Mouth opening is mediated by separation of dorsal and ventral daughter cells of the lip precursor cells in the larvacean, Oikopleura dioica

Transcriptomes of a fast-developing chordate uncover drastic differences in transcription factors and localized maternal RNA composition compared with those of ascidians

The larvacean Oikopleura dioica is a fast-developing chordate because of its small number of cells (∼4500 in juveniles) and rapid development to complete morphogenesis by 10 h after fertilization. Strikingly, most of its blastomeres are restricted to give rise to a single cell-type by the 32-cell stage of embryogenesis, unlike cell fate determination at the 110-cell stage in ascidians. In this study, RNA-sequencing (RNA-seq) revealed non-canonical properties of O. dioica: (1) an initial zygotic gene expression of 950 genes at the 16- to 32-cell stage; (2) 25 transcription factors (TFs) are expressed in the 32-cell stage (fewer than half of the TFs underlying gene regulatory networks in ascidian embryogenesis were lost or not expressed); (3) five maternal mRNAs localized in the vegetal-posterior blastomeres in animal and vegetal hemispheres; and (4) three maternal mRNAs localized in the small vegetal pole region of unfertilized eggs. These observations indicate that this fast-developing chordate lacks the first phase of development in ascidians: fertilization-driven ooplasmic movements that drive postplasmic RNAs toward the vegetal pole. These data have been deposited in ANISEED (https://www.aniseed.fr/) as transcriptome resources.

The oral sensory organs in Bathochordaeus stygius (Tunicata Appendicularia) are unique in structure and homologous to the coronal organ

BACKGROUND

Appendicularia consists of approximately 70 purely marine species that belong to Tunicata the probable sister taxon to Craniota. Therefore, Appendicularia plays a pivotal role for our understanding of chordate evolution. In addition, appendicularians are an important part of the epipelagic marine plankton. Nevertheless, little is known about appendicularian species, especially from deeper water.

RESULTS

Using µCT, scanning electron microscopy, and digital 3D-reconstruction techniques we describe three pairs of complex oral sensory organs in the mesopelagic appendicularian Bathochordaeus stygius. The oral sensory organs are situated at the anterior and lateral margin of the mouth and inside the mouth cavity. A single organ consists of 22-90 secondary receptor cells that project apical cilia through a narrow hole in the epidermis. The receptor cells are innervated by branches of the second brain nerve.

CONCLUSIONS

Based on position, morphology, and innervation we suggest that the oral sensory organs are homologues of the coronal organs in other tunicates. We discuss the hypothesized homology of coronal organs and the lateral line system of primary aquatic vertebrates. The complex oral sensory organs of B. stygius are unique in tunicates and could be adaptations to the more muffled environment of the mesopelagic.

Evolutionary traces of miniaturization in a giant-Comparative anatomy of brain and brain nerves in Bathochordaeus stygius (Tunicata, Appendicularia)

Appendicularia comprises 70 marine, invertebrate, chordate species. Appendicularians play important ecological and evolutionary roles, yet their morphological disparity remains understudied. Most appendicularians are small, develop rapidly, and with a stereotyped cell lineage, leading to the hypothesis that Appendicularia derived progenetically from an ascidian-like ancestor. Here, we describe the detailed anatomy of the central nervous system of Bathochordaeus stygius, a giant appendicularian from the mesopelagic. We show that the brain consists of a forebrain with on average smaller and more uniform cells and a hindbrain, in which cell shapes and sizes vary to a greater extent. Cell count for the brain was 102. We demonstrate the presence of three paired brain nerves. Brain nerve 1 traces into the epidermis of the upper lip region and consists of several fibers with some supportive bulb cells in its course. Brain nerve 2 innervates oral sensory organs and brain nerve 3 innervates the ciliary ring of the gill slits and lateral epidermis. Brain nerve 3 is asymmetric, with the right nerve consisting of two neurites originating posterior to the left one that contains three neurites. Similarities and differences to the anatomy of the brain of the model species Oikopleura dioica are discussed. We interpret the small number of cells in the brain of B. stygius as an evolutionary trace of miniaturization and conclude that giant appendicularians evolved from a small, progenetic ancestor that secondarily increased in size within Appendicularia.

Formation of the brain by stem cell divisions of large neuroblasts in Oikopleura dioica, a simple chordate

Stem cell division contributes to the generation of various cell types during animal development, especially a diverse pool of neural cells in the nervous system. One example is reiterated unequal stem cell divisions, in which a large stem cell undergoes a series of oriented unequal divisions to produce a chain of small daughter cells that differentiate. We show that reiterated unequal stem cell divisions are involved in the formation of the brain in simple chordate appendicularians (larvaceans). Two large neuroblasts in the anterior and middle of the brain-forming region of hatched larvae were observed. They produced at least 30 neural cells out of 96 total brain cells before completion of brain formation at 10 hours after fertilization by reiterated unequal stem cell divisions. The daughter cells of the anterior neuroblast were postmitotic, and the number was at least 19. The neuroblast produced small daughter neural cells posteriorly every 20 min. The neural cells first moved toward the dorsal side, turned in the anterior direction, aligned in a single line according to their birth order, and showed collective movement to accumulate in the anterior part of the brain. The anterior neuroblast originated from the right-anterior blastomeres of the eight-cell embryos and the right a222 blastomere of the 64-cell embryo. The posterior neuroblast also showed reiterated unequal stem cell divisions, and generated at least 11 neural cells. Sequential unequal stem cell divisions without stem cell growth have been observed in protostomes, such as insects and annelids. The results provide the first examples of this kind of stem cell division during brain formation in non-vertebrate deuterostomes.

Differentiation of endostyle cells by Nkx2-1 and FoxE in the ascidian Ciona intestinalis type A: insights into shared gene regulation in glandular- and thyroid-equivalent elements of the chordate endostyle

Due to similarities in iodine concentrations and peroxidase activities, the thyroid in vertebrates is considered to originate from the endostyle of invertebrate chordates even though it is a glandular (mucus-producing) organ for aquatic suspension feeding. Among chordates with an endostyle, urochordates are useful evolutionary research models for the study of vertebrate traits. The ascidian Ciona intestinalis forms an endostyle with specific components of glandular- and thyroid-related elements, and molecular markers have been identified for these components. Since we previously examined a simple endostyle in the larvacean Oikopleura dioica, the expression of the thyroid-related transcription factor genes, Ciona Nkx2-1 and FoxE, was perturbed by TALEN-mediated gene knockout in the present study to elucidate the shared and/or divergent features of a complex ascidian endostyle. The knockout of Ciona Nkx2-1 and FoxE exerted different effects on the morphology of the developing endostyle. The knockout of Nkx2-1 eliminated the expression of both glandular and thyroidal differentiation marker genes, e.g., vWFL1, vWFL2, CiEnds1, TPO, and Duox, while that of FoxE eliminated the expression of the differentiation marker genes, TPO and CiEnds1. The supporting element-related expression of Pax2/5/8a, Pax2/5/8b, FoxQ1, and β-tubulin persisted in the hypoplastic endostyles of Nkx2-1- and FoxE-knockout juveniles. Although the gene regulation of ascidian-specific CiEnds1 remains unclear, these results provide insights into the evolution of the vertebrate thyroid as well as the urochordate endostyle.

Developmental biology of the larvacean Oikopleura dioica: Genome resources, functional screening, and imaging

The larvacean Oikopleura dioica is a cosmopolitan planktonic chordate and is closely related to vertebrates. It is characterized by a tadpole-shaped morphology with notochord flanked by muscle in the tail and brain on the dorsal side, a short life cycle of five days, a compact genome of approximately 56 Mb, a simple and transparent body with a small number of cells (~4000 in functional juveniles), invariant embryonic cell lineages, and fast development that ensures complete morphogenesis and organ formation 10 h after fertilization. With these features, this marine chordate is a promising and advantageous animal model in which genetic manipulation is feasible. In this review, we introduce relevant resources and modern techniques that have been developed: (1) Genome and transcriptomes. Oikopleura dioica has the smallest genome among non-parasitic metazoans. Its genome databases have been generated using three geographically distant O. dioica populations, and several intra-species sequence differences are becoming evident; (2) Functional genetic knockdown techniques. Comprehensive screening of genes is feasible using ovarian microinjection and double-strand DNA-induced gene knockdown; and (3) Live imaging of embryos and larvae. Application of these techniques has uncovered novel aspects of development, including meiotic cell arrest, left-right patterning, epidermal cell patterning, and mouth formation involving the connection of ectoderm and endoderm sheets. Oikopleura dioca has become very useful for developmental and evolutionary studies in chordates.

Germline development during embryogenesis of the larvacean, Oikopleura dioica

Germ cells develop into eggs and sperms and represent a lineage that survives through multiple generations. Germ cell specification during embryogenesis proceeds through one of two basic modes: either the cell-autonomous mode or the inductive mode. In the cell-autonomous mode, specification of germ cell fate involves asymmetric partitioning of the specialized maternal cytoplasm, known as the germplasm. Oikopleura dioica is a larvacean (class Appendicularia) and a chordate. It is regarded as a promising animal model for studying chordate development because of its short life cycle (5 days) and small genome size (∼60 ​Mb). We show that their embryos possess germplasm, as observed in ascidians (class Ascidiacea). The vegetal cytoplasm shifted towards the future posterior pole before the first cleavage occurred. A bilateral pair of primordial germ cells (PGC, B11 ​cells) was formed at the posterior pole at the 32-cell stage through two rounds of unequal cleavage. These B11 ​cells did not undergo further division before hatching of the tadpole-shaped larvae. The centrosome-attracting body (CAB) is a subcellular structure that contains the germplasm and plays crucial roles in germ cell development in ascidians. The presence of CAB with germplasm was observed in the germline lineage cells of larvaceans via electron microscopy and using extracted embryos. The CAB appeared at the 8-cell stage and persisted until the middle stage of embryogenesis. The antigen for the phosphorylated histone 3 antibody was localized to the CAB and persisted in the PGC until hatching after the CAB disappeared. Maternal snail mRNA, which encodes a transcription factor, was co-localized with the antigen for the H3S28p antibody. Furthermore, we found a novel PGC-specific subcellular structure that we call the germ body (GB). This study thus highlights the conserved and non-conserved features of germline development between ascidians and larvaceans. The rapid development and short life cycle (five days) of O. dioica would open the way to genetically analyze germ cell development in the future.

Nkx2-1 and FoxE regionalize glandular (mucus-producing) and thyroid-equivalent traits in the endostyle of the chordate Oikopleura dioica

The endostyle is a ventral pharyngeal organ used for internal filter feeding of basal chordates and is considered homologous to the follicular thyroid of vertebrates. It contains mucus-producing (glandular) and thyroid-equivalent regions organized along the dorsoventral (DV) axis. Although thyroid-related genes (Nkx2-1, FoxE, and thyroid peroxidase (TPO)) are known to be expressed in the endostyle, their roles in establishing regionalization within the organ have not been demonstrated. We report that Nkx2-1 and FoxE are essential for establishing DV axial identity in the endostyle of Oikopleura dioica. Genome and expression analyses showed von Willebrand factor-like (vWFL) and TPO/dual oxidase (Duox)/Nkx2-1/FoxE as orthologs of glandular and thyroid-related genes, respectively. Knockdown experiments showed that Nkx2-1 is necessary for the expression of glandular and thyroid-related genes, whereas FoxE is necessary only for thyroid-related genes. Moreover, Nkx2-1 expression is necessary for FoxE expression in larvae during organogenesis. The results demonstrate the essential roles of Nkx2-1 and FoxE in establishing regionalization in the endostyle, including (1) the Nkx2-1-dependent glandular region, and (2) the Nkx2-1/FoxE-dependent thyroid-equivalent region. DV axial regionalization may be responsible for organizing glandular and thyroid-equivalent traits of the pharynx along the DV axis.

3D reconstruction of structures of hatched larva and young juvenile of the larvacean Oikopleura dioica using SBF-SEM

The larvacean Oikopleura dioica is a planktonic chordate and an emerging model organism with a short life cycle of 5 days that belongs toTunicata (Urochordata), the sister clade of vertebrates. It is characterized by the rapid development of a tadpole-shaped body. Organ formation in the trunk proceeds within 7 h after the hatching of the tailbud larvae at 3 h after fertilization (hpf) and is completed at 10 hpf, giving rise to fully functional juveniles as miniature adult form. Serial block face scanning electron microscopy was used to acquire ~ 2000 serial transverse section images of a 3 hpf larva and a 10 hpf juvenile to characterize the structures and cellular composition of the trunk and organs using 3D images and movies. Germ cells were found to fuse and establish a central syncytial cell in the gonad as early as 10 hpf. Larval development gave rise to functional organs after several rounds of cell division through trunk morphogenesis. The feature would make O. dioica ideal for analyzing cellular behaviors during morphogenetic processes using live imaging. The detailed descriptions of the larvae and juveniles provided in this study can be utilized as the start and end points of organ morphogenesis in this rapidly developing organism.