Choanoflagellates

11 Key excerpts on "Choanoflagellates"

• 

  eBook - ePub

  Epigenetic Mechanisms of the Cambrian Explosion

  • Nelson R Cabej, Nelson R. Cabej(Authors)
  • 2019(Publication Date)
  • Academic Press

    (Publisher)

  From Anderson, D.P., Whitney, D.S., Hanson-Smith, V., Woznica, A., Campodonico-Burnett, W., Volkman, B.F., et al., 2016. Evolution of an ancient protein function involved in organized multicellularity in animals. eLife 5 (2016), e10147.

  Choanoflagellates are not derived from metazoans; as a separate lineage, they evolved before the emergence of metazoans (King et al., 2008 ).

  Choanoflagellates form colonies of several cells. The development of colonies in this group results from reproduction of similar cells (Figs. 1.4 and 1.5 ) rather than from aggregation of similar cells as demonstrated by the experimental evidence that the use of the cell cycle inhibitor aphidicolin prevents formation of choanocyte colonies. There is no evidence that Choanoflagellates may form colonies via aggregation of similar cells (Fairclough et al., 2010 ).

  Figure 1.3  Comparative ultrastructural schematics of a choanoflagellate and a sponge choanocyte, modified from Maldonado, M., 2004. Choanoflagellates, choanocytes, and animal multicellularity. Invertebr. Biol. 123, 1–22 , following (Woollacott and Pinto, 1995 ) for the microtubule cytoskeleton and (Karpov and Coupe, 1998 ) for the actin cytoskeleton. mt , mitochondria; fl , flagellum; fv , food vacuole.  From Brunet, T., King, N., 2017. The origin of animal multicellularity and cell differentiation. Dev. Cell 43, 124–140.

  A characteristic of all multicellular organisms, beginning from the simplest ones, is the presence of epithelium, a one-cell thick tissue that separates and protects underlying tissues from the harmful environmental agents. It also lines the inner cavities and free surfaces of tubular organs. Formation of epithelia required emergence of a stable mechanism of cell adhesion that seems to have independently evolved in several groups of living forms and connect epithelial cells with each other and with the intercellular matrix. The last common ancestor of metazoans is inferred to have been in possession of various intercell adhesion junctions (Abedin and King, 2010 ) (Fig. 1.6

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  The Evolution of Multicellularity

  • Matthew D. Herron, Peter L. Conlin, William C. Ratcliff, Matthew D. Herron, Peter L. Conlin, William C. Ratcliff(Authors)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  Wan and Jékely, 2020 ), as are the mechanisms of their patterning and morphogenesis (Marshall, 2020). Properly understood as an independent and unique evolutionary experiment in achieving levels of size and morphological complexity that rival those of small animals, ciliates remain as fascinating as ever.

  13.6 20th CENTURY: THE COLLARED FLAGELLATE/CHOANOBLASTAEA MODEL

  Although it had to compete with the ciliate hypothesis for part of the 20th century, Metchnikoff’s concept of a choanoflagellate-like ancestor for all animals – and not just for sponges – was continuously supported by some authors (Hyman, 1940; Rieger, 1976 ; Salvini-Plawen, 1978 ; Nielsen and Norrevang, 1985). These researchers were each convinced about the monophyly of animals based on shared features such as sperm and eggs, epithelia, and gastrulation. This implied that all animals had evolved from a single lineage of protist, of which Choanoflagellates were considered the most plausible living representative as their similarity to choanocytes was so strong. This view received further support from the discovery of choanocyte-like collar cells by electron microscopy in diverse animal phyla other than sponges (Nerrevang and Wingstrand, 1970 ; Lyons, 1973 ; Rieger, 1976; Brunet and King, 2017). Claus Nielsen named this revised Blastaea model – starting from a collared ancestor – the “Choanoblastaea” (Nielsen, 2008) (Figure 13.6 ).

  FIGURE 13.6

  The choanoblastaea model of animal origins (Nielsen, 2008). (A) a modern choanoflagellate rosette colony proposed to resemble early stem-animals. Cells are arranged as a sphere surrounding a shared core of extracellular matrix (dark grey). (B) a hypothetical later stem-animal (“Choanoblastaea”), in which cells have become adjacent and have evolved intercellular junctions and now form a sealed epithelial sphere. (C) a later hypothetical stem-animal (“Advanced choanoblastaea”) in which some cells have become amoeboid and populated the inner space of the colony (compare P. haeckelii

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Cells in Evolutionary Biology

  Translating Genotypes into Phenotypes - Past, Present, Future

  • Brian K. Hall, Sally A. Moody, Brian K. Hall, Sally A. Moody(Authors)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  To understand such transition, we first need to understand how the unicellular ancestor of animals was. Given that we do not have that ancestor among us anymore, we can only infer how it was by comparing animals with their extant closest unicellular relatives. However, which of all extant protists are the closest unicellular relatives of animals is something that only became clear a few years ago with the advent of phylogenomic (multigene phylogenetic) analyses. Based on morphology, a group of flagellate protists, known as Choanoflagellates, had already been proposed on the nineteenth century to be the closest unicellular relatives to animals (King 2005; Leadbeater 2015). The reason for uniting Choanoflagellates with animals was a suggested homology of Choanoflagellates with a specific cell type of sponges (the choanocyte) (although the homology has been recently disputed; see Mah et al. 2014). Given that sponges were thought to be the earliest animal lineage, the homology was easily explained if Choanoflagellates were the sister-group to animals. Indeed, the first ribosomal phylogenies (Medina et al. 2003), and subsequent multigene or phylogenomic analyses (Lang et al. 2002; Steenkamp et al. 2006; Ruiz-Trillo et al. 2004, 2006, 2008; Shalchian-Tabrizi et al. 2008; Carr et al. 2008), supported this view, so that it is now clear that Choanoflagellates are the sister-group to animals. These trees showed that animals and Choanoflagellates, together with fungi, shared a closer ancestor than with plants or algae, forming a clade known as the opisthokonts.

  Further molecular data from other potential opisthokont protists demonstrated that the tree of opisthokonts had additional lineages (Torruella et al. 2012, 2015; Ruiz-Trillo et al. 2004; Shalchian-Tabrizi et al. 2008; Steenkamp et al. 2006). Three of those lineages, the Filasterea, the Ichthyosporea, and the Corallochytrea, were subsequently shown to be close relatives to Metazoa. The clade composed of animals and their closest unicellular relatives is known as the Holozoa (Lang et al. 2002) (Figure 4.3 ). The most taxon-rich and gene-rich phylogenetic analysis of the Holozoa shows that Teretosporea (Ichthyosporea + Corallochytrea) represents the earliest branching lineage, followed by Filasterea, and then Metazoa and Choanoflagellata (Torruella et al. 2015; Figure 4.3

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Thorp and Covich's Freshwater Invertebrates

  Ecology and General Biology

  • James H. Thorp, D. Christopher Rogers(Authors)
  • 2014(Publication Date)
  • Academic Press

    (Publisher)

  Fenchel and Finlay, 1995 ).

  Most Choanoflagellates are small (<10  μm), with a feeding filter that forms a “collar” around the single anterior flagellum (Figures 7.1 and 7.3 ). The flagellum creates the water current that flows through the filter. Choanoflagellates are exclusively phagotrophic and either solitary (e.g., Monosiga ) or colonial (e.g., Sphaeroeca ). They are important grazers of the smallest suspended bacteria in fresh waters and in the marine plankton (e.g., Salpingoeca , Stephanoeca , and Parvicorbicula ).

  FIGURE 7.3  A selection from the variety of form and function in flagellated protozoa.

  (a) A choanoflagellate with bacterial food trapped on the external surface of the collar filter (Monosiga ; body  +  collar 0.015  mm). (b) A bacteria-feeding helioflagellate (Pteridomonas ; body ∼0.007  mm). (c) A phagotrophic euglenid (Dinema ; ∼0.03  mm) with an ingested diatom. (d) A marine heterotrophic dinoflagellate (Protoperidinium ; ∼0.06  mm) with a diatom trapped in its feeding veil. (e) A free-living, bacteria-feeding, anaerobic diplomonad (Hexamita ; body ∼0.008  mm). (f) A small heterotrophic (bacteria-feeding) cryptomonad flagellate (Goniomonas ; ∼0.008  mm). (g) A heterotrophic, typically bacteria-feeding, chrysomonad flagellate (Spumella : body ∼0.008  mm). (h) A bacteria-feeding bodonid (Rhynchomonas ; body ∼0.005  mm).

  Euglenids usually have two flagella that emerge from a small anterior pocket. This is a large group of planktonic and benthic protozoa, common in both fresh and marine waters. Species associated with surfaces typically keep one trailing flagellum in contact with the substrate, whereas the other pulls the cell forward. Many species are green and phototrophic (e.g., Euglena ), but there is a great diversity of phagotrophs (e.g., Astasia , Petalomonas , Entosiphon , and Notosolenus ) that feed on bacteria. All bodonids are small flagellates with characteristic heterodynamic flagella (e.g., Bodo and Rhynchomonas,  Figure 7.3 ). They are almost always found in organically polluted (even anoxic) waters or soils that are rich in bacteria, on which they feed. Many closely related (“kinetoplastid”) flagellates are parasites: Ichthyobodo

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Ecology and Classification of North American Freshwater Invertebrates

  • James H. Thorp, Alan P. Covich(Authors)
  • 2009(Publication Date)
  • Academic Press

    (Publisher)

  [172] . Choanoflagellates attach to the substrate, and many have an external, loose-fitting covering or lorica (although, again, it may be difficult to see with the light microscope). In one marine family (Acanthoecidae), the lorica is basket-like.

  Bicoecids (Fig. 3.14i ) resemble Choanoflagellates, although they lack a collar. Like Choanoflagellates, they are enclosed in a lorica and have a flagellum that is used to create a feeding current. A second flagellum lies along the cell and continues posteriorly to become an attachment to the base of the lorica.

  Kinetoplastids (Fig. 3.14j, n–p ) are known mostly as parasites, especially Trypanosoma and its relatives, but many members of the suborder Bodina live in freshwater[301] . They have a unique single mitochondrion which runs the length of the cell and one or more DNA-rich organelles, kinetoplasts, associated with this mitochondrion near the flagella. The best-known genus is Bodo , which, like other bodonids, has two flagella (Fig. 3.14j ). One trails, often in contact with the substrate, while the other extends ahead. The trailing flagellum may attach temporarily to the substratum. In the related but sessile and colonial genus Cephalothamnium (Fig. 3.14l) , the attachment is permanent.

  Diplomonads and cercomonads are relatively unimportant groups to the freshwater ecologist; diplomonads are largely parasitic, with a few free-living forms that may be found in organically enriched water (e.g., Hexamita , Fig. 3.14m) . There is one freshwater cercomonad, Cercomonas (Fig. 3.14k ), and a common soil flagellate, Heteromita. Cercomonas , like the diplomonadida, tends to be found in organically enriched environments.

  The cryptomonads include many common heterotrophs and autotrophs; mixotrophy has been reported, but is not common. The two flagella are unequal in length and different in appearance (Fig. 3.7

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Animals Without Backbones

  An Introduction to the Invertebrates

  • Ralph Buchsbaum, Mildred Buchsbaum, John Pearse, Vicki Pearse(Authors)
  • 2013(Publication Date)
  • University of Chicago Press

    (Publisher)

  collar flagellates, which often live attached by a stalk to the substrate. In this group as well, the cells may occur singly or as small colonies. Each cell has a collar made up of many delicate cytoplasmic projections, from the center of which emerges a single flagellum. The beating of the flagellum draws a current of water toward the cell. Bacteria and other food particles in the current encounter the sides of the collar and main cell body, and are taken up into food vacuoles. The collar flagellates are of special interest because a strikingly similar type of cell occurs as the food-ingesting cell of sponges. Thus it is often suggested that sponges, which are many-celled animals, evolved from collar flagellates.

  Most notorious among the animal-like flagellates are the trypanosomes that cause Chagas’ disease in South America and sleeping sickness in Africa (not the same as the sleeping sickness that is a viral encephalitis). The trypanosomes and their relatives live as parasites in insects, certain plants, and many vertebrates in Africa without causing much inconvenience to their hosts; over millions of years of exposure these hosts have presumably evolved the ability to resist or to tolerate the parasites. But when humans and their domestic animals become infected, incapacitating and sometimes fatal disease results.

  Trypanosome. The flagellum is attached by a membrane for most of its length but extends free at the anterior end of the organism.

  The trypanosomes that cause African sleeping sickness are transmitted by blood-sucking tsetse flies. Practically all wild game in Africa harbor trypanosomes in their blood; and when a tsetse fly sucks the blood of an antelope, for example, or of an infected human, the blood taken into the intestine of the fly will contain these trypanosomes. From the intestine they invade the salivary glands of the insect, meanwhile multiplying and undergoing changes in form until they reach a stage in which they are infective to a vertebrate host. If a fly carrying such infective stages bites a human, the trypanosomes are injected into the blood of the victim with the saliva of the fly. In the blood they multiply rapidly and wriggle about among the blood cells, propelled by the undulations of the flagellum.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  The Invertebrate Tree of Life

  • Gonzalo Giribet, Gregory D. Edgecombe(Authors)
  • 2020(Publication Date)
  • Princeton University Press

    (Publisher)

  Likewise, such characters are unlikely to be recognized in the fossil record and thus if the last common ancestor of all animals looked like a comb jelly, a sponge, or a placozoan they would be recognized as stem groups of each of those three lineages, but probably not as the so-called Urmetazoan. Only one scenario, that of sponge paraphyly at the base of the animal tree, would provide the necessary power to say something about such an ancestor, as proposed by Nielsen (2008) in his “choanoblastaea” hypothesis. Sponge paraphyly is, however, disfavored in most recent phylogenetic analyses of sponges and metazoans.

  Two facts are important for this book. First is the position of Metazoa in the broader tree of life within a clade of Opisthokonta named Holozoa. Holozoa includes, in addition to animals, Choanoflagellates, filastereans and ichthyosporideans. Metazoa is well supported in all molecular phylogenetic analyses as sister group of Choanoflagellatea (e.g., Torruella et al., 2015) [fig. 1.1 ]. The resemblance of Choanoflagellates to sponge choanocytes is striking and has been used a synapomorphy for the clade containing Choanoflagellates and metazoans (= Choanozoa), reinforced in those topologies that suggest sponge paraphyly at the base of animals (Nielsen, 2012a). However, few real comparisons have been made between Choanoflagellates and choanocytes until recently (Mah et al., 2014), and these authors indicated that although these cells are similar in some aspects, they differ in others, concluding that homology cannot be taken for granted. Similarities in collar-flagellum systems separated by 600 million years of evolution, whether homologous or convergent, suggest that these form important adaptations for optimizing fluid flow at microscale levels (Mah et al., 2014).

  Irrespective of whether or not these two cell types are homologous, animal biologists have much to learn from animals’ closest relatives. The first choanoflagellate genome, for the unicellular species Monosiga brevicollis

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Introduction to Marine Micropaleontology

  • B.U. Haq, A. Boersma(Authors)
  • 1998(Publication Date)
  • Elsevier Science

    (Publisher)

  ORGANIC-WALLED MICROFOSSILS Scanning electron micrographs reveal different pat-terns or ornamental ribbing in closely related chitino-zoans of the genus Cyathochitina. From type Caradoc (Upper Ordovician), England, x 180. 13 DINOFLAGELLATES, ACRITARCHS AND TASMANITIDS GRAHAM L. WILLIAMS INTRODUCTION Dinoflagellates In modern times man has become increas-ingly aware of a natural phenomenon called red tide and the disastrous effect which it can have on the ecology of marine coastal areas. Red tides are blooms or high con-centrations (up to 6 X 10^ organisms per liter of water) of small unicellular algae called dinoflagellates. In some areas, presumably where the water is richer in nutrients, the dinoflagellates multiply so rapidly and become so concentrated that their distinctive cell pigments impart a red hue to the water. Red tides can be a serious threat to life, be-cause the contained dinoflagellates secrete a lethal toxin known as paralytic shellfish poison. Other marine dinoflagellates are well known to man because they are responsible for most of the luminescence in the seas, such as that often seen in a ship's wake or in the breaking surf at night. Living dinoflagellates are unicellular bi-flagellate algae ranging in size from 5 |Lim to 2 mm. They constitute the Division Pyrrho-phyta (from the Greek pyrrhos = flame-colored, and phyta = plants) of the Algae. They have a varied habit; some are planktonic organisms in marine and fresh-water environ-ments, where they are an important part of the food chain; but a few are marine sand dwellers, and some are symbionts or parasites. Most dinoflagellates are autotrophic: they contain chromatophores and carry out photo-synthesis. Some are heterotrophic: they are devoid of chromatophores and feed like animals by ingesting their foods; either holo-zoic (ingesting whole food particles) or saprophytic (absorbing dissolved food material as parasites).

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  The Open Sea

  The World of Plankton

  • Alister Hardy(Author)
  • 2012(Publication Date)
  • Collins

    (Publisher)

  In this way they may stave off the inevitable decline due to that nitrate and phosphate shortage in the water which we discussed in Chapter 4. Some of these have gone further, losing their chlorophyll and power of photosynthesis, to live entirely as animals. Yes, it is among the flagellated organisms that we must look for the most primitive animals and not to Amoeba which, in spite of its apparent simplicity, must have travelled much further along the line of animal evolution than have the flagellates. There are actually some flagellates which become amoebalike in form at one stage in their life-cycles. By the beginning of the Cambrian period, which produced the first sedimentary rocks giving us really reliable fossils, we find a considerable variety of invertebrate animals already in existence; they include highly organised crustaceans (the trilobites) which are now extinct, and equally elaborate aquatic snail-like animals. Those early Cambrian rocks were, according to the latest estimates, laid down some 500 million years ago and by then most of the invertebrate groups were already well developed; their evolution from primitive protozoa must surely have taken at least as long again. It seems likely that the first animals must have come into existence in a marine plankton of something like a thousand million years ago. Most of the marine flagellates are either true plants or forms which live both as plants and animals; these we have already dealt with in Chapter 3. One dinoflagellate, however, has been reserved for treatment here, for it is entirely animal in nature: the remarkable Noctiluca shown on Plate III. There appears to be only one species in northern waters, which has unfortunately been called by two names: Noctiluca miliaris or N

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Emerging Protozoan Pathogens

  • Naveed Ahmed Khan(Author)
  • 2008(Publication Date)
  • Taylor & Francis

    (Publisher)

  Gonyaulax.

  3.10 Diplomonadida

  Protozoa in this taxon lack mitochondria, Golgi bodies and peroxisomes. The organisms have two equal-sized nuclei and multiple flagella, for example, Giardia.

  3.11 Euglenozoa

  This group is further subdivided into two groups. (i) The Euglenids are photoautotrophic unicellular microbes with chloroplasts containing pigments (historically thought to be plants). However, they possess flagella, lack cell wall and are chemoheterotrophic phagocytes (in the dark), for example, Euglena. (ii) The Kinetoplastids have a single large mitochondrion that contains a unique region of mitochondrial DNA, called a kinetoplast. Kinetoplastids live inside animals and some are pathogenic, for example, Trypanosoma and Leishmania.

  3.12 Stramenopila

  This group is a complex assemblage of ‘botanical’ protists with both heterotrophic and photosynthetic representatives. The evolutionary history of this group is unclear. Generally, the organisms included in this group are slime nets, water molds and brown algae, and are characterized by possessing flagella. Recent molecular phylogenetic studies revealed that Blastocystis belongs to this group.

  It is important to indicate that no single classification scheme has gained universal support and future studies will almost certainly dictate changes in the above scheme. The representative protozoa pathogens that are covered in this book are indicated in Table 1.

  4 Locomotion

  Motility in protozoa is usually mediated by cilia, flagella, or cellular appendages adapted for propulsion, or amoeboid movement. Other modes of protozoa locomotion involve gliding movements in which no changes in body shape are observed. The various protozoa have evolved to exhibit distinct movements depending on where they normally live. For example, protozoans with amoeboid movements using pseudopodia are normally present in environments with abundant organic matter or in flowing water with plant life. Cilia or flagella are used to travel longer distances per se

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  The Story of Evolution

  • Joseph McCabe(Author)
  • 1997(Publication Date)
  • Perlego

    (Publisher)

  For the moment, however, we must glance at the operation of this and other natural principles in the evolution of the one-celled animals and plants, which we take to represent the primitive population of the earth. As there are tens of thousands of different species even of "microbes," it is clear that we must deal with them in a very summary way. The evolution of the plant I reserve for a later chapter, and I must be content to suggest the development of one-celled animals on very broad lines. When some of the primitive cells began to feed on each other, and develop mobility, it is probable that at least two distinct types were evolved, corresponding to the two lowest animal organisms in nature to-day. One of these is a very minute and very common (in vases of decaying flowers, for instance) speck of plasm, which moves about by lashing the water with a single oar (flagellum), or hair-like extension of its substance. This type, however, which is known as the Flagellate, may be derived from the next, which we will take as the primitive and fundamental animal type. It is best seen in the common and familiar Amoeba, a minute sac of liquid or viscid plasm, often not more than a hundredth of an inch in diameter. As its "skin" is merely a finer kind of the viscous plasm, not an impenetrable membrane, it takes in food at any part of its surface, makes little "stomachs," or temporary cavities, round the food at any part of its interior, ejects the useless matter at any point, and thrusts out any part of its body as temporary "arms" or "feet."

  Now it is plain that in an age of increasing microbic cannibalism the toughening of the skin would be one of the first advantages to secure survival, and this is, in point of fact, almost the second leading principle in early development. Naturally, as the skin becomes firmer, the animal can no longer, like the Amoeba, take food at, or make limbs of, any part of it. There must be permanent pores in the membrane to receive food or let out rays of the living substance to act as oars or arms. Thus we get an immense variety amongst these Protozoa, as the one-celled animals are called. Some (the Flagellates) have one or two stout oars; some (the Ciliates) have numbers of fine hairs (or cilia). Some have a definite mouth-funnel, but no stomach, and cilia drawing the water into it. Some (Vorticella, etc.), shrinking from the open battlefield, return to the plant-principle, live on stalks, and have wreaths of cilia round the open mouth drawing the water to them. Some (the Heliozoa) remain almost motionless, shooting out sticky rays of their matter on every side to catch the food. Some form tubes to live in; some (Coleps) develop horny plates for armour; and others develop projectiles to pierce their prey (stinging threads).

  Sign up to read

  Learn more about book