Overview of metazoan Diversity

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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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Top

Bottom

Back

Front

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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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A New Look At Metazoans

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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

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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