During the metamorphosis of tunicate ascidians, the swimming larva uses its three anterior papillae to detect the substrate for settlement, reabsorbs its chordate-like tail, and becomes a sessile oozooid. In view of the crucial role played by the anterior structures and their nerve relations, we applied electron microscopy and immunocytochemistry to study the larva of the colonial ascidian Botryllus schlosseri, following differentiation of the anterior epidermis during late embryogenesis, the larval stage, and the onset of metamorphosis. Rudiments of the papillae appear in the early tail-bud stage as ectodermic protrusions, the apexes of which differentiate into central and peripheral bipolar neurons. Axons fasciculate into two nerves direct to the brain. Distally, the long, rod-like dendritic terminations extend during the larval stage, becoming exposed to sea water. After the larva selects and adheres to the substrate, these neurons retract and regress. Adjacent to the papillae, other scattered neurons insinuate dendrites into the tunic and form the net of rostral trunk epidermal neurons (RTENs) which fasciculate together with the papillary neurons. Our data indicate that the papillae are simple and coniform, the papillary neurons are mechanoreceptors, and the RTENs are chemoreceptors. The interpapillary epidermal area, by means of an apocrine secretion, provides sticky material for temporary adhesion of the larva to the substrate.
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.
Background: An important question behind vertebrate evolution is whether the cranial placodes originated de novo, or if their precursors were present in the ancestor of chordates. In this respect, tunicates are of particular interest as they are considered the closest relatives to vertebrates. They are also the only chordate group possessing species that reproduce both sexually and asexually, allowing both types of development to be studied to address whether embryonic pathways have been co-opted during budding to build the same structures. Results: We studied the expression of members of the transcriptional network associated with vertebrate placodal formation (Six, Eya, and FoxI) in the colonial tunicate Botryllus schlosseri. During both sexual and asexual development, each transcript is expressed in branchial fissures and in two discrete regions proposed to be homologues to groups of vertebrate placodes. Discussion: Results reinforce the idea that placode origin predates the origin of vertebrates and that the molecular network involving these genes was co-opted in the evolution of asexual reproduction. Considering that gill slit formation in deuterostomes is based on similar expression patterns, we discuss possible alternative evolutionary scenarios depicting gene cooption as critical step in placode and pharynx evolution. Developmental Dynamics 242:752-766, 2013. V C 2013 Wiley Periodicals, Inc.Key words: Botryllus schlosseri; budding; embryogenesis; pharynx; placodes; co-option; evolution Key findings:We here characterize B. schlosseri orthologues of Six1/2, Six3/6, Eya, and FoxI, and report their spatiotemporal expression patterns during both embryogenesis and blastogenesis in the colonial tunicate B. schlosseri. Our results show that these genes are expressed both in larva and bud during branchial fissure formation, and in two domains along the anterior-posterior axis. We hypothesize that the latter are placodal homologue territories that can be recognized during not only sexual but also asexual development of tunicates. We discuss the idea that placode origin predates the origin of vertebrates and the molecular network involving these genes was co-opted in the evolution of asexual reproduction. We consider possible alternative evolutionary scenarios depicting gene co-option as critical step in placode and pharynx evolution.
It has long been known that extracellular Ca2+ and Mg2+ modulate synaptic transmission at the neuromuscular junction, acting both pre- and post-synaptically. Relevant questions concerning the modulation of acetylcholine (ACh) receptors (AChRs) are however still open: are the fetal (gamma-AChR) and adult (epsilon-AChR) receptors modulated differently? Does the ACh concentration influence the effect of divalent cations? Is the effect on channel open duration dependent on type and concentration of divalent cation? These questions were addressed by studying the modulation of the single-channel behaviour of gamma- and epsilon-AChRs by Ca2+ and Mg2+ at the endplate of muscle fibres acutely dissociated from 12- to 14-day-old mice. Ca2+ reduced the conductances of the two receptor channels comparably. Mg2+ had a stronger effect than Ca2+ and reduced the conductance of epsilon-AChR significantly more than that of gamma-AChR. With 0.1 microM ACh, Ca2+ and Mg2+ increased the mean open duration of gamma- and epsilon-AChR channels comparably. At 100 microM ACh, gamma- and epsilon-AChR channels opened in bursts of strikingly similar duration, which was unaffected by divalent cations. These findings indicate that Ca2+, and even more so Mg2+, may regulate synaptic transmission by modulating the function of AChRs in addition to the well-established effects on transmitter release.
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