Workshop: The Evolution of Animalia body symmetry embryonic germ layers ontogenetic origins I. What is an Animal?

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1 Workshop: The Evolution of Animalia by Dana Krempels Perhaps even more than the other Eukarya, Animalia is characterized by a distinct progression of complexity in form and function as one moves from the more primitive to the more derived taxa. Early in animal evolution, major changes in body symmetry, embryonic germ layers, and ontogenetic origins of major anatomical structures diverge in the nascent monophyletic groups. Over the course of this workshop, you will review the major changes that occurred during the evolution of Kingdom Animalia. By the end of the workshop, you should be able to 1. List the synapomorphies that distinguish animals from other eukaryotes 2. Understand the meanings of asymmetry, radial symmetry and bilateral symmetry 3. Be able to recognize the major animal phyla on the basis of a. body symmetry b. embryonic germ layers c. presence or absence of an internal body cavity d. ontogeny and morphology of the internal body cavity e. ontogenetic differences between protostomes and deuterostomes. 4. Be able to recognize acoelomate, pseudocoelomate and coelomate body plans 5. Distinguish between a. spiral and radial cleavage b. determinate and indeterminate cleavage c. schizocoely and enterocoely I. What is an Animal? Animals are eukaryotes, and hence may share a distant common ancestor with some or all other eukaryotes. However, there are several characteristics that set animals apart from all other types of organisms. List those characteristics below: eukaryotic, multicellular ingestive heterotrophs no cell walls surrounding plasma membrane tissues, organs and organ systems nervous system and muscular system primitively sexually reproductive (some species secondarily parthenogenic) characteristic embryonic development (blastula, gastrulation & subsequent morphogenesis)

2 II. Germ Layers Comparisons of ontogeny are sometimes used to devise phylogenies. 1. At what stage in an embryo's development are germ layers first present? gastrulation 2. Which germ layer(s) form first, and where are they located? ectoderm (lining the outside of the gastrula) and (lining the archenteron, or primitive gut) 3. Complete the following table PHYLUM Germ Laye Name of middle layer Derivation of middle layer (ectoderm, or neither) Porifera none mesohyl (nonal mesenchyme Cnidaria ectoderm, Platyhelminthes ectoderm, Nematoda Annelida Echinodermata ectoderm, ectoderm, ectoderm, mesogloea (acellular proteinaceous matrix) mesenchyme not a true tissue layer; not derived from or ectoderm; some migratory cells not a true tissue layer; not derived from or ectoderm; no cellular component cellular components are derived from ectoderm ectoderm \ a. What other Phylum has germ layer development characteristics similar to Cnidaria's? Ctenophora b. What other Phylum/a show(s) germ layer development similar to Nematoda's? Rotifera, Gastrotricha, Nematomorpha, Priapulida & other pseudocoelomates. c. What other Phylum/a show(s) germ layer development similar to Annelida s? Mollusca, Arthropoda, Tardigrada & other coelomates. d. What other Phylum/a show(s) germ layer development similar to Echinodermata s? Hemichordata, Chordata Do you think that these ontogenetic similarities indicate monophyly in each of these cases? If not, why not? Discuss. Not necessarily. Being a persistent blastopore, the pseudocoelom might have evolved several times independently, once evolved from. The coelom might have arisen independently in the two coelomate lineages (Ecdysozoa and Lophotrochozoa), but it is also possible that one mechanism of coelom formation is derived from the other. Not enough data are available yet for a clear picture.

3 III. Body Symmetry: Define the following. 1. asymmetry lacking a true plane of symmetry (usually as defined by true tissues) 2. radial symmetry - plane of body symmetry central to a "pie shaped" symmetry, in which the organism can be divided into identical wedges. May be biradial, quadriradial, pentaradial, etc.) 3. bilateral symmetry - body halves form mirror images of each other 4. oral surface in radially symmetrical animals ONLY, the surface of the animal containing the mouth. 5. aboral surface - in radially symmetrical animals ONLY, the surface of the animal NOT containing the mouth. 6. dorsal In bilaterally symmetrical animals, the back surface 7. ventral - In bilaterally symmetrical animals, the belly surface 8. lateral - In bilaterally symmetrical animals, the side surfaces 9. medial - In bilaterally symmetrical animals, located at the central body axis 10. cephalic/anterior the head end (cephalon is Greek for head ) 11. caudal/posterior - the tail end, away from the head (caudum is Greek for "tail") 12. sagittal section a longitudinal cut or view through the organism 13. mid-sagittal section cut/view through the center of the longitudinal plane 14. transverse section cross (horizontal) cut/view through organism (perpendicular to the longitudinal plane) 15. cephalization concentration of sensory organs in a distinct head, the first part of the animal to meet the environment in bilaterally symmetrical, motile species. Draw and Discuss 1. Asymmetry a. In the space below, sketch an example of an animal with no plane of body symmetry. b. Which animals are characterized by the lack of true body symmetry? Are the monophyletic? Is the absence of a character useful for developing phylogenies? Discuss. Poriferans are most likely polyphyletic. Though they likely share a choanoflagellate-like ancestor, their lineages have likely been separate for a long time. Lack of a morphological character (in this case, symmetry) is a poor basis for establishing monophyly.

4 2. Radial Symmetry a. In the space below, sketch examples of animals radial symmetry, including biradial, quadriradial, pentaradial and hexaradial symmetry. b. Which animal phyla are characterized by radial symmetry? Cnidaria, Ctenophora c. Do you think radial symmetry in these phyla is homologous? Discuss. Since symmetry can be convergent, and both of these taxa are very ancient, it would be wise to reserve judgment about the homology of their radial symmetry without supporting data (e.g., molecular) that also indicates monophyly of this clade. 3. Bilateral Symmetry a. In the space below, sketch an example of an animal exhibiting bilateral symmetry. b. Which animal phyla are characterized by bilateral symmetry? All triploblastic animal phyla are bilaterally symmetrical, from Platyhelminthes through the pseudocoelomate phyla (Nematoda, Rotifera, etc.), through the protostomes (Annelida, Mollusca, Arthropoda, etc.) and deuterostomes (Echinodermata, Hemichordata, Chordata, etc.) c. What is the functional significance of cephalization? Why do you think the majority of animals are bilaterally symmetrical? Discuss. Cephalization allows greater motility and the ability to exploit a greater variety of environments in a more active than passive way. The vast diversity of the bilateria suggests that bilateral symmetry with its cephalization may have conferred a selective advantage over radially symmetrical, sessile (or passively planktonic) forms in terms of colonizing new environments. Adaptive radiation due to genetic drift and natural selection are more likely to occur when organisms are spread widely into different types of habitats where they are more likely to undergo physical reproductive isolation leading to speciation.

5 IV. Internal Body Cavity Your textbook, course notes and other resources often provide you with a crosssectional view of the three animal body plans. To demonstrate your understanding of the anatomy of acoelomate, pseudocoelomate and coelomate animals, sketch each of the three plans in LONGITUDINAL section. Label ectoderm,, and /mesenchyme, intestinal lumen, parietal and visceral surfaces. (In the Workshop Leader version germ layers are labeled as follows in color coding: ectoderm,,, mesenchyme. Students should also be able to recognize cross sections of each body plan.) ACOELOMATE: PSEUDOCOELOMATE: 1. What is the function of the pseudocoelom? hydrostatic skeleton; housing and cushioning of internal organs/organ systems 2. Why is the pseudocoelom considered a persistent blastocoel? Mesoderm in the pseudocoelomates buds off from the ectoderm, proliferates and grows inward to line only the parietal side of the gastrula's blastocoel. The pseudocoelom is not a secondary cavity, but merely the remnants of the embryonic blastocoel, which never became filled with. (This is best shown with a diagram)

6 COELOMATE: 3. What is the function of the coelom in the following coelomate phyla? a. Annelida - hydrostatic skeleton; housing and cushioning of internal organs/organ systems. The circulatory system is closed, and is contained within the coelom. b. Mollusca - reduced to a vestigial space around the heart, gonads, part of the intestine and reproductive organs (gonocoel). A secondarily developed body space, the hemocoel, is the main body cavity. It is part of the open circulatory system c. Arthropoda - reduced to a vestigial space around the heart, gonads, part of the intestine and reproductive organs (gonocoel). A secondarily developed body space, the hemocoel, is the main body cavity. It is part of the open circulatory system d. Echinodermata - coelom gives rise to the water-vascular system, used for locomotion and in some species, prey capture e. Chordata; Vertebrata - houses the internal organs, and provides fluid cushioning for organs and organ systems. (Your own coelom is the space lined by your peritoneum (the al tissues anchoring your internal organs in their proper place in the abdomen) as well as the ally lined space in your thoracic cavity.) 4. Do your answers to #2 and #3 tell you anything about possible evolutionary relationships and monophyly of the taxa involved? If you re not sure, then give your reasons. Discuss well! Not long ago, Molluscs, Annelids and Arthropods were considered to be closely related sister taxa, partly because of morphological similarities in the coelom and ontogenetic similarities in coelom development. More recently, molecular data support a clade consisting of Nematoda and Arthropoda (Ecdysozoa) and another consisting of Annelida, Mollusca and several other coelomate protostomes (Lophotrochozoa). Is the superficial similarity of the mollusc and arthropod gonocoel and haemocoel (and open circulatory system) convergent, due to common ancestry, or due simply to similar action of homeotic genes during development? The answer is not yet known. When morphological/ontogenetic data and molecular data suggest conflicting hypotheses of evolutionary relationships, more data are needed to clarify the picture.

7 V. Protostomes and Deuterostomes The most derived lineages of eumetazoans have an internal body cavity (coelom; pronounced see-lome ) lined on both the parietal and visceral surfaces with. However, the two major (putatively) monophyletic groups of coelomates achieve this adult anatomy in different ways. Other ontogenetic features also suggest that although the protostomes and deuterostomes share a common ancestor, the taxa within each lineage are distinct unto themselves. Consider the following and discuss. 1. What phylum might be an appropriate outgroup you could use to determine which protostome and deuterostome character states are primitive? The only phyla within Animalia clearly rooted outside the Bilaterian clade are the Radiata. These are not particularly helpful for establishing primitive character states in triploblastic organisms, other than the blastopore giving rise to the mouth. When some of the problematic taxa (e.g., Platyhelminthes) are more clearly seated in the tree, outgroups might be easier to establish. For now, we can group the Ecdysozoa and Lophotrochozoa on the basis of synapomorphies. The Deuterostomes are likely a monophyletic assemblage. But the same cannot be said for the Protostomes. Recent rrna data suggest that a particular group of flatworms, the Acoelomorpha (not related to the Platyhelminthes, which are more derived) may be the most basal Bilaterians, and may reflect many ancestral character states of the clade. 2. What might a hypothetical common ancestor of protostomes and deuterostomes have looked like? Choose the characters that both taxa share, and put them together: triploblasty, bilateral symmetry, blastopore giving rise to the mouth. But little else can be established for certain. There is a very nice overview in the Most Excellent Biology Blog, Pharyngula, on this topic: Go read it now! Or read it in workshop! 3. Describe some possible ontogenetic origins of the following in a hypothetical common ancestor of protostomes and deuterostomes: a. origin of (i.e., from ectoderm or?) Embryonic origin of (i.e., from ectoderm or ) is often not clear, even in extant taxa. But it is believed that all Bilaterians have and muscle cells/tissue derived from. Some have contractile elements developed from ectoderm, but al origin of muscle in Bilaterians appears to be the primitive condition, even in the acoelomate flatworms. b. fate of the blastopore (mouth or anus?) The blastopore becomes the mouth in the most primitive lineages of eumetazoans (Radiates, Acoelomates), so this is likely the primitive condition that was present in the common ancestor of the Bilateria. c. cleavage at the 4- to 8-cell stage (i.e., spiral or radial?) Similarly, the most primitive bilaterians (likely acoelomates) have spiral cleavage, so the ancestral Bilaterian likely also underwent spiral cleavage. d. determinate or indeterminate cleavage?

8 Cleavage in the most primitive bilaterians is determinate, so the ancestral bilaterial likely exhibited this condition. e. embryonic location of the circulatory system This is a little bit more tricky. Acoel flatworms lack a circulatory system, as do many Bilaterians. Because the protostomes share so many other characters with the basal bilaterians, one might reasonably suspect that the dorsal circulatory system came first. But this has not yet been established for certain. f. embryonic location of the nervous system Acoel flatworms have primitively ventral nervous system, so it s a good bet this is the ancestral condition. 4. Do you think the coelom of an Annelid is homologous or analogous to that of a Chordate? Discuss. Schizocoely and enterocoely are somewhat different mechanisms by which coelomates build a coelom. But it is possible that the same housekeeping genes are responsible for triggering either process. VI. Diversification and Progression of Complexity You should now have a good grasp of the progression of complexity in ontogeny and anatomy of the animals. Using the phylogenetic tree on the following page, place each of the characters listed at the proper place where it originated in an ancestral lineage, giving rise to today's extant animal phyla. At the root of the tree, begin with a hypothetical ancestral colonial flagellate. (Note that this phylogenetic tree does not include all animal phyla, and it's only the most recent hypothesis. It could change as new data become available.) a. diversification of cell types b. gastrulation c. ectoderm & (diploblasty) d, mesenchyme (mesogloea with cellular components) e. true (triploblasty) f. pseudocoelom The following taxa have a pseudocoelom: Nematoda (roundworms) Rotifera (rotifers) Kinorhyncha Nematomorpha, nematomorphs, or horsehair worms Gastrotricha Loricifera Priapulida Acanthocephala (spiny-headed worms) Aschelminth animals Entoprocta It is not impossible that pseudocoelomates evolved from coelomates, with mutations giving rise to a body cavity lined only on the parietal side by. g. coelom derived via schizocoely

9 h. coelom derivedc via enterocoely i. blastopore becomes the mouth j. blastopore becomes the anus k. circulatory system dorsal in the embryo l. circulatory system ventral in the embryo m. nervous system ventral in the embryo n. nervous system dorsal in the embryo o. spiral, determinate cleavage p. radial, indeterminate cleavage Hypothetical Phylogeny of Animalia (

10 1. The tree above is based on molecular (ribosomal RNA) data. Are the morphological characters you placed on the tree consistent with a most parsimonious explanation for the evolutionary relationships shown? Not always. But this may be because we have not yet determined which groups are truly basal to the Bilateria. Also, some characters that appear primitive (e.g., the pseudocoelom) may actually be derived. Workshop leaders should moderate a discussion about how different lineages are quite capable of evolving new, derived characters even if they have branched off the main taxon relatively long ago! 2. How is it possible that morphological data and molecular data might not produce trees that are congruent with each other? Discuss. This can be a continuation of the discussion above. Characters that appear primitive may not always be. Characters can be secondarily lost. And in some cases evolution may simply not have proceeded in a parsimonious fashion. This is why it s especially important to consider multiple lines of information (morphology, molecular information, etc.) in order to find congruence among hypothetical cladograms generated by each type of data. The search for what really happened may never end. DISCUSSION QUESTION Why is it important for humans to understand phylogenetic relationships of animals? Can you think of any practical applications, or is this just "science for science's sake?" I don t think you need me to provide an answer to this one. But you can at least start with this: Knowing the embryonic derivations and evolutionary links among animalia enables us to place our own species in the evolutionary continuum. You may expound on the philosophical, social, and even spiritual benefits of this type of knowledge. As with any taxonomic group, new species are more effectively classified if we know the characteristics of the major groups, and of what economic and medical importance they might be. If a particular taxon has many members that humans use for their own purposes, then a new species within that taxon might potentially be useful, as it will likely share the synapomorphies of that group, some of which might be useful to humans. Help your group think of examples, concentrating on animals that humans use for food, protection, etc.