involve all of the surface cells of the planula. The spatial behavior of calcium transients is the same in entoderm free planulae (lacking ganglion cells) as in normal planulae. The propagation of these calcium transients most probably occurs via epithelial conduction. This metamorphic step involving calcium transients is probably the intercellular communication system that informs the cells of the planula that metamorphosis will commence.Metamorphosis inMitrocomella planulae can also be induced with phorbol esters. Calcium transients do not occur during phorbol ester-induced metamorphosis, indicating that they act at a different point in the metamorphic pathway. Calcium channel blockers do not inhibit phorbol ester-induced metamorphosis. Inhibitors of protein kinase-C, inhibit both phorbol ester-induced metamorphosis and Cs⁺- and bacteria-induced metamorphosis, but have no effect on the calcium transients induced by Cs⁺. This indicates that the calcium transient mediated step in the metamorphic pathway occurs prior to protein kinase-C activation. Calcium transients probably play a major role in activating protein kinase-C.     

Visualization of the initiation and sequential expansion of the metamorphic conversion of anuran larval skin into the precursor of adult type

A tadpole of bullfrog, Rana catesbeiana , is originally covered with the larval skin over its entire body. Drastic changes arise in both the epidermis and the subcutaneous connective tissue at an early developmental stage, producing the precursor of adult type skin (pre-adult skin). It was found that calcium is a useful probe to detect the region where the precursor formation has occurred because its deposition in the upper part of subcutaneous collagen bundles coincides with the appearance of the pre-adult skin. Whole-mount in situ staining of tadpoles with alizarin red S revealed the initiation site of the premetamorphic transformation of the larval skin into the adult precursor and its ensuing region-dependent expansion. The pre-adult skin first emerged at TK II to III (TK, Taylor and Kollros staging) t lateral sides of the body, which led us to postulate that 'the center for premetamorphic skin transformation' is formed at the specific site in this region. This center moved dorsally and then ventrally, then reached to the most proximal region of the tail, yielding a unique sequential conversion pattern by around TK V when the conversion was completed in the trunk. The present study also visualized the process of the hindlimb skin transformation.