Download 3. Evolution of a body cavity

Overview of Animal Diversity
General Features of Animals
Animals are a diverse group of consumers
that share major characteristics
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All are heterotrophs
All are multicellular
Cells do not have cell walls
Most are able to move
All are very diverse in form and habitat
Most reproduce sexually
Have a characteristic patterns of embryonic
development
• Cells of all animals (except sponges) are organized
into tissues
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Evolution of the Animal Body Plan
Five key transitions in animal evolution
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2.
3.
4.
5.
Tissues
Symmetry
Body cavity
Development
Segmentation
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Evolution of the Animal Body Plan
1. Evolution of tissues
– Parazoa (Sponges - the simplest animals) lack
defined tissues and organs
• Have the ability to disaggregate and aggregate their
cells
– Eumetazoa (all other animals) have distinct
and well-defined tissues
• Have irreversible differentiation for most cell types
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Evolution of the Animal Body Plan
2. Evolution of symmetry
– Parazoa (sponges) lack any definite symmetry
– Eumetazoa have a symmetry defined along
imaginary axes drawn through the animal’s
body
There are two main types of symmetry
– Radial symmetry
– Bilateral symmetry
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Evolution of the Animal Body Plan
• Radial symmetry
– Body parts arranged around central axis
– Can be bisected into two equal halves in any
2D plane perpendicular to that axis
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Evolution of the Animal Body Plan
• Bilateral symmetry
– Body has right and left halves that are mirror
images
– Body has distinct anterior/posterior and
dorsal/ventral divisions
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Evolution of the Animal Body Plan
• Bilaterally symmetrical animals have
two main advantages over radially
symmetrical animals
1. Cephalization
• Evolution of a definite brain area
2. Greater mobility
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Evolution of the Animal Body Plan
3. Evolution of a body cavity
• Eumetazoa produce three germ layers
– Outer ectoderm (body coverings and nervous
system)
– Middle mesoderm (skeleton and muscles)
– Inner endoderm (digestive organs and
intestines)
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Evolution of the Animal Body Plan
3. Evolution of a body cavity
• Three basic kinds of body plans
a. Acoelomates have no body cavity
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Evolution of the Animal Body Plan
b. Pseudocoelomates have a body cavity
between mesoderm and endoderm
• Called the pseudocoel
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Evolution of the Animal Body Plan
c. Coelomates have a body cavity entirely within
the mesoderm
• Called the coelom
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Evolution of the Animal Body Plan
• The body cavity made possible the
development of advanced organs systems
• Coelomates developed a circulatory
system to flow nutrients and remove wastes
– Open circulatory system: blood passes from
vessels into sinuses, mixes with body fluids and
reenters the vessels
– Closed circulatory system: blood moves
continuously through vessels that are separated
from body fluids
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Evolution of the Animal Body Plan
4. Evolution of different patterns of
development
• The basic bilaterian pattern of development
– Mitotic cell divisions of the egg form a hollow ball
of cells, called the blastula
– Blastula indents to form a 2-layer-thick ball called
a gastrula with:
• Blastopore - Opening to outside
• Archenteron - Primitive body cavity
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Evolution of the Animal Body Plan
Bilaterians can be divided into two groups
• Protostomes develop the mouth first from or
near the blastopore
– Anus (if present) develops either from blastopore
or another region of embryo
• Deuterostomes develop the anus first from
the blastopore
– Mouth develops later from another region of the
embryo
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Evolution of the Animal Body Plan
• Deuterostomes differ from protostomes in three
other embryological features:
– Cleaveage pattern of embryonic cells
• Protostomes - Spiral cleavage
• Deuterostomes - Radial cleavage
– Developmental fate of cells
• Protostomes - Determinate development
• Deuterostomes - Indeterminate development
– Origination of coelom
• Protostomes - Forms simply and directly from the mesoderm
• Deuterostomes - Forms indirectly from the archenteron
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Evolution of the Animal Body Plan
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Evolution of the Animal Body Plan
5. Evolution of segmentation
– Segmentation provides two advantages
• 1. Allows redundant organ systems in adults such as
occurs in the annelids
• 2. Allows for more efficient and flexible movement
because each segment can move independently
– Segmentation appeared several times in the
evolution of animals
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Traditional Classification of Animals
• Multicellular animals, or metazoans, are
traditionally divided into 36 or so distinct phyla
based on shared anatomy and embryology
• Metazoans are divided into two main
branches:
– Parazoa - Lack symmetry and tissues
– Eumetazoa - Have symmetry and tissues
• Diploblastic - Have two germ layers
• Triploblastic - Have three germ layers
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Going From A single cell to
Multicellular Organism
This is an example
of Phylogenic Tree
As an A & P class
will work from left to
right throughout
the 1st marking
period
•Do you see any
patterns?
Cleavage
Blastula
Gastrulation
Gastrula
Cell Cycle: G1, S, G2, and Mitosis
Fertilization (1n) egg + (1n) sperm=(2n)
zygote
• The initial event in development in sexual
reproduction is fertilization, the union of
male and female gametes to form a
zygote.
• Recombinant of parental and maternal
genes
• Parthenogenesis-development without
fertilization (Example some fish and
salamanders)
Cleavage and Early Development
• During cleavage the embryo divides repeatedly to
convert the large cytoplasmic mass into a large
cluster of small maneuverable cells called
Blastomeres.
Cleavage and Early Development
• No growth during this period, only
subdivisions of mass, which continues
until somatic cell size is attained. Think of
it a origami!!!!
• So, what are somatic cells?
• At the end of cleavage 100s to 1000s of
cells
Cleavage and Early
Development
Look at the Diagram
to see an example of
a VEGETAL POLE
AND ANIMAL POLE
•What is the role of
the yolk?
•Do the cells at the
poles divide at the
same rate?
What can we learn from Development?
• Developmental Biology is a growing field!!
• How can a zygote, a single layer cell,
produce a multitude of body parts in an
organism and how gene expressions
proceeds.
• Search for commonalities among
organisms.
II. An overview of Development Following
Cleavage
A. Blastulation
• Cleavage subdivides the mass of zygote
until a cluster of cells called a blastula is
formed (looks like a hollow mass of cells).
• In most animals, the cells are arranged
around a central fluid-filled cavity called a
blastocoel.
• Formation of a blastula stage, with its one
layer of germ cells, occurs in all
multicellular animlas.
B. Gastrulation and Formation of Two Germ Layers
a. Gastrulation converts the spherical blastula into
a more complex configuration and forms a
second germ layer.
– One side of the blastula bends inward in a
process called invagination, forming a new
internal cavity. Picture pushing in a beach
ball-the inward region forms a pouch.
– The internal pouch is the gut cavity called an
archenteron or gastrocoel.
– The opening to the gut, where the inward
bending began, is the blastopore.
Gastrulation and Formation of Two Germ Layers
C. The gastrula stage has two layers:
a. An outer layer of cells surrounding the blastocoel,
called ectoderm
b. An inner layer of cells is called endoderm
– The gut opens only at the blastopore it is called a
blind or incomplete gut. Animals with a blind gut must
consume food completely digested, or the remains of
the food egested through the mouth. Ex sea
anemones and flatworms.
– Most animals have a complete gut with a second
opening, the anus.
c. Formation of the Mesoderm, a
Third layer
i. Multicellular
animals (not
sponges)
proceed
blastula to
gastrula
• Two germ
layers called
DIPLOBLASTI
C
• Three germ
layers called
C. Formation of the Coelom
• Coelomates are animals
with a true coelom, a fluid
filled body cavity
completely lined by
tissues derived from
mesoderm.
• The inner and outer
layers of tissue that
surround the cavity
connect dorsally and
ventrally to form
mesenteries that
suspend the
internal organs.
Animals can be divided
into three body type:
1. Acoelomate
2. Pseudocoelomate
3. Coelomate
D. Protostome vs Deuterostome in Coelomates
• Mollusks, annelids, arthropods and some other
phyla are collectively called protostomes.
• Echinoderms and chordates are called
deuterostomes.
4 Fundamental differences between the two
groups:
***** See Class Handout or next slide
A New Look At Metazoans
• The traditional animal phylogeny is being
reevaluated using molecular data
• Myzostomids are marine animals that are
parasites of echinoderms
• Have no body cavity and only incomplete
segmentation and so have been allied with
annelids
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A New Look At Metazoans
• Recent analysis of the translation machinery
revealed that myzostomids have no close
link to the annelids at all
• Instead, they are
more closely allied
with the flatworms
(planaria and
tapeworms)
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A New Look At Metazoans
• It seems that key morphological characters
used in traditional classification are not
necessarily correct
• Molecular systematics uses unique
sequences within certain genes to identify
clusters of related groups
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A New Look At Metazoans
• Most new phylogenies agree on two
revolutionary features:
1. Separation of annelids and arthropods into
different clades
2. Division of the protostome group into
Ecdysozoa and Spiralia
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The latter is then broken down into
Lophotrochozoa and Platyzoa
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A New Look At Metazoans
Examples can be found in Table 32.2 of Raven et al.
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Evolutionary Developmental Biology
• Most taxonomists agree that the animal
kingdom is monophyletic
• Three prominent hypotheses have been
proposed for the origin of metazoans from
single-celled protists
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Evolutionary Developmental Biology
1. The multinucleate hypothesis
2. The colonial flagellate hypothesis
3. The polyphyletic origin hypothesis
• Molecular systematics using rRNA sequences
settles this argument in favor of the colonial
flagellate hypothesis
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