overview of metazoan diversity. 2 learning outcomes identify three features that characterize all...
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LEARNING OUTCOMES• Identify three features that characterize all
animals and four that characterize only some types of animals.
• Understand how the metazoans are organized and how this organization is different from that of plants, fungi, protists, and prokaryotes.
• Know the five key innovations in body plans.• Compare and contrast Parazoa and
Eumetazoa in terms of evolution, complexity, symmetry, and organization of embryonic cell layers.
• Compare and contrast asymmetry, radial symmetry, and bilateral symmetry.
• Differentiate among acoelomate, pseudocoelomate, and coelomate organisms; indicate how they are evolutionarily related and give examples of each.
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• Differentiate between protostomes and deuterostomes.
• Understand the advantages of segmentation; give at least one example of segmentation in each of the coelomate phyla.
• Compare the tradition methods using morphology in classification of metazoans to the new molecular systematics using DNA and RNA analysis to classify related metazoan groups. What is the problems of classification regarding homology and analogy?
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General Features of metazoansAre “metazoans” monophyletic?• Animals are so diverse that few criteria fit
them all. But some, such as metazoans being eaters, or consumers, apply to all.
• ALL:– Are heterotrophs– Are multicellular (It was such a great
move, it evolved at least 16 different times. Animals, land plants, fungi and algae all joined in)
– Have cells without cell walls
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General Features of Animals
• MOST:– Most are able to move– Are very diverse in form and habitat – Most reproduce sexually– Have a characteristic pattern of
embryonic development– Cells of all metazoans (except sponges)
are organized into tissues
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Traditional Classification of MetazoansFive key innovations can be noted in animal
evolution:1. The evolution of symmetry2. The evolution of tissues, allowing specialized
structuresand functions3. The evolution of a body cavity4. The evolution of various patterns of embryonicdevelopment5. The evolution of segmentation, or repeated body
units
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• Traditional methods of classification:– Morphology– Embryology– Symmetry– Germ layers
Problem: these comparisons can be analogous or homologous.– What’s the difference?– What is the difference in
classification and phylogenetics?
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Classification of Animals 5 Key Transitions
(1) Tissues
(2) Body Symmetry
(3) Body Cavity
(4) Development
(5) Segmentation
Figure 31.3
Chapter 31
PhylogenyMetazoans are divided into two main
branches:
• Parazoa = Lack symmetry and tissues– These “simplest” metazoans lack defined
tissues and organs – Have the ability to disaggregate and
aggregate their cells
• Eumetazoa = Have symmetry and tissues– Diploblastic = Have two germ layers– Triploblastic = Have three germ layers
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Evolution of the Animal Body Plan
1. Evolution of tissues—Parazoa/Eumetazoa split
• Have irreversible differentiation for most cell types
• The evolution of tissues allowed for specialized structures and functions
• Eumetazoa (all other metazoans) have distinct and well-defined tissues
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Evolution of the Animal Body Plan
2. Evolution of symmetry• Radiata/Bilateria split.• Sponges lack any definite symmetry• Eumetazoa have a symmetry defined along
an imaginary axis drawn through the metazoan’s body
• There are two main types of symmetry
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Evolution of the Animal Body Plan
-Radial symmetry (The Radiata)
-Body parts arranged around central axis
-Can be bisected into two equal halves in any 2-D plane-Bilateral symmetry (The Bilateria)
-Body has right and left halves that are mirror images-Only the sagittal plane bisects the
animal into two equal halves
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Evolution of the Animal Body Plan
Bilaterally symmetrical metazoans have two main advantages over radially symmetrical ones
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 two or three germ layers• Body cavity = Space surrounded by
mesoderm tissue that is formed during development
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Evolution of the Animal Body Plan
3. Evolution of a body cavityThree basic kinds of body plans
• Acoelomates = No body cavity• Pseudocoelomates = Body cavity between
mesoderm and endoderm– Called the pseudocoel
• Coelomates = Body cavity entirely within the mesoderm– Called the coelom
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Diploblastic vs. Triploblastic – Cell Layers
Diploblastic – two cell layers– Ectoderm – outer layer– Endoderm – inner layer– The Radiata
Triploblastic – three cell layers– Ectoderm, endoderm– Mesoderm – layer between ectoderm and endoderm– The Bilateria
Ectoderm – outer covering of the body; nervous system
Endoderm – digestive organs and intestinesMesoderm – skeleton and muscles
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Evolution of the Animal Body Plan
The body cavity made possible the development of advanced organs systems
• Pseudocoelomates use pseudocoel for circ.• 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
• Why do you think closed is more advanced?
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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 two-layer-thick ball
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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Embryonic development in protostomes and deuterostomes
Mouth formsfrom blastopore
Mouth
CoelomAnusMesoderm
BlastoporeBlastula
CoelomArchenteron
Mesoderm splits
BlastoporeBlastula
Mouth Coelom
AnusAnus formsfrom blastopore
CoelomArchenteron
Archenteron outpocketsto form coelom
Protostomes
Deuterostomes
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Evolution of the Animal Body Plan
Deuterostomes differ from protostomes in three other fundamental embryological features: -1. Cleaveage pattern of embryonic cells
-Protostomes = Spiral cleavage-Deuterostomes = Radial cleavage
-2. Developmental fate of cells -Protostomes = Determinate
development
-Deuterostomes = Indeterminate development
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Evolution of the Animal Body Plan
-3. Origination of coelom • Protostomes = Forms simply and
directly from the mesoderm• Deuterostomes = Forms indirectly
from the archenteron
Deuterostomes evolved from protostomes more than 500 MYA
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(5) - Segmentation
Segmentation-Body is assembled from
succession of similar segments
• Each segment may develop into complete set of adult organs
• Damage to one segment is less fatal
• Locomotion is easier when segments can move independently
• Earthworms, Arthropods, and Chordates
• Originated multiple times in metazoans.
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A New Look At Metazoans
The traditional metazoan phylogeny is being reevaluated using molecular data. (Remember the homology/analogy problem.)
Therefore, key morphological characters used in traditional classification are not necessarily conservative
Molecular systematics uses unique sequences within certain genes to identify clusters of related groups
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A New Look At Metazoans
Molecular data has helped to clarify the relationship of different groups with the animals (metazoans) for example annelids and arthropods
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Evolutionary Developmental Biology
Most taxonomists agree that the metazoan kingdom is monophyletic
Three prominent hypotheses have been proposed for the origin of metazoans from single-celled protists