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BIOL 305 > LECTURE 02 - Metazoa > Flashcards

LECTURE 02 - Metazoa Flashcards

1
Q

What are animals?

A

Animals are motile multicellular organisms with somatic differentiation (usually)

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2
Q

What is “Metazoa”?

A

Metazoa is a monophyletic group

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3
Q

What is the sister taxon of metazoa?

A

Choanoflagellata

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4
Q

What do the horizontal lines in a phylogenetic tree represent?

A
  • The horizontal lines are branches that represent the lineage leading to the terminal taxon
  • The length of the branch represents the amount of evolutionary change that has taken place from the most recent common ancestor of a taxon and its sister taxon
  • The unit is substitutions per sequence site
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5
Q

What is choanoflagellata?

A
  • Choanoflagellates have a single flagellum whose base is surrounded by a collar formed of actin fibres (microvilli)
  • The undulation of the flagellum draws water through the microvilli, which trap edible particles such as bacteria
  • The flagellum may also propel the organism through the water, cell forward (i.e., the single flagellum is posterior)
  • In other flagellated organisms the propulsive flagellum is usually anterior and pulls the cell through the water
  • May be either free-swimming or sessile and attached to the substrate by a thin pedicel
    • sessile forms in particular are often colonial, with cells embedded in an extracellular matrix
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6
Q

What is reasonable to infer about the common ancestor of Metazoa?

A

It is reasonable to infer that the common ancestor of Metazoa resembled a colonial choanoflagellate

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7
Q

What is the somatic differentiation in the sponge body?

A

The sponge animal is a sessile benthic filter feeder, extracting edible particles from the water stream created by the choanocytes
- the outer surface of the sponge is a pinacoderm of protective cells
- Between the pinacoderm and the choanocyte chambers is a gelatinous layer, the mesohyl
- the mesohyl has a population of totipoten archeocytes (amoebocytes)
- the mesohyl is bridged by porocytes which allow water to flow from outside into the interior of the sponge
- the sponge body is stiffened, strengthened and protected by spicules made of calcites, silicate or protein

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8
Q

How many cells do sponges consists of, and what are they?

A

4
- Choanocytes
- Pinacocytes
- Porocytes
- Archaeocytes
Organized in a matrix of mesohyl invested with spicules

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9
Q

What differentiates Porifera from Choanoflagellata?

A
  • somatic differentiation
  • unique metazoan features such as fibrillar collagen and sperm
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10
Q

What are the epithelium and mesenchyme?

A
  • Epithelial cells are polarized, with their axes aligned in parallel with each other
    • they are joined by belt-form junctions
    • only their basal and apical surfaces associate with extracellular matrix (basal lamina of a basement membrane basally and cuticle apically)
  • the mesenchymal cell below has no particular alignment with other mesenchymal cells, bears only spot-form junctions, and is essentially surrounded by the extracellular matrix
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11
Q

What is the blastula?

A
  • equal radial cleavage in lecithotrophic (yolk-bearing) eggs leads eventually to a hollow ball of cells, the blastula
  • the cells bear cilia directed outwards
  • in many animals the blastula is released into the water as a motile free-swimming organism
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12
Q

In order for the blastula to develop further, what difficulties must it overcome?

A
  • It cannot feed because it has no mouth (a motile choanocyte colony could feed, however)
  • It cannot develop further because of the ciliation constraint that applies to all animals
    • no metazoan cell can divide while ciliated
    • if the cilia were shed in order to permit development, the embryo would sink
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13
Q

What happens during gastrulation?

A
  • During gastrulation, cells from the surface of the blastula move to its interior by invagination or introgression
  • This evolves in response to an ancient constrain in Metazoa
    • cells cannot divide while flagellated, because the basal body cannot act simultaneously as a centriole for the flagellum and as a microtubule organizing centre for the mitotic spindle
  • if the surface cells lose their flagella, the blastula will sink
  • cells in the interior can divide and differentiate without compromising motility
  • this is why gastrulation is a fundamental feature of metazoan development
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14
Q

What is a crucial feature in the evolution of individuals with stable development?

A

Somatic cell lineages lose the capacity to reproduce and develop into new individuals

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15
Q

What are somatic cell linages vulnerable to?

A

Somatic cell lineages are vulnerable to invasion by selfish cell lineages which revert to being totipotent cells capable of reproduction, since any mutation causing reversion will tend to spread by virtue of its reproductive advantage

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16
Q

For somatic cell lineages, what does attaining a definite complex mutlicellular form require and how is it achieved?

A
  • It requires restricting the heritability of somatic variation
  • This is very generally achieved through three mechanisms acting during early development
    • Reproduction by eggs: this shifts selection from involving competition between cell lineages within an individual to competition between individuals that favour an integrated body plan
    • Maternal control of early development: determinate development controlled by morphogens provided in the egg suppresses the proliferation of selfish cell lineages
    • Early segregation of the germ line: when the germ line is phyiscally separated from somatic lineages early in development, it is difficult for it to be invaded by selfish cell lineages
  • These ideas were clearly expressed by Leo Buss in “The Evolution of Individuality”
17
Q

Grades of organization in Metazoa

A
  • Embryonic development of epithelia in lower metazoans (centre pathway) and grades of organization reflected at the various embryonic stage
  • The morula and early blastula stages are non-epithelial, lacking a basal lamina, and can be taken as reflecting cel organization in the lower two phyla, Porifera and Placozoa, because of the lack of a basal lamina
  • The cells of the late blastula (lower side of dividing line) become epithelial by forming a basal lamina and sealing junctions
  • Gastrulation produces two epithelial layers - epidermis and gastrodermis - and from such a stage the diploblasts, with a single internal compartment, the gastrovascular cavity, are derived
  • Triploblasts develop by setting off a third epithelial germ layer that delimits another internal compartment, the coelem
  • Mesenchymal cells derive from this epithelium in particular
  • Epithelia and mesenchyme are the building blocks from which organs form, so true organs arise only in the triploblasts
18
Q

What differentiates Placozoa from Ctenophora/Cnidaria?

A
  • Gastrulation
  • Epithelia with basement membrane
  • Muscle and nerve
19
Q

What differentiates Ctenophora/Cnidaria from Bilateria?

A
  • Secondary somatic differentiation
  • Cephalization
  • Central nervous system
  • New organ systems e.g., excretory system
  • Complete set of Hox genes
20
Q

All metazoans can be grouped into how many clades? What are they?

A

5 clades
- Porifera: sponges
- Placozoa: ‘flat animals’
- Cnidaria: jellyfish, corals, hydroids and others
- Ctenophora: comb jellies
- Bilateria: everything else

21
Q

What are integrins?

A

Integrins are proteins that function mechanically, by attaching the cell cytoskeleton to the extracellular matrix (ECM), and biochemically, by sensing whether adhesion has occured

  • happens between the evolution of Fungi to Filasterea
22
Q

What are cadherins?

A

Cadherins (names for “calcium-dependent adhesion”) are a type of cell adhesion molecule (CAM) that is important in the formation of adherent junctions to bind cell with each other

  • happens between the evolution of Filasterea to Choanoflagellata
23
Q

What are tyrosine kinases?

A

A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to a protein in a cell
It functions as an “on” or “off” switch in many cellular functions

  • Happens between the evolution of Filasterea and Choanoflagellata
24
Q

What is the conventional theory of ancestry of Metazoa?

A

Choanocytes homologous with choanoflagellate cell
- muscle and nerve evolve once
- from Porifera to Ctenophora/Cnidaria

25
Q

What is the unconventional theory of ancestry of Metazoa?

A

Choanocytes an innovation in Porifera
- muscle and nerve evolve twice
- In Ctenophora and from Porifera to all other Metazoa

26
Q

What did the earliest metazoans eat?

A
  • Must have consumed bacteria or unicellular eukaryotes
  • They had traps or filters adapted to collecting unicells
  • Sponges and sponge-like creatures can do this
  • They inherited the basic device, the choanocyte, from their unicellular ancestors
27
Q

What are armed tentacles adapted for?

A

Armed tentacles are adapted to capturing large multicellular prey
- they cannot be used to collect unicells
- hence, cnidarians and ctenophores are unlikely body plans for the earliest metazoans

There are kinds of tentacles able to collect unicells (the ciliated lophophore of bryozoans, etc.), but they are unknown in cnidarians and ctenophores
- Hence, Vendian impressions unlikely to be cnidarians

28
Q

What is the first direct evidence of zookplankton?

A

Klausmuelleria (phosphatocopid) from lower Cambrian
- there were also chaetognaths (or protoconulids) around at that time, which certainly ate zooplankton

29
Q

What is the ecologically correct conclusion for what the earliest Metazoans were?

A

The earliest metazoans were sponges that filtered bacteria from seawater
- Zooplankton > 1mm in size appeared in the lowest Cambrian
- Cnidarians and ctenophores evolved to eat them

30
Q

What are some enigmatic parasites that have turned out to be cnidarians?

A
  • Polypodium
    • Endoparasite of sturgeon eggs
  • Buddenbrockia
    • Endoparasite of bryozoans
  • Myxozoa
    • Cellular endoparasites of annelids
31
Q

What are Dicyemids?

A
  • Dicyemids are among the simplest of all animals, with a tiny, worm-like adult body that consists of between 10 and 40 cells
  • They have no organs, body cavities or even guts
    • A structural simplicity which is a consequence of where and how they live
  • The only place you will find adult dicyemids is inside the bodies of cephalopods, typically octopuses and cuttlefish where large numbers of them cling to the inner wall of the kidney
32
Q

What are Orthonectida?

A
  • The adults are microscopic wormlike animals, consisting of a single layer of ciliated outer cells surround a mass of sex cells
  • They swim freely within the bodies of their hosts, which include flatworms, polychaete worms, bivalve molluscs, and echinoderms
  • They are gonochoristic, with separate male and female individuals
33
Q

What do Orthonectida do when they are ready to reproduce?

A
  • The adults leave the host, and sperm from the males penetrate the bodies of the females to achieve internal fertilization
  • The resulting zygote develops into a ciliated larva that escapes from the mother to seek out new hosts
  • Once it finds a host, the larva loses its cilia and develops into a syncytial plasmodium larva
  • This, in turn, break up into numerous individual cells that become the next generation of adults
34
Q

What are some cryptic worms from marine sand?

A
  • Lobatoerebrum
    • a vermiform, fully ciliate animal without setae or segmentation
    • Body cavity filled with mesenchyme, but with open digestive tube and remnants of a blood vascular system
  • Diurodrilus
    • Unsegmented worm with ventral creeping foot
    • Pedal glands reminiscent of gastrotrichs
  • Jennaria
    -Acoelomate worm; unicellular longitudinal muscles, no circular muscle
35
Q

What is the Xenoturbella?

A
  • Xenoturbella is a marine worm about 1-3 cm long, possessing a very simple body plan
  • Externally, it has brownish to pale yellow colour with a whitish anterior
  • The circumferential furrow encircles the body about at the middle, and side furrows are seen running down both lateral sides from the anterior tip to around the circumferential furrow
  • The mouth opens in the ventral side, slightly anterior to the circumferential furrow
  • The outermost layer is a ciliated epidermis
  • The innermost structure is the intestine, which is connected to the outside via the mouth, but there is no anus
  • Structures such as a central nervous system (brains or ganglia), reproductive organs (gonads or gonopores), excretory organs or coelom are lacking in Xenoturbella
36
Q

What is the Salinella?

A
  • Salinella salve, an organism described as a single layer of cells, ciliated on both inner and outer surfaces and surrounding a central hollow sac open at both ends, has been collected only once, by German biologist Johannes Frenzel, who claimed to have found it in salt pans of Argentina in the late 19th century
  • Its body plan, which suggests a central digestive tract with a mouth and anus, was unlike that of any known life-form
  • Composed of just one cell layer, Salinella seemed intermediate between single-celled organisms that perform all digestion inside the cell, and multicellular life-forms that excrete digestive juices into a central sac

BIOL 305 (25 decks)

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