Comparative Reproduction

Fabio Gasparini , Loriano Ballarin , in Encyclopedia of Reproduction (Second Edition), 2018

Abstract

Tunicates or Urochordates are the closest relatives of vertebrates. They are marine filter-feeding animals, found at all the latitudes, and can assume a planktonic or benthic lifestyle. They are, with few exceptions, hermaphroditic animals with an indirect development: almost all the tunicate species have a tadpole swimming larva. Coloniality is widespread and a larval stage followed by metamorphosis is the rule in the class Ascidiacea. Blooms of pelagic tunicates are quite common in warm seasons: their ecological relevance relies on the key role they play in the alimentary chain of open seas.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128096338206018

Molecular Breeding of Woody Plants

Satoshi Kimura , Takao Itoh , in Progress in Biotechnology, 2001

INTRODUCTION

The tunicates (urochordates) are the only animals known to produce highly crystalline cellulose 1 . The name "Tunicata" is derived from the unique integumentary tissue called the "tunic", which contains the cellulose microfibrils. To date, cellulose I microfibrils have been found in almost all of the ascidians and thaliaceans 2,3 . Thus, the cellulosic composition of the tunic is considered to be a characteristic common to ascidians and thaliaceans in animals belonging to the subphylum Tunicata.

The appendicularians is another group in the Tunicata. Although molecular phylogeny based on 18S rDNA sequences suggested that appendicularians share a common ancestor with the other groups of tunicates 4,5 , they do not possess the tunic as an integumentary tissue. On the other hand, the appendicularians secrete a balloon-like, gelatinous structure called a "house" that acts as a feeding apparatus 6 . It is possible that the house corresponds to a kind of tunic in the appendicularians, but it is not yet clear whether the house contains cellulose. Therefore, it is necessary to clarify the existence of cellulose in the appendicularian house to understand the evolutionary pathway of cellulose biosynthesis in the tunicates.

The present study focused on the existence and characterization of cellulose in the appendicularian house. Our investigation provides a key to understanding whether the ability for cellulose synthesis is universal in the tunicates.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/S0921042301800630

Tunicate Immunology

John DeFilippo , Gregory Beck , in Reference Module in Life Sciences, 2018

Tunicate Characteristics

Tunicates are a diverse clade of filter-feeding marine invertebrates, with a diet subsisting mostly of phytoplankton. Their name is descriptive of their hard, leathery outer covering, or "tunic", which is made of cellulose and serves as a protective exoskeleton. Their size and shape are quite variable, and they can be solitary or colonial (compound), and sessile, benthic, or pelagic. The paraphyletic Ascidiacea is by far the largest and most diverse of the Tunicata classes. There are some three thousand species of these benthic or sessile ascidians, over twenty times as many species as the other two classes of pelagic tunicates, Thaliacea (which include sea salps, doliolids, and pyrosomes), and Appendicularia or Larvacea, combined (Kocot et al., 2018; Holland, 2016). Ascidians are also known as sea squirts owing to how they contract and squirt water out of siphons on contact. Most tunicates are hermaphroditic, allowing for both sexual as well as asexual means, which facilitates their population expansion. In solitary ascidians sexual reproduction is more common, while colonial ascidians can reproduce by both sexual as well as asexual reproduction via budding. Hermaphroditic ascidians display self-incompatibility (SI) at fertilization (Nonaka and Satake, 2010). Appendicularians have a cosmopolitan distribution, and unusual for tunicates, have separate sexes, reproducing sexually. The asexual reproductive component of tunicates is also thought to be involved in their ability to regenerate body parts, possibly through budding (Holland, 2016). For example, while solitary tunicates do not possess a defined vascular system (Lemaire, 2011), the invasive colonial star ascidian Botryllus schlosseri has the ability to regenerate the entirety of its body solely from its vasculature. Colonies of compatible tunicates can also recognize each other and fuse into a single chimeric organism through fusion of their vasculature. And in a manner akin to mammalian allograft rejection incompatible colonies will reject each other (see below) and not fuse (Voskoboynik et al., 2013).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128096338902887

Gastrulation: From Embryonic Pattern to Form

Konner M. Winkley , ... William C. Smith , in Current Topics in Developmental Biology, 2020

Abstract

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.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/S0070215319300742

Origin and Functions of Tunicate Hemocytes

Francesca Cima , ... Loriano Ballarin , in The Evolution of the Immune System, 2016

1 Introduction

Tunicates, or urochordates, are a subphylum of the phylum Chordata, sharing with other members of the phylum: (1) a permanent or temporary notochord, in the form of a dorsal rod; (2) a central nervous system, in the form of a dorsal tube; (3) a pharynx provided with gill slits or pharyngeal pouches, and a ventral gland secreting iodoproteins (endostyle or thyroid); and (4) a muscular tail.

Tunicates are considered the sister group of vertebrates, 1 forming with the latter the clade Olfactoria. Recently, it has been proposed to classify them as a phylum within the superphylum Chordata. 2 They are traditionally subdivided in three classes: (1) Ascidiacea (benthic and sessile), (2) Thaliacea, and (3) Larvacea or Appendicularia (pelagic).

Ascidians have a free-swimming, tadpole-like larva, an adult sac-like body with two siphons that allow water flux, and a large branchial basket, provided with a ventral endostyle that secretes the mucous net required for filtration. They comprise two orders: Enterogona (including the suborders Phlebobranchia and Aplousobranchia) and Pleurogona (with the suborder Stolidobranchia). 3 Thaliaceans include three orders: the colonial Pyrosomida, and the solitary/colonial Doliolida and Salpida. They have a barrel-like adult body, and, with the exception of Doliolida, are devoid of larval stages. 4,5 Larvaceans or appendicularians resemble the ascidian larvae and use the tail to create the water current for filtration; filters are included in the gelatinous house secreted by the animals themselves. 4 Most of the recent authors consider larvaceans as a sister group of the other tunicates, and thaliaceans as a sister group of Enterogona 5–10 (Fig. 2.1).

Figure 2.1. Phylogenetic tree of Tunicates.

Ascidians include about 2300 species, and most of the information on tunicate hemocytes comes from studies on this group of organisms. This review will, then, focus mainly on ascidian hemocytes and will discuss their role in immunity. Where possible, information on circulating cells of pelagic tunicates will be added.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128019757000025

International Review of Cell and Molecular Biology

Valentina P. Gallo , ... Enrico Crivellato , in International Review of Cell and Molecular Biology, 2016

6.2 Chordata

Tunicates or urochordates (appendicularians, salps, and sea squirts), cephalochordates (lancelets) and vertebrates constitute the three extant groups of chordate animals. Traditionally, cephalochordates are considered as the closest living relatives of vertebrates, while tunicates should represent the earliest chordate lineage. Nevertheless phylogenetic analyses of genomic data indicate a strong affinity between tunicates and vertebrates whereas cephalochordates should be more distantly related (Delsuc et al., 2006). To investigate the evolution of the catecholaminergic system in Chordata, the presence and distribution of CA and OA were studied in the central nervous system of adult amphioxus (Branchiostoma lanceolatum) belonging to cephalochordates. The results evidenced in amphioxus head the presence of significant amounts of DA and OA, as in most protostomian species, but not of N (Moret et al., 2004). The absence of N suggested that, as regards Chordata, the vertebrate noradrenergic system could be an innovation appeared along with the neural crest, a multipotent embryonic cell population unique to vertebrates. Neural crest cells produce both epithelial and mesenchymal derivatives as peripheral neurons, glia, and neurosecretory cells of the thyroid and adrenal medulla, which synthesizes and secretes the CA, after migrating from the dorsal neural tube to definitive positions. No migrating neural crest cells have been discovered in amphioxus to date, even if the expression of specific early neural crest genes was detected in lateral neural plate or in the surrounding ectoderm in amphioxus embryos (Meulemans and Bronner-Fraser, 2002). On the other hand, in the ascidian Ecteinascidia turbinata belonging to urocordates, Jeffery et al. (2004) demonstrated the presence of cells emerging from the neural tube and migrating into the body wall and subsequently differentiating into pigment cells. These cells express HNK-1 antigen and Zic gene, markers of vertebrate neural crest cells, suggesting that migratory cells with some of the features of neural crest cells are present in urochordates. Subsequently Jeffery et al. (2008) investigated the embryonic origin, migratory activity, and neural crest related gene expression patterns of neural crest-like cells (NCLC) in the ascidian Ciona intestinalis. The results suggest that NCLC of tunicates and neural crest cells of vertebrates may be homologous cell types originating in a common ancestor and support the possibility that a putative regulatory network governing NCLC development was coopted to produce neural crest cells during vertebrate evolution. As regards a catecholaminergic system in urochordates, in Ciona was demonstrated the presence of genes encoding for TH and DβH, but not of a gene encoding for PNMT, so adrenergic neurons are probably absent in Ciona (Horie et al., 2009). In the ascidian Phallusia nigra, De Barros et al. (2012) studied the effect of N on the production of nitric oxide (NO) by hemocytes. NO is an important modulator of the immune system, and the results suggested that N may induce a decrease in the immune function via specific hemocyte receptors. No data are available regarding OA in urochordates.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/S1937644815001227

Cytotoxic Cells of Compound Ascidians

Nicola Franchi , Loriano Ballarin , in Lessons in Immunity, 2016

Introduction

Tunicates or Urochordates are a subphylum of the phylum Chordata (Fig. 14.1). The realization that they represent the sister group of vertebrates 1 led to a renewed interest toward this group of organisms and stimulated a flourishing of research dealing with them. Tunicates traditionally include three classes: the benthic and sessile Ascidiacea (ascidians), the pelagic Thaliacea, and Larvacea (or Appendicularia).

Figure 14.1. Phylogenetic relationships among tunicates. Ascidians, marked by filled rectangles, are polyphyletic. According to Delsuc et al., 1 vertebrates represent the tunicate sister group. Light gray: solitary ascidian species; dark gray: colonial ascidian species.

 From Voskoboynik A, Neff NF, Sahoo D, Newman AM, Pushkarev D, Koh W, et al. The genome sequence of the colonial chordate, Botryllus schlosseri. Elife 2013;2:e00569, modified according to Tsagkogeorga G, Turon X, Hopcroft RR, Tilak MK, Feldstein T, Shenkar N, et al. An updated 18S rRNA phylogeny of tunicates based on mixture and secondary structure models. BMC Evol Biol 2009;9:187 and Tatián M, Lagger C, Demarchi M, Mattoni C. Molecular phylogeny endorses the relationship between carnivorous and filter-feeding tunicates (Tunicata, Ascidiacea). Zool Scr 2011;40:603–12.

Ascidians have an indirect development, with some exceptions in Molgulids, 2–4 and a free-swimming, tadpolelike larva, which metamorphoses into a saclike adult. With the exception of the carnivorous species, once considered a separate class of tunicates (Sorberacea 5 ) and today included within the family Molgulidae, 4 adult ascidians are provided with two siphons, which allow water flux, and a voluminous branchial basket provided with a ventral endostyle secreting the mucous net required for filtration. Ascidians comprise two orders: Enterogona, with two suborders (Aplousobranchia and Phlebobranchia), and Pleurogona, with the order Stolidobranchia (Burighel and Cloney 5 ). Thaliaceans include three orders: the colonial Pyrosomida, Doliolida, and Salpida, which alternate solitary and colonial phases in their life cycles; they have a barrel-like adult body and, with the exception of Doliolida, are devoid of larval stages. 6,7 Larvaceans or appendicularians resemble the ascidian larvae in morphology and use the tail to create the water current for filtration; filters are included in the gelatinous house secreted by the animals themselves. 6 Ascidians include about 2300 species and coloniality developed independently many times within the taxon: Aplousobranchia are all colonial, while Phlebobranchia and Stolidobranchia include both solitary and colonial species (Fig. 14.1). 8,9

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B978012803252700014X

Intestinal Regeneration

José E. García-Arrarás , in Regenerative Medicine Applications in Organ Transplantation, 2014

35.4.3 Tunicates

Tunicates or ascidians are deuterostomes closely related to chordates. Some of these organisms show amazing regenerative properties. For example, colonial species are known to regenerate their complete body from a group of cells named "blood cells." In terms of the digestive tract, experiments have been done where animals were cut in half and gut regeneration was determined [126]. RA inhibition either by drugs or by RNAi against RA synthesizing enzymes inhibited gut regeneration. It has been suggested that in tunicates, RA plays a role in the transdifferentiation of the atrial epithelium into the gut tissue. Nonetheless, although RA appears to be involved in initiating regeneration, its role is not exclusive to initial regeneration but might also be involved in different roles at other regeneration stages. The modulation by RA also shows the importance of RA in modulating activity of Hox genes and in establishing anterior–posterior polarity in most animals.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123985231000355

Inflammatory Response of the Ascidian Ciona intestinalis

Parrinello Nicolò , ... Vizzini Aiti , in Lessons in Immunity, 2016

Introduction

Tunicates (urochordates) are retained as the closest living relatives of vertebrates. 1,2 They share components of innate immune responses with vertebrates. 3 The sea squirt Ciona intestinalis is a noncolonial ascidian that lives mainly in clusters fixed in natural and artificial substrates. It is a simultaneous hermaphrodite, and the swimming small (about 2500   cells) tadpole is comprised of a notochord and dorsal neural tube. Recent reports have shown genetic divergence between populations, suggesting a species-divergence process. 4 Phylogenies inferred from mitochondrial and nuclear DNA markers accredited the existence of two cryptic species: C. intestinalis sp. A, genetically homogeneous, distributed in the Mediterranean Sea, Northeast Atlantic, and Pacific, and C. intestinalis sp. B in the North Atlantic. The recent papers by Pennati et al. 5 and Brunetti et al., 6 based on morphological comparisons between adults and larvae of the types A and B, distinguish two species, Ciona robusta and C. intestinalis, respectively. We study the innate immune system of ascidians from the Mediterranean Sea, and it is reasonable that structures and functions of this system could be largely conserved 7,8 in microevolution. The Ciona whole genome has been sequenced and analyzed (about 16,000 protein-coding genes annotated), and a large number of them have single copies. 9 Further molecular analysis between immune genes from the two species and populations could disclose the divergence level of their sequences. In accordance with the previously published papers, in the present review the species nomenclature will be reported as C. intestinalis.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128032527000138

Brains of Primitive Chordates

J.C. Glover , B. Fritzsch , in Encyclopedia of Neuroscience, 2009

Urochordates

The urochordate subphylum includes the ascidians, the thaliaceans, and the appendicularians. Nearly all ascidians, some thaliaceans, and all appendicularians have a free-swimming larval stage with a notochord and a small central nervous system that appear homologous to the same structures in craniates. In contrast to the rather uniform appearance of the neuraxis of cephalochordates, the larval ascidian and larval and adult appendicularian central nervous systems are organized into an obvious tripartite structure ( Figure 2 ). The three divisions are (1) a rostral ganglion (totaling approximately 215 cells in the ascidian Ciona and approximately 75 cells in the appendicularian Oikopleura), which contains sensory receptor structures, including an ocellus and/or an otolith, followed by (2) a caudal ganglion (containing approximately 45 cells in Ciona and approximately 25 neurons in Oikopleura), from which extends (3) a caudal nerve cord (containing approximately 65 cells, mostly ependymal, in Ciona, and approximately 30 neurons and 25 support cells in Oikopleura). In addition, the rostral and caudal ganglia are connected by a nerve trunk in appendicularians or a slender 'neck' region containing nerve cells in ascidians. Other urochordate taxa seem to exhibit primarily variations in size but not in structure. The ganglia of urochordates have the organization of an invertebrate ganglion, with cell bodies at the periphery and a neuropil in the center. Several nerves that vary considerably between species have been traced from adult ganglia, reaching up to 75 nerves in certain salps. These nerves appear to be mixed sensory and motor nerves and are asymmetric in several species, potentially related to the overall body asymmetry.

The organization of motor projections varies among urochordates. In ascidian larvae, all the motor neurons (three to five pairs in different species) are located in the caudal ganglion and project along the predominantly aneural nerve cord before exiting to innervate the peripheral muscle. In appendicularians, both the caudal ganglion and the nerve cord contain motor neurons, whose axons project laterally to the peripheral muscle either directly or after extending some distance in the cord.

Comparisons between urochordates and cephalochordates claim similarities in sensory organs within the central nervous system, in particular the infundibular sensory cells and otolith. The ciliated funnel, which extends to one side of the rostral ganglion, is believed to be homologous to the pituitary. However, the interpretation of homologies for most sensory cells and sensory organs outside the central nervous system is contended. Some adult urochordates have fairly complex eyes attached to the cerebral ganglion. There is a marked asymmetry in the organization of central and peripheral sensory structures.

No large neurons with descending axons that might be homologous to the large reticulospinal neurons of craniates have been described in urochordates, although there are some large cells within the rostral ganglion that could subserve a similar function.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780080450469009451