Lecture III.4. Animals – I.

Molecular-Based Phylogeny of Animalia. Note poly-tomies (unresolved branching sequences) leading to sponges and Coelenterates (= Cnidaria+Ctenophera).

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Overview.

Sister group to choanoflagellates.

Characteristics include 1. Multicellularity. 2. Differentiated cells, tis-

sues, organs. 3. Motility at some stage. 4. Heterotrophy. 5. Muscular and neuro-

sensory systems. 6. Complex development including blastula formation

(all) and gastrulation (most).

A simplified animal phyloge-ny.

Simplified animal phylogeny.

Sponges are basically choanoflagellates embedded in a gelati-nous mass strengthened by calcareous / siliceous spicules and / or a protein called spongin.

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Some Terminology.

Metazoa – all animals.

Eumetazoa – all animals excluding sponges and a few others.

1. Two or three cell layers. 2. Radial or bilateral symmetry.

Bilateria – all animals excluding sponges, cnidaria and a few others.

1. Three cell layers.

2. Bilateral symmetry.1

1 Includes groups, e.g., echinoderms, in which bilateral symmetry lost sec-ondarily lost.

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Evolutionary Trends.

Increasing size and morphological complexity.

Exploitation of new (benthic / terrestrial) environments.

Homeostasis – Increasing ability to

1. Perceive, respond to and control conditions of life.

2. Maintain constant internal conditions in the face of ex-ternal environmental fluctuations.

Nervous System – Increas-ing

1. Nervous system complex-

ity. Nerve nets replaced by CNS

2. Cephalization: concentra-tion of sense organs at the front of the animal.

3. Intelligence: Behavior; social interaction; cognitive abil-ity; self-awareness.

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Principle Groups.

Porifera (sponges) 1. Radial symmetry or none. 2. No cell layers as such.

Cnidaria (hydra, jellyfish, corals). 1. Radial symmetry. 2. Two cell layers.

Bilateria 1. Bilateral symmetry. 2. Three cell layers.

Major Bilaterian Groups. 1. Lophotrochozoans. Include

a. Mollusks – snails, clams, cephalopods. b. Annelids – segmented worms.

2. Ecdysozoans

a. “Molting animals.” b. Arthropods and their allies.

3. Deuterostomes

a. Echinoderms – starfish, etc. b. Chordates – “fish”, tetrapods.

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Phyla Distinguished by Body Plans.

Symmetry: 1. Radial. 2. Bilateral. 3. Other.

Number of embryonic cell (tissue) layers: 1. Two – diploblastic. 2. Three – triploblastic.

Embryonic cell layers:

1. Endoderm ⇒ lining of the gut.

2. Mesoderm ⇒ muscles, connective tissue.

3. Ectoderm ⇒ skin, nerv-ous system.

Type of body cavity: 1. None. 2. Pseudocoelom. 3. True coelom.

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Pseudocoelomate Locomotion (Round worm).

Coelomate Body Plan (Earthworm).

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Phyla Distinguished by Body Plans (continued).

Embryonic development.

1. Protostomes. a. Blastopore becomes the mouth.2 b. Coelom (body cavity) forms within mesoderm. c. Cleavage spiral in some. d. Embryonic cell fate determined early.

2. Deuterostomes. a. Blastopore becomes the anus. b. Coelom forms from an out-pocketing of the gut. c. Cleavage radial. d. Embryonic cell fate determined late.

2 In some protostomes both mouth and anus form from the blastopore.

Germ Layers and Tissues Ectoderm Epidermis; nervous system

Mesoderm Muscle, bone, blood and other connective tissues

Endoderm Mucous membranes lining diges-tive tract and respiratory system

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Development in Protostomes and Deuterostomes.

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Origin of Embryonic Tissue Types.

Traditional view: Germ layers and gastrulation evolved af-ter sponges (cladogram on page 1).

More recent suggestion: 1. Beginnings of germ layer differentiation seen in some

sponges.

2. Based on observation in sponges of a transcription factor important in regulating endodermal develop-ment in Eumetazoa.

Alternative opinions regarding the origins of germ layers and gas-trulation. From Nakanishi et al. (2014); See also Adamska (2016).

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Embryonic Tissues and Autoimmune Disease.

Antigenic properties of adult tissue determined by embryonic tissue of origin.

Example: Often fatal Stevens Johnson Syndrome (SJS). 1. Auto-immune disorder trig-

gered by drugs and infection. 2. Antibodies attack epithelial

tissue (skin, cornea, etc.), i.e., tissue that derives from ectoderm.

3. Skin dies, sloughs off

4. Dermis largely unaffected –

antigenically distinct; derives from mesoderm.

5. Sequelae include permanent damage to eyes.

6. But – if patient survives, essentially normal skin often

replaces tissue that was lost.

SJS survivor after 7 years.

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Animal Feeding Strategies.

How 1. Filter feeders.

a. Strain out food particles. b. Clams; krill (crustaceans); baleen whales.

2. Deposit feeders

a. Eat their way through the environment. b. Earthworms.

3. Fluid feeders.

a. Suckers and drinkers. b. Hummingbirds, adult mosquitoes, vampire bats.

4. Mass feeders – bite / tear off chunks - us.

What 1. Predators: eat other animals.

2. Herbivores: plants.

3. Detritivores/Saprovores: dead/decomposing matter.

4. Omnivores: anything.

Parasites: live in (endo-) or on (ecto-) living hosts.

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Self-Quiz. Deposit and Filter Feeders. Enter the taxonomic group (phylogeny on page 1) to which each of the following examples belongs.

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Self-Quiz (continued). Fluid and Mass Feeders. Enter the taxonomic group to which each of the following examples belongs.

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Locomotion.

Whole body undulates.

1. Nematode – muscles + pseudocoelom. 2. Fish, snakes – muscles + skeleton.

Limbs for walking, paddling, flying.

1. Sac-like – Onycophorans. 2. Tube feet – echinoderms. 3. Jointed (external skeleton) – arthropods 4. Not jointed (internal skeleton) – vertebrates.

Are animal limbs homologous?

1. No. 2. Often limb development controlled by same genes.

a. But the limbs themselves arose independently. b. In such cases, genes are homologous, the struc-

tures are not.

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Possible phylogeny of genes that control limb development and other functions in Bilateria (Panganiban and Rubenstein, 2002). Note gene duplications on the branch leading to vertebrates. Urochordate ampul-lae are blood filled sacs that mediate an immune response to para-sites and when colonial tunicates come into contact (Franchi and Bal-larin. 2017. Immunity in Protochordates: The tunicate perspective. Front. Immunol. 8: (674): 1-16).

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Figure Next Page. Top. Experimental set-up and results implicating the importance of Dll (distal-less*) homologs in animal limb development. Dll itself is a homeobox gene**, the expression of which is necessary for normal limb de-velopment in Drosophila. Dll homologs, called Dlx genes, are expressed in developing limbs in at least six different phyla. The figure is from your text (4th edition), with the conclusion suitably modified to reflect the fact that the ex-periment shows that the genes, but not the structures for which they presently code, are homologous. Not noted in the write-up is the fact that that, while Polychaete worms have leg-like parapodia, other segmented worms do not, i.e., not all segmented worms have appendages. Bottom. The lancelet, Amphioxus, is close to the ancestry of verte-brates. While various vertebrate characters (gill slits, noto-chord, dorsal nerve cord, segmented trunk musculature) are present, there is no suggestion of appendages. This argues against the claim that all animal appendages (as opposed to the genes that control their development) are homologous; likewise, the fact that Cambrian chordates (next lecture) also lacked appendages. ________ * In Drosophila, limb development is aborted in Dll mutants. ** Homeobox genes contain so-called homeobox sequences (coding for transcription factors that regulate morphogenesis in animals and fungi.

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Limbs.

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Life Cycles.

Diplontic (diploid except for gametes) with a few excep-tions.

Multiple stages facilitate

1. Dispersal; 2. Exploitation of different

environments, resources.

Stages can be represented by different individuals or the same individual.

Reproduction. 1. Sexual or asexual. 2. Fertilization internal or

external.

3. Birth – live or from eggs deposited into the envi-ronment.

Animal reproduction. Top. Damsel flies mating. Bottom. Giant clam releasing sperm.

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Complete (left) and incomplete metamorphosis (right) in in-sects. In these cases, the same individual passes through the different life cycle stages.

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The broad fish tapeworm, Diphyllobothrium latum, has two in-termediate hosts through which it must pass before infecting a fish-eating mammal, the primary host in which it reproduces. The cycle is completed when the primary host defecates in or near fresh water. In this case, life cycle stages are represented by different individuals.

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Simple Animals.

Sponges – not a monophy-letic group – see p. 1. 1. Radial symmetry or

asymmetrical. 2. Sessile. Support provid-

ed by a “skeleton” con-sisting of spines called spicules and/or an elas-tic network of fibers

3. Organized about a series

of water canals (pores).

4. Specialized collar cells (choanocytes) a. Suck water in. b. Extract food parti-

cles. c. Excrete waste products.

5. "Used" water exits through one or more oscula.

Water is sucked into the sponge through pores and exits through the osculum. Currents are generated by the beating of collar cell fla-gella.

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Cnidaria.

1. Hydras, jellyfish, corals. a. Two cell layers; b. Radial symmetry. c. "Blind" guts. d. Almost all marine.

2. Carnivorous. Use nema-

tocysts to paralyze prey.

3. Basic structure a two-layered, tentacle bear-ing cup that is either a polyp (sessile) or a me-dusa (floating).

4. Life cycle involves both

a. Polyps reproduce by budding.

b. Medusas reproduce sexually.

c. Fertilized eggs de-

velop into ciliated, free-swimming plan-ula larva.

Generalized cnidarian life cy-cle showing polyp & medusa.

Nematocyst (here called “cnid-oblast”) structure.

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5. Three principal groups.

a. Hydrozoans - hydras (solitary and colonial)

b. Scyphozoans – jelly-

fish

c. Anthozoans – anem-ones (solitary); corals (colonial)

6. Portuguese man-of-war,

Physalia physalis, is a floating hydrozoan col-ony.

a. Consists of differenti-

ated polyps special-ized for different func-tions: flotation (“sail”), stinging, feeding and reproduction.

b. Tentacles pack a wal-

lop.

Top. Portuguese man-of-war is a floating hydrozoan colo-ny. Bottom. Victim.

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Planula larvae of a jellyfish (Scyphozoa) are basi-cally ciliated gastrulas.

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Corals (Anthozoans).

1. Famous for reef formation.

2. Many contain symbiotic pho-tosynthetic dinoflagellates.

3. Accounts for coral’s a. Ability to live in nutrient

poor waters.

b. Restriction to surface wa-ters.

4. But –

a. Coral reefs often found far out to sea where they form ring-like atolls around shal-low lagoons.

b. Adjacent waters very deep: How do atolls form in the first place?

3. Darwin’s conjectured solu-

tion: balance between sea floor subsidence and reef growth.

Three stages in coral reef formation. From top to bot-tom: fringing reef; barrier reef; atoll.

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Ctenophora (comb jellies).

1. Name derives from locomotor organ consisting of eight comb-like rows of fused plates of cilia called ctenes.

2. Previously grouped with Cnidaria in Coelenterata. 3. Complete gut. 4. Two cell layers separated by a gelatinous mesoglea.

5. Tentacles, but no nematocysts – use sticky filaments.

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Protostomes vs. Deuterostomes.

Fundamental split among Bilateria.

Recall developmental differences – see pp. 9-10.

Both

1. Triploblastic (three cell layers)

2. Have free-floating, ciliated larvae (dispersal stage) that differ with regard to number and anatomy of cilia.

Left. Protostome larva (annelid) larva. Right. Deuterostome (echi-noderm) larva.

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Two Great Groups of Protosomes.

Lophotrochozoa. 1. Include:

a. Flatworms;

b. Rotifers;

c. Lophophorates (Bry-

ozoans, brachiopods, phoronid worms);

d. Spiralians (Nemertean worms, annelids, mollusks).

2. Name derives from lophophore (a feeding organ) + trochophore larva (previous page). a. The word lophophore from the Greek - lophos =

'crest of a helmet' + phoros = 'bearing.' Hence, 'crest-bearing.'

b. U-shaped ridges around the mouth bear one or two rows of hollow, ciliated tentacles – video.

c. Occur in Lophotrochozoan groups that collectively

form the Lophophorates.

Lophotrochozoan phylogeny. Only Lophophorates have a lophophore.

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3. When there is a skeleton (shell), growth by accretion.

Lophotrochozoans. Left. Shell of the pearly nautilus grows by accretion, Right. A phoronid worm and its lophophore. Below. A bryozoan extends its lophophore.

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Ecdysozoa ("Worms”, Lobopods and arthropods).

1. Grow by molting a stiff external skeleton

2. Locomotion often by jointed appendages.

Ecdysozoan phylogeny based principally on molecular data. Four of the five worm-like phyla – Kinorhyncha, Priapulida, Nematomorpha and Nematoda – have pseudocoeloms and were formerly grouped to-gether; the fifth, Chaetognatha (arrow worms), was traditionally grouped with deuterostomes on the basis of embryology. Of the re-maining Ecdysozoan phyla, those with jointed appendages are placed in the phylum Arthropoda, the subphylum Uniramia being a fusion of insects and the myriapodous arthropods – millipedes and centipedes. Tardigrades and onycophorans, sometimes called Lobopods, are al-lied to the arthropods with which they share the derived traits of hav-

ing appendages for walking and chitinous exoskeletons.

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3. Ecdysozoa shed their "skins" in order to grow. a. In Lophotrochozoans, the shell is open at one end

of both. Shell grows by accretion.

b. Presence of an exoskeleton makes this impossi-ble. Instead, the animal molts.

4. Arthropods have the most advanced ecdysozoan skele-

tons – essentially suits of armor.

a. Consist of plates joined by articular membranes; secreted by underlying hypodermis.

b. At molting, the hypodermis secretes an enzyme that

erodes the base of the overlying cuticle; new cuticle forms beneath it.

c. The old cuticle splits, and the animal crawls out. d. New cuticle soft, pliable – allows animal to grow. e. Growth stage between molts called instars. Meta-

bolic reserves accumulate between molts. 5. Ecdysozoan monophyly based on molecular evidence

(small ribosomal subunit and other genes).

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Next page. Arthropod exoskeleton (A) consists of overlapping cuticular plates joined by thin, flexible articular membranes (B) often folded beneath the plates. Arthropods are primitively seg-mental, and the cuticle of each segment consists of four primary plates: a dorsal tergum, two lateral pleura and a ventral ster-num. The skeleton of the appendages (C) (primitively one pair per segment) is similar, with the membranes in some cases sup-plemented by vertebrate-like condyles. Muscles (inside the skeleton) attach to sub-cuticular “bumps” called apodemes that are analogous to vertebrate tendons. The cuticle is secreted (D) by a single layer of cells called the hy-podermis that consists of two layers, a thin outer epicuticle and the much thicker procuticle beneath it. The procuticle contains chitin, which in some cases is strengthened by impregnation with calcium salts. Like vertebrate epithelial tissue, the cuticle derives from embryonic ectoderm that forms other structures in addition to the outer covering. In particular, arthropod fore- and hind-guts are lined with cuticle, as are the respiratory organs and parts of the reproductive system.

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Uniramians.

Include insects (three body re-gions, three pairs of legs at-tached to the thorax).

The most successful of all Meta-zoa. Many have wings.

“Advanced” traits such as the ability to maintain constant body temperatures – some moths and bees.

“An inordinate fondness for beetles.”

Competing hypotheses for the evolutionary origin of wings. Di-agrammatic cross sections show (A) the ancestor of winged in-sects according to the wings-from-legs theory; (B) an extant winged insect; (C) the ancestor of winged insects according to the wings from tergal extensions theory. From Jockusch and Ober (2004).

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Some social hymenoptera (ants, wasps, bees), have complex societies with 1. Individuals specialized behaviorally and morphologi-

cally for different tasks.

2. Sterile workers.

Explanations for the evolution of worker sterility include 1. Haplodiploid sex determination (males haploid; females

diploid);

2. Kin selection

3. See HW questions 11-12.

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Questions. 1. Referring to the cladogram on page 1 and with regard to

mono- / polyphyly, vertebrates are ______ group; inver-tebrates are _______ .

2. Referring to the cladogram on page 1, which of the fol-

lowing traits arose just once? a. Coelom b. Limbs c. Pseudocoelom d. Radial symmetry e. Segmentation

3. Referring to the cladogram on page 1, the acoelomate

condition of flatworms is a(n) _____ trait. 4. The mass of cells that results from division of the original

fertilized egg is called a blastula. A ______ results when the blastula invaginates.

5. If all animal limbs really are homologous (as opposed to

the genes that control their development), cephalochor-date ancestors must ______ .

6. SJS patients often require intubation. Why? (You may

have to look up “intubation.”)

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7. A general evolutionary trend among animals has been increased size and complexity. What would Lamarck have had to say about this? What would Darwin have said? (Not covered in lecture)

8. Baleen whales are huge; their prey, tiny. Most predators that eat small prey are themselves small. Why?

9. Strategies that have been employed in the fight against

malaria include the following: a. insecticides; b. draining ponds and marshes; c. anti-malarial drugs; d. window screens; e. bed nets often impregnated with insecticide. Which of these target(s) the larval stage?

10. The classifications / phylogenies given in this lecture are

quite different from those that your instructor learned way back when. What principal development(s) might have caused the changes?

11. In many social hymenoptera (ants, bees), workers (all diploid females) are generally sterile. In these species, males are haploid. From an evolutionary point of view, how might these observations be related? (Requires outside reading.)

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12. Regarding your answer to #11, how would your answer be affected by the observation that queens often mate with multiple males? Likewise, how does sex determina-tion in termites affect your argument? (Requires outside reading.)