1 Chap 32: The Diversity of Animal Forms. 2 What is an animal: the basic characteristics 1.Animals are: Heterotrophic (as opposed to plants that are autotrophic)

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Chap 32: The Diversity of Animal Forms

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What is an animal: the basic characteristics

1. Animals are:

• Heterotrophic (as opposed to plants that are autotrophic)

• So they ingest their food

• Multicellular

• Protists such as amoeba and paramecium are unicellular

• Plants can be unicellular or multicellular

•Eukaryotes

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2. Animals lack cell walls

• So how do they support themselves: structural proteins such as collagen

3. Animals possess nervous and muscle tissue

4. Most animals reproduce sexually with a dominate diploid stage.

5. Early embryonic development (next slide)

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Figure 32.1 Early embryonic development (Layer 1)

Cleavage: a series of rapid cell divisions and in most cases leads to the formation of a blastula or hollow ball of cells.

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Figure 32.1 Early embryonic development (Layer 2)

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Figure 32.1 Early embryonic development (Layer 3)

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6. Hox genes: these are genes that control development. There is

diversity when these genes are expressed in a developing embryo

and this produces the variety of animal forms.

7. Most biologist (systematists) are able to trace animal origins to a

single common ancestor (monophyletic)

• A colony-based flagellated protist from Precambrian era

• Probably related to the choanoflagellates

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Figure 32.2 A modern choanoflagellate colony

Choanoflagellate colony inhabits ponds, lakes and marine environments.

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Figure 32.3 One hypothesis for the origin of animals from a flagellated protist

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Figure 32.4 A traditional view of animal diversity based on body-plan grades

The circled numbers represent branch points for each “major branch” or grade.

So for example, the presence of tissues is a point of distinction between groups of animals and is therefore a “grade.”

So what is really fun here is this is the “traditional” view of animal phylogeny or organization. There is a second type of phylogeny based on rRNA.

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Branch Point #1: Tissues or No Tissues

1. Tissue: a group of cells performing the same function.

• Parazoans (“beside the animals”): lack true tissues

• Example: sponges

• Eumetazoans: possess true tissues of some sort.

•Examples: nervous tissue, muscle tissue, digestive

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Branch Point #2: Type of Symmetry

So the eumetazoans, possessing tissues, can have their body plan arranged one of two ways:

1. Radial Symmetry: hydras, jellies, sea anemones (Cnidarians)

• Called radiata

• Possesses a top (dorsal) and bottom (ventral) but no head or rear end or left or right sides

• Picture trying to slice a jelly fish into right and left sides

• Diploblastic: possess only two germ layers (ectoderm and endoderm)

• Ectoderm: outer covering and CNS, hair, nails, sense organs

• Endoderm: lines digestive tube, liver, lungs, pancreas,

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Figure 32.5 Body symmetry

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Branch Point #2: Symmetry

2. Bilateral Symmetry: humans, crayfish, earthworms

• Called bilateria

• Possesses dorsal and ventral sides, head end (anterior) as well as a rear end (posterior), and right and left side

• Cephalization: concentration of sensory tissue near the anterior end.

• Triploblastic: possesses a third germ layer, the mesoderm that gives rise to muscles and bones, circulatory system, kidneys

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Branch Point #3: Body Cavity or No Body Cavity

The presence / absence of a body cavity is a division of the bilaterally symmetrical animals.

1. Some of these animals do not have a cavity between the digestive tract and the outer body wall

• Acoelomates: solid body

• Examples: flatworms such as planaria (Platyhelminthes)

2. Most of these triploblastic animals possess a body cavity

• Coelomates: possess a fluid-filled space between digestive tract and outer body wall.

• The body wall is lined with mesoderm

• Mesenteries suspend internal organs (like we have)

• Earthworms (annelida), Chordates (humans)

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Branch Point #3: Triploblastic organisms can be divided into coelomates, acoelomates and psuedocoelomates

2. (cont’d)

• Pseudocoelomates: since the body cavity develops differently, it is not completely lined by tissue from the mesoderm.

• Daphnia (rotifers) and roundworms

3. The value of a coelom

i. Internal fluid cushions

ii. Internal fluid of earthworms is under pressure yet is incompressible and serves as an internal skeleton

iii. Allows for independent movement of internal organs

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Figure 32.6 Body plans of the bilateria

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Branch Point #4: Protostome vs. Deuterostome

So those organisms with body cavities (coelomates) can have differences. It is in their development where the differences lie. There are 3 main differences:

1. Cleavage

• Protostomes

• Determinate cleavage: cell’s fate is determined early (4 cell stage)

• Spiral cleavage: new cells lie in the grooves of the older, lower cells.

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Branch Pt #4: Protosome vs. Deuterostome

1. (cont’d)

• Deuterostomes

• Indeterminate cleavage: cells retain the capacity to develop into a complete individual longer. This is how humans can have identical twins and why embryonic stem cells are so important.

• Radial cleavage: cells align directly on top of each other.

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Branch Pt #4: Proto- vs. Deuterostomes

2. Coelom Formation: begins in the gastrula stage

• Protostomes

• These animals form splits in the mesoderm to form the coelom

• Deuterostomes

• These animals form the coelom from folds in the archenteron

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Figure 32.10 Ecdysis

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Branch Pt #4: Protostome vs Deuterostome

3. Fate of the Blastopore

• The archenteron forms as the invagination continues during gastrulation

• The archenteron will extend through the entire blastula and make another opening becoming the second opening of the digestive tract.

• Protostomes: blastopore forms the mouth, second opening, the anus

• Deuterostomes: mouth is the second opening, anus is the blastopore.

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Figure 32.7 A comparison of early development in protostomes and deuterostomes

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Constantly Changing Animal Phylogeny

The small subunit ribosomal RNA or SSU-rRNA has been used to show similarities and differences between views of animal phylogeny

Where are there similarities between animal phylogeny based on the traditional body plan arrangement and the more modern molecular biology-based SSU-rRNA?

1. Branch #1: Presence (Eumetazoa) / Absence of Tissues (Parazoa)

2. Branch #2: Radial vs. Bilateral Symmetry

3. Within the coelomates the deuterostomes are made of the echinoderms (sea stars) and chordates (vertebrates like us)

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What are the differences between between the traditional “body plan” arrangement of animal classification and the SSU-rRNA method?

The differences lie in the protostomes (animals without body cavities). With the SSU-rRNA method:

1. The protostomes are divided into two groups or clades:

a. Lophotrochozoa: annelids and mollusks

Possess a structure called a lophophore, ciliated tentacles used for feeding (next slide)

b. Ecdysozoa: arthropods (secrete external skeleton)

From “ecdysis”: shedding of old skeleton

Basic Concept: The annelids and mollusks go through a similar larval stage called the trochophore larva as well as having similar SSU-rRNA.

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Figure 32.11 A lophophorate

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Figure 32.9 A trochophore larva

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Figure 32.8 Animal phylogeny based on sequencing of SSU-rRNA

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2. Flatworms, which were classified as acoelomates and having no body cavity, have a molecular-based similarity to protostomes, which possess a body cavity.

a. It is thought that maybe they once had a coelom and then lost it.

b. The pseudocoelomates, rotifers and nematodes, also are placed within the protostomes

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3. A New Phyla: Lophophorate

a. Examples: flatworms, roundworms, mollusks, segmented worms.

b. Share characteristics with protostomes and deuterostomes

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Figure 32.8 Animal phylogeny based on sequencing of SSU-rRNA

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Figure 32.12 Comparing the molecular based and grade-based trees of animal phylogeny- EGADS!!!!!

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Origins of Animal Diversity

Occurred over a brief period of time

1. Lasted about 40 million years (565 to 525 million years ago)

2. During late Precambrian and early Cambrian periods

a. Fossils as old as 565 to 543 million yrs old have been found in the Ediacara Hills of Australia (field trip?)

b. Fossilized animal embryos in China (570 million yrs ago)

c. Other fossils and molecular data even suggest 1 billion years ago.

d. Types of animals fossilized: cnidarians (hydra-like) and soft-bodied mollusks

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3. Cambrian Explosion

a. 543 – 525 million years ago

b. First hard bodied animals

Burgess Shale in British Columbia

Greenland

China

4. Because there were so many animals it is hard to sort out their sequence of branching.

5. But why such an explosion of animal types, forms, etc.?

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Figure 32.13x Burgess Shale fossils

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Cambrian Diversification because of:

1. Change in biological communities: this lead to diverse adaptations such as protective shells, new means of locomotion all as an “attempt” to deal with a changing environment.

2. Oxygen levels may have increased to such a level that new and more animals could survive and under aerobic metabolism. This meant more active / aerobic means of locomotion so new adaptations could be “tried.”

3. Evolution of Hox genes: these regulatory genes can lead to diverse forms of body plans based on when they are active during embryonic development of an animal form.