Placode formation and generation of gonadotropin-releasing hormone (GnRH) neurons in ascidians

De novo neurogenesis in a budding chordate: Co-option of larval anteroposterior patterning genes in a transitory neurogenic organ

During metamorphosis of solitary ascidians, part of the larval tubular nervous system is recruited to form the adult central nervous system (CNS) through neural stem-like cells called ependymal cells. The anteroposterior (AP) gene expression patterning of the larval CNS regionalize the distribution of the ependymal cells, which contains the positional information of the neurons of the adult nervous system. In colonial ascidians, the CNS of asexually developed zooids has the same morphology of the one of the post-metamorphic zooids. However, its development follows a completely different organogenesis that lacks embryogenesis, a larval phase and metamorphosis. In order to describe neurogenesis during asexual development (blastogenesis), we followed the expression of six CNS AP patterning genes conserved in chordates and five neural-related genes to determine neural cell identity in Botryllus schlosseri. We observed that a neurogenesis occurs de novo on each blastogenic cycle starting from a neurogenic transitory structure, the dorsal tube. The dorsal tube partially co-opts the AP patterning of the larval CNS markers, and potentially combine the neurogenesis role and provider of positional clues for neuron patterning. This study shows how a larval developmental module is reused in a direct asexual development in order to generate the same structures.

Two novel gonadotropin-releasing hormones (GnRHs) from the urochordate ascidian, Halocynthia roretzi: implications for the origin of vertebrate GnRH isoforms

Three forms of gonadotropin-releasing hormone (GnRH) are found in vertebrates; these differ in amino acid sequence, localization, distribution, and embryological origin. We used northern blot analysis, and in situ hybridization to detect GnRH transcripts in various tissues in the large ascidian Halocynthia roretzi. We cloned a cDNA encoding two novel GnRHs, termed tGnRH-10 and tGnRH-11, from H. roretzi, with deduced amino acid sequences of QHWSYGFSPG and QHWSYGFLPG, respectively. Both GnRHs are highly similar to those of teleosts and tetrapods. For example, the tGnRH-10 sequence is 90% identical to seabream GnRH1, and tGnRH-11 is 90% identical to salmon GnRH3. The primary structure of the deduced preprotein is similar to that of chordate GnRHs and consists of a signal peptide, two decapeptides, up- and downstream processing sequences (containing lysine and arginine), and a GnRH-associated peptide. The transcripts of the H. roretzi GnRH gene were expressed in all tissues examined. Comparison of the signal peptide of the lamprey GnRH-II precursor with those of three forms from representative vertebrates revealed homology to GnRH2 precursors. These novel ascidian GnRHs offer a new perspective on the origin of vertebrate GnRH subtypes. We hypothesize that gnathostome GnRH2 was derived only from lamprey GnRH-II and that ancestral gnathostome GnRH, which produces neurons that originate in peripheral organs, gave rise to vertebrate GnRH1 and GnRH3 through whole-genome duplication.

Expression of neuropeptide- and hormone-encoding genes in the Ciona intestinalis larval brain

Despite containing only approximately 330 cells, the central nervous system (CNS) of Ciona intestinalis larvae has an architecture that is similar to the vertebrate CNS. Although only vertebrates have a distinct hypothalamus-the source of numerous neurohormone peptides that play pivotal roles in the development, function, and maintenance of various neuronal and endocrine systems, it is suggested that the Ciona brain contains a region that corresponds to the vertebrate hypothalamus. To identify genes expressed in the brain, we isolated brain vesicles using transgenic embryos carrying Ci-β-tubulin(promoter)::Kaede, which resulted in robust Kaede expression in the larval CNS. The associated transcriptome was investigated using microarray analysis. We identified 565 genes that were preferentially expressed in the larval brain. Among these genes, 11 encoded neurohormone peptides including such hypothalamic peptides as gonadotropin-releasing hormone and oxytocin/vasopressin. Six of the identified peptide genes had not been previously described. We also found that genes encoding receptors for some of the peptides were expressed in the brain. Interestingly, whole-mount in situ hybridization showed that most of the peptide genes were expressed in the ventral brain. This catalog of the genes expressed in the larval brain should help elucidate the evolution, development, and functioning of the chordate brain.