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)
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 endoderm (lining the
archenteron, or primitive gut)
3. Complete the following table
PHYLUM
Germ Layers
Name of middle layer
Derivation of middle layer
(ectoderm, endoderm or neither)
“Porifera”
none
mesoderm
not a true tissue layer; not derived from
endoderm or ectoderm; some migratory
cells
not a true tissue layer; not derived from
endoderm or ectoderm; no cellular
component
cellular components are derived from
ectoderm
ectoderm
mesoderm
endoderm
mesoderm
endoderm
Cnidaria
Platyhelminthes
mesohyl (nonmesodermal
mesenchyme
ectoderm, mesogloea (acellular
endoderm proteinaceous matrix)
ectoderm,
endoderm
Nematoda
ectoderm,
endoderm
mesoderm
Annelida
ectoderm,
endoderm
mesoderm
Echinodermata ectoderm,
endoderm
mesoderm
mesenchyme
\
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 mesoderm evolved from
endoderm.
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.
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.
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.
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, endoderm, and mesoderm/mesenchyme, intestinal lumen, parietal and
visceral surfaces.
(In the Workshop Leader version germ layers are labeled as follows in color
coding: ectoderm, endoderm, mesoderm, 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 mesoderm. (This is best shown with a
diagram)
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
mesodermal tissues anchoring your internal organs in their proper place in the
abdomen) as well as the mesodermally 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.
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 mesoderm. 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
deuterostomes have looked like?
common
ancestor
of
protostomes
and
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:
http://scienceblogs.com/pharyngula/2006/06/acoelomorph_flatworms_and_prec.php
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 mesoderm (i.e., from ectoderm or endoderm?)
Embryonic origin of mesoderm (i.e., from ectoderm or endoderm) is
often not clear, even in extant taxa. But it is believed that all Bilaterians
have mesoderm and muscle cells/tissue derived from mesoderm. Some
have contractile elements developed from ectoderm, but mesodermal
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?
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 & endoderm (diploblasty)
d, mesenchyme (mesogloea with cellular components)
e. true mesoderm (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 mesoderm.
g. coelom derived via schizocoely
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 (www.tolweb.org)
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.