overview of animal diversity. 2 general features of animals animals are a diverse group of consumers...
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Overview of Animal Diversity
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General Features of Animals
Animals are a diverse group of consumers that share major characteristics
• 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
1. Tissues
2. Symmetry
3. Body cavity
4. Development
5. 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 Plan2. 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 plansa. 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
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
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?
Cleavage and Early Development
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 CleavageA. 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
ectodermb. 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 DIPLOBLASTIC
• Three germ layers called TRIPLOBLASTIC
– The third layer is called mesoderm
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
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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
• 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