LECTURE 02 - Metazoa Flashcards
What are animals?
Animals are motile multicellular organisms with somatic differentiation (usually)
What is “Metazoa”?
Metazoa is a monophyletic group
What is the sister taxon of metazoa?
Choanoflagellata
What do the horizontal lines in a phylogenetic tree represent?
- 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
What is choanoflagellata?
- 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
What is reasonable to infer about the common ancestor of Metazoa?
It is reasonable to infer that the common ancestor of Metazoa resembled a colonial choanoflagellate
What is the somatic differentiation in the sponge body?
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
How many cells do sponges consists of, and what are they?
4
- Choanocytes
- Pinacocytes
- Porocytes
- Archaeocytes
Organized in a matrix of mesohyl invested with spicules
What differentiates Porifera from Choanoflagellata?
- somatic differentiation
- unique metazoan features such as fibrillar collagen and sperm
What are the epithelium and mesenchyme?
- 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
What is the blastula?
- 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
In order for the blastula to develop further, what difficulties must it overcome?
- 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
What happens during gastrulation?
- 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
What is a crucial feature in the evolution of individuals with stable development?
Somatic cell lineages lose the capacity to reproduce and develop into new individuals
What are somatic cell linages vulnerable to?
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
For somatic cell lineages, what does attaining a definite complex mutlicellular form require and how is it achieved?
- 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”
Grades of organization in Metazoa
- 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
What differentiates Placozoa from Ctenophora/Cnidaria?
- Gastrulation
- Epithelia with basement membrane
- Muscle and nerve
What differentiates Ctenophora/Cnidaria from Bilateria?
- Secondary somatic differentiation
- Cephalization
- Central nervous system
- New organ systems e.g., excretory system
- Complete set of Hox genes
All metazoans can be grouped into how many clades? What are they?
5 clades
- Porifera: sponges
- Placozoa: ‘flat animals’
- Cnidaria: jellyfish, corals, hydroids and others
- Ctenophora: comb jellies
- Bilateria: everything else
What are integrins?
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
What are cadherins?
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
What are tyrosine kinases?
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
What is the conventional theory of ancestry of Metazoa?
Choanocytes homologous with choanoflagellate cell
- muscle and nerve evolve once
- from Porifera to Ctenophora/Cnidaria
