T system in ascidian muscle: organization of the sarcotubular system in the caudal muscle cells of Botryllus schlosseri tadpole larvae

Structural Diversity of Arthropod Biophotonic Nanostructures Spans Amphiphilic Phase-Space

Many organisms, especially arthropods, produce vivid interference colors using diverse mesoscopic (100-350 nm) integumentary biophotonic nanostructures that are increasingly being investigated for technological applications. Despite a century of interest, precise structural knowledge of many biophotonic nanostructures and the mechanisms controlling their development remain tentative, when such knowledge can open novel biomimetic routes to facilely self-assemble tunable, multifunctional materials. Here, we use synchrotron small-angle X-ray scattering and electron microscopy to characterize the photonic nanostructure of 140 integumentary scales and setae from ∼127 species of terrestrial arthropods in 85 genera from 5 orders. We report a rich nanostructural diversity, including triply periodic bicontinuous networks, close-packed spheres, inverse columnar, perforated lamellar, and disordered spongelike morphologies, commonly observed as stable phases of amphiphilic surfactants, block copolymer, and lyotropic lipid-water systems. Diverse arthropod lineages appear to have independently evolved to utilize the self-assembly of infolding lipid-bilayer membranes to develop biophotonic nanostructures that span the phase-space of amphiphilic morphologies, but at optical length scales.

Muscle differentiation in a colonial ascidian: organisation, gene expression and evolutionary considerations

BACKGROUND

Ascidians are tunicates, the taxon recently proposed as sister group to the vertebrates. They possess a chordate-like swimming larva, which metamorphoses into a sessile adult. Several ascidian species form colonies of clonal individuals by asexual reproduction. During their life cycle, ascidians present three muscle types: striated in larval tail, striated in the heart, and unstriated in the adult body-wall.

RESULTS

In the colonial ascidian Botryllus schlosseri, we investigated organisation, differentiation and gene expression of muscle beginning from early buds to adults and during zooid regression. We characterised transcripts for troponin T (BsTnT-c), adult muscle-type (BsMA2) and cytoplasmic-type (BsCA1) actins, followed by in situ hybridisation (ISH) on sections to establish the spatio-temporal expression of BsTnT-c and BsMA2 during asexual reproduction and in the larva. Moreover, we characterised actin genomic sequences, which by comparison with other metazoans revealed conserved intron patterns.

CONCLUSION

Integration of data from ISH, phalloidin staining and TEM allowed us to follow the phases of differentiation of the three muscle kinds, which differ in expression pattern of the two transcripts. Moreover, phylogenetic analyses provided evidence for the close relationship between tunicate and vertebrate muscle genes. The characteristics and plasticity of muscles in tunicates are discussed.

Cross-coupling between voltage-dependent Ca2+ channels and ryanodine receptors in developing ascidian muscle blastomeres

1. Ascidian blastomeres of muscle lineage express voltage-dependent calcium channels (VDCCs) despite isolation and cleavage arrest. Taking advantage of these large developing cells, developmental changes in functional relations between VDCC currents and intracellular Ca2+ stores were studied. 2. Inactivation of ascidian VDCCs is Ca2+ dependent, as demonstrated by two pieces of evidence: (1) a bell-shaped relationship between prepulse voltage and amplitude during the test pulse in Ca2+, but not in Ba2+, and (2) the decay kinetics of Ca2+ currents (ICa) obtained as the size of tail currents. 3. During replacement in the external solution of Ca2+ with Ba2+, the inward current appeared biphasic: it showed rapid decay followed by recovery and slow decay. This current profile was most evident in the mixed bath solution (2 % Ca2+ and 98 % Ba2+, abbreviated to '2Ca/98Ba'). 4. The biphasic profile of I2Ca/98Ba was significantly attenuated in caffeine and in ryanodine, indicating that Ca2+ release is involved in shaping the current kinetics of VDCCs. After washing out the caffeine, the biphasic pattern was reproducibly restored by depolarizing the membrane in calcium-rich solution, which is expected to refill the internal Ca2+ stores. 5. The inhibitors of endoplasmic reticulum (ER) Ca2+-ATPase (SERCAs) cyclopiazonic acid (CPA) and thapsigargin facilitated elimination of the biphasic profile with repetitive depolarization. 6. At a stage earlier than 36 h after fertilization, the biphasic profile of I2Ca/98Ba was not observed. However, caffeine induced a remarkable decrease in the amplitude of I2Ca/98Ba and this suppression was blocked by microinjection of the Ca2+ chelator BAPTA, showing the presence of caffeine-sensitive Ca2+ stores at this stage. 7. Electron microscopic observation shows that sarcoplasmic membranes (SR) arrange closer to the sarcolemma with maturation, suggesting that the formation of the ultrastructural machinery underlies development of the cross-coupling between VDCCs and Ca2+ stores.

Developmental autonomy of muscle fine structure in muscle lineage cells of ascidian embryos

We have observed ultrastructural features of muscle differentiation in the muscle lineage cells of cleavage-arrested whole embryos and partial embryos of ascidians. Whole embryos of Ciona intestinalis and Ascidia ceratodes were cleavage-arrested with cytochalasin B at the 8-cell stage and reared to an age equivalent to several hours after hatching; these embryos formed extensive myofilaments which were often further organized into myofibrils of different sizes and densities in the peripheral cytoplasm of the two muscle lineage blastomeres (B4.1 pair). Developing myofibrils in cleavage-arrested embryos resembled the muscle elements observed in normal hatched larvae, but were less uniformly organized. A similar development of myofilaments and myofibrils occurred in the muscle lineage cells of multicellular partial embryos reared to "hatching" age. These partial embryos resulted from the isolated muscle lineage pair (B4.1) of blastomeres of the 8-cell stage (Ciona and Ascidia), and from a muscle lineage blastomere pair (B5.2) isolated at the 16-cell stage (Ascidia). Muscle lineage cells in the partial embryos were readily identified by the dense aggregates of mitochondria in their cytoplasm. Taken together, these results from the two kinds of partial embryo effectively eliminate inductive interactions with embryonic tissues other than mesodermal as a necessary factor in the onset of self-differentiation in muscle lineage cells. The relative complexity of muscle phenotype expressed in cleavage-arrested and partial embryos attests to an unusually strong developmental autonomy in the ascidian muscle lineages. This autonomy lends further support to the theory that a localized and segregated egg cytoplasmic determinant is responsible for larval muscle development in ascidian embryos.

The fine structure of developing locomotor muscles of the pelagic tunicate, cyclosalpa affinis (Thaliacea: Salpidae)

Salps are free-swimming tunicates whose peculiar life history renders them ideal for developmental studies. The solitary salp reproduces asexually by budding a stolon containing the complete developmental sequence of the aggregate generation. The ultrastructure of developing locomotor muscle of the aggregate generation of Cyclosalpa affinis was studied. The early muscle contains essentially non-striated myofibrils. However, in transverse sections, , areas indicating early I-band A-bands can be recognized. As development continues, the number of fibrils increases, the Z-line appear, and the fibrils contain more recognizable striations. The fully developed muscle has the caracteristic structure of striated muscle. Longitudinal sections show sarcomeres with irregular and discontinuous (perforated) Z-lines; H-zones are not apparent. No M-lines are seen. Throughout development, the ratio of thin to thick myofilaments is always 2:1, the ratio found in all vertebrate striated muscle. Other finding in C affinis suggest that: (1) multinucleated muscle cells are formed by the fusion of mononucleated cells, (2) membranes of adjacement mononucleated cells destined to fuse form myelin figures, and (3) these myelin figures become closely associated with mitochondria.