Review

Authors: Konner M Winkley, Matthew J Kourakis, Anthony W DeTomaso...

Tunicates are a diverse group of invertebrate marine chordates that includes the larvaceans, thaliaceans, and ascidians. Because of their unique evolutionary position as the sister group of the vertebrates, tunicates are invaluable as a comparative model and hold the promise of revealing both conserved and derived features of chordate gastrulation. Descriptive studies in a broad range of tunicates have revealed several important unifying traits that make them unique among the chordates, including invariant cell lineages through gastrula stages and an overall morphological simplicity. Gastrulation has only been studied in detail in ascidians such as Ciona and Phallusia, where it involves a simple cup-shaped gastrula driven primarily by endoderm invagination. This appears to differ significantly from vertebrate models, such as Xenopus, in which mesoderm convergent extension and epidermal epiboly are major contributors to involution. These differences may reflect the cellular simplicity of the ascidian embryo.

Topics: Animals; Body Patterning; Cell Lineage; Embryo, Nonmammalian; Endoderm; Evolution, Molecular; Gastrula; Gastrulation; Gene Expression Regulation, Developmental; Morphogenesis; Urochordata

PubMed: 31959289
DOI: 10.1016/bs.ctdb.2019.09.001

Authors: Linda Z Holland

Tunicates, also called urochordates, are an extremely diverse subphylum of the Chordata, a phylum that also contains the vertebrates and cephalochordates. The tunicates seem to have undergone especially rapid evolution: while remaining exclusively marine, they have radiated to occupy habitats ranging from shallow water, to near shore to the open ocean and the deep sea. Furthermore, they have evolved a variety of remarkable reproductive strategies, combining asexual and sexual modes of reproduction that allow for very rapid expansion of populations. An outstanding question is what happened to allow tunicates to evolve so much faster than their nearest relatives, cephalochordates and vertebrates.

Topics: Animals; Biological Evolution; Phylogeny; Reproduction; Urochordata

PubMed: 26906481
DOI: 10.1016/j.cub.2015.12.024

Review

Authors: Qiongxuan Lu, Punit Bhattachan, Bo Dong...

The elongation of embryo and tissue is a key morphogenetic event in embryogenesis and organogenesis. Notochord, a typical chordate organ, undergoes elongation to perform its regulatory roles and to form the structural support in the embryo. Notochord elongation is morphologically similar across all chordates, but ascidian has evolved distinct molecular and cellular processes. Here, we summarize the current understanding of ascidian notochord elongation. We divide the process into three phases and discuss the underlying molecular mechanisms in each phase. In the first phase, the notochord converges and extends through invagination and mediolateral intercalation, and partially elongates to form a single diameter cell column along the anterior-posterior axis. In the second phase, a cytokinesis-like actomyosin ring is constructed at the equator of each cell and drives notochord to elongate approximately two-fold. The molecular composition and architecture of the ascidian notochord contractile ring are similar to that of the cytokinetic ring. However, the notochord contractile ring does not impose cell division but only drives cell elongation followed by disassembly. We discuss the self-organizing property of the circumferential actomyosin ring, and why it disassembles when certain notochord length is achieved. The similar ring structures are also present in the elongation process of other organs in evolutionarily divergent animals such as Drosophila and C. elegans. We hereby propose that actomyosin ring-based circumferential contraction is a common mechanism adopted in diverse systems to drive embryo and tissue elongation. In the third phase, the notochord experiences tubulogenesis and the endothelial-like cells crawl bi-directionally on the notochord sheath to further lengthen the notochord. In this review, we also discuss extracellular matrix proteins, notochord sheath, and surrounding tissues that may contribute to notochord integrity and morphogenesis.

Topics: Actomyosin; Animals; Biological Evolution; Cell Movement; Models, Biological; Notochord; Urochordata

PubMed: 30458170
DOI: 10.1016/j.ydbio.2018.11.009

Authors: Joel C Glover

The appendicularian tunicate Oikopleura epitomizes the degree to which evolution can constrain both genome and cellular composition, while at the same time unleashing fantastic specializations.

Topics: Animals; Brain; Genome; Movement; Polysaccharides; Urochordata

PubMed: 33080189
DOI: 10.1016/j.cub.2020.07.075

Authors: Michael T Veeman, Shota Chiba, William C Smith...

Ascidians, such as Ciona, are invertebrate chordates with simple embryonic body plans and small, relatively non-redundant genomes. Ciona genetics is in its infancy compared to many other model systems, but it provides a powerful method for studying this important vertebrate outgroup. Here we give basic methods for genetic analysis of Ciona, including protocols for controlled crosses both by natural spawning and by the surgical isolation of gametes; the identification and propagation of mutant lines; and strategies for positional cloning.

Topics: Animals; Chromosome Mapping; Cloning, Molecular; Cryopreservation; DNA; DNA Mutational Analysis; Female; Fertilization in Vitro; Genetic Techniques; Hybridization, Genetic; Larva; Male; Microinjections; Mutation; Polymerase Chain Reaction; Polymorphism, Single Nucleotide; Urochordata

PubMed: 21805273
DOI: 10.1007/978-1-61779-210-6_15

Review

Authors: Linxia Xiao

Meridianins are a family of indole alkaloids derived from Antarctic tunicates with extensive pharmacological activities. A series of meridianin derivatives had been synthesized by drug researchers. This article reviews the extraction and purification methods, biological activities and pharmacological applications, pharmacokinetic characters and chemical synthesis of meridianins and their derivatives. And prospects on discovering new bioactivities of meridianins and optimizing their structure for the improvement of the ADMET properties are provided.

Topics: Animals; Indole Alkaloids; Urochordata; Antarctic Regions

PubMed: 36557848
DOI: 10.3390/molecules27248714

Authors: Scott Barolo

Topics: Animals; Base Sequence; Enhancer Elements, Genetic; Transcription Factors; Urochordata

PubMed: 27247414
DOI: 10.1073/pnas.1606109113

Review

Authors: Alexander C A Fodor, Megan M Powers, Kristin Andrykovich...

Ascidians are invertebrate chordates, with swimming chordate tadpole larvae that have distinct heads and tails. The head contains the small brain, sensory organs, including the ocellus (light) and otolith (gravity) and the presumptive endoderm, while the tail has a notochord surrounded by muscle cells and a dorsal nerve cord. One of the chordate features is a post-anal tail. Ascidian tadpoles are nonfeeding, and their tails are critical for larval locomotion. After hatching the larvae swim up toward light and are carried by the tide and ocean currents. When competent to settle, ascidian tadpole larvae swim down, away from light, to settle and metamorphose into a sessile adult. Tunicates are classified as chordates because of their chordate tadpole larvae; in contrast, the sessile adult has a U-shaped gut and very derived body plan, looking nothing like a chordate. There is one group of ascidians, the Molgulidae, where many species are known to have tailless larvae. The Swalla Lab has been studying the evolution of tailless ascidian larvae in this clade for over 30 years and has shown that tailless larvae have evolved independently several times in this clade. Comparison of the genomes of two closely related species, the tailed Molgula oculata and tailless Molgula occulta reveals much synteny, but there have been multiple insertions and deletions that have disrupted larval genes in the tailless species. Genomics and transcriptomics have previously shown that there are pseudogenes expressed in the tailless embryos, suggesting that the partial rescue of tailed features in their hybrid larvae is due to the expression of intact genes from the tailed parent. Yet surprisingly, we find that the notochord gene regulatory network is mostly intact in the tailless M. occulta, although the notochord does not converge and extend and remains as an aggregate of cells we call the "notoball." We expect that eventually many of the larval gene networks will become evolutionarily lost in tailless ascidians and the larval body plan abandoned, with eggs developing directly into an adult. Here we review the current evolutionary and developmental evidence on how the molgulids lost their tails.

Topics: Animals; Biological Evolution; Larva; Notochord; Tail; Urochordata

PubMed: 33881514
DOI: 10.1093/icb/icab022

Authors: Steven Q Irvine, Filomena Ristoratore, Anna Di Gregorio...

Topics: Animals; Embryonic Development; Models, Biological; Notochord; Urochordata

PubMed: 30803728
DOI: 10.1016/j.ydbio.2019.02.007

Review

Authors: John W Blunt, Brent R Copp, Murray H G Munro...

This review covers the literature published in 2004 for marine natural products, with 693 citations (491 for the period January to December 2004) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, coelenterates, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (716 for 2004), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies (8), and syntheses (80), including those that lead to the revision of structures or stereochemistries, have been included.

Topics: Animals; Biological Products; Bryozoa; Cnidaria; Echinodermata; Eukaryota; Marine Biology; Molecular Structure; Mollusca; Phytoplankton; Porifera; Urochordata

PubMed: 16453031
DOI: 10.1039/b502792f