1. Heterochrony is—
a. the alteration of the spatial pattern of gene expression
b. a change in the relative position of a body part
c. a change in the relative timing of developmental events
d. a change in a signaling pathway
The correct answer is c—
A. Answer a is incorrect. It refers to a developmental change called homeosis.
The correct answer is c—
B. Answer b is incorrect. It refers to a developmental change called homeosis.
The correct answer is c—a change in the relative timing of developmental events
C. Answer c is correct. Heterochrony literally means “different timing.”
The correct answer is c—
D. Answer d is incorrect. A change in a signaling pathway is not a change in the
timing of a developmental event.
2. Vast differences in the phenotypes of organisms as different as fruit flies and
humans—
a. must result from differences among many thousands of genes controlling
development
b. have apparently arisen largely through manipulation of the timing and
regulation of expression of probably less than 100 highly conserved genes
c. can be entirely explained by heterochrony
d. can be entirely explained by homeotic factors
The correct answer is b—
A. Answer a is incorrect. To the best of our knowledge, genes controlling major
patterns of development number less than 100; it is expression differences that
produce large variation.
The correct answer is b— have apparently arisen largely through manipulation of the
timing and regulation of expression of probably less than 100 highly conserved genes
B. Answer b is correct. There are many examples of profound phenotypic changes
brought about by changes in a single gene of major effect (such as, stop codon in
CAL), or in multiple developmental timing or gene expression profiles (for
example, Brachyury, Tbx5).
The correct answer is b—
C. Answer c is incorrect. Although heterochrony is one important mechanism in
developmental variation, there are other important mechanisms.
The correct answer is b—
D. Answer d is incorrect. Although homeotic factors are one important type of
mechanism in developmental variation, there are other important mechanisms.
3. Homoplastic structures—
a. can involve convergence of completely unrelated developmental pathways
b. always are morphologically distinct
c. are produced by divergent evolution of homologous structures
d. are derived from the same structure in a shared common ancestor
The correct answer is a—can arise by convergence of completely unrelated
developmental pathways
A. Answer a is correct. There are examples where different developmental pathways
have evolved to produce a common phenotype.
The correct answer is a—
B. Answer b is incorrect. Homoplastic structures look the same.
The correct answer is a—
C. Answer c is incorrect. Homologous structures subjected to divergent evolution
will change in form and no longer be analogous or homoplastic.
The correct answer is a—
D. Answer d is incorrect. It is the definition of homologous not homoplastic
structures.
4. Why was it important to create transgenic plants to determine the role of AP3 in
petal formation?
a. It provided a functional test of the role of AP3 in petal development
b. Duplication of AP3 could not be resolved on the phylogeny
c. To check if the phylogenetic position of AP3 is really derived
d. Because tests already established the role of paleoAP3 in stamen development
The correct answer is a—It provided a functional test of the role of AP3 in petal
development
A. Answer a is correct. The experiments allowed investigators to establish a
functional and positional role within the gene that represents a new function
subsequent to the gene duplication.
The correct answer is a—
B. Answer b is incorrect. The origination of the gene duplication event is established
from other data; the experiments take the phylogenetic position as a given.
The correct answer is a—
C. Answer c is incorrect. The experiments (which demonstrate function) don’t help
investigators discriminate between a derived or basal origination of the gene
The correct answer is a—
D. Answer d is incorrect. The role of paleoAP3 is not in question relative to the role
of AP3 in the duplication event.
5. Eyes of vertebrates and invertebrates—
a. are two examples of a structure that has probably evolved completely
independently more than 10 times
b. are similar in structure due to convergence
c. share no similarities in their pattern of development
d. may be homologous at the level of the initiation of development of a visual
receptor
The correct answer is d—
A. Answer a is incorrect. Evidence concerning the role of Pax6 suggests that there
are probably many fewer independent origins of eyes in the animal kingdom.
The correct answer is d—
B. Answer b is incorrect. A common gene seems to be responsible for initiating eye
development.
The correct answer is d—
C. Answer c is incorrect. Similarities are so great that transgenic animals can produce
remarkably intact eyes with genes mixed from taxa as separate phylogenetically
as flies and mice.
The correct answer is d—may be homologous at the level of the initiation of development
of a visual receptor
D. Answer d is correct. Experiments with Pax6 suggest a common organizing role
for eye development across a broad range of taxa. It is more parsimonious, given
new evidence from functional genomics, that the genetic program for eyes
evolved at a very ancient point in the history of animals.
6. Hox genes—
a. are found in both plants and animals
b. are found only in animals
c. are found only in plants
d. are only associated with genes in the MADS complex
The correct answer is a—are found in both plants and animals
A. Answer a is correct. Hox genes appeared before the divergence of plants and
animals.
The correct answer is a—
B. Answer b is incorrect.
The correct answer is a—
C. Answer c is incorrect.
The correct answer is a—
D. Answer d is incorrect.
7. The Brachyury gene in vertebrates and the Ap3 gene in flowering plants—
a. are examples of Hox genes
b. are examples of co-opting a gene for a new function
c. are homologues for determining the body plan of eukaryotes
d. help regulate the formation of a photoreceptor organ
The correct answer is b—
A. Answer a is incorrect. Neither Ap3 or Brachyury are Hox genes.
The correct answer is b—are examples of co-opting a gene for a new function
B. Answer b is correct. Brachyury is central to the formation of a notochord, but
existed in invertebrates, but with a different function; Ap3 arose from a
duplication event with part of the new copy, eventually taking on a role in the
control of petal development.
The correct answer is b—
C. Answer c is incorrect. The two genes are unrelated.
The correct answer is b—
D. Answer d is incorrect. Neither gene is involved in the development of a
photoreceptor.
8. Which of the following organisms is not considered to be a model genetic system?
a. Mice
b. Fruit flies
c. Humans
d. Yeast
The correct answer is c—
A. Answer a is incorrect. Mice are used as a model genetic system. Interestingly,
since mice and humans are both mammals; many of the genes in mice have very
similar functions to those they have in humans.
The correct answer is c—
B. Answer b is incorrect. Fruit flies are considered to be a quite powerful model
genetic system. Many tools are available for manipulating fly DNA to study gene
expression and function.
The correct answer is c—Humans
C. Answer c is correct. Due to a variety of moral, ethical, and practical reasons,
humans are not commonly used as a model genetic system. Instead, we use other
organisms to learn about gene function and then use much of that information to
further our understanding of the human body.
The correct answer is c—
D. Answer d is incorrect. Yeast is a simple eukaryote and an excellent model for
studying many basic cellular processes.
9. Which of the following statements about Pax6 is false?
a. Pax6 has a similar function in mice and flies.
b. Pax6 is involved in eyespot formation in ribbon worms.
c. Pax6 is required for eye formation in Drosophila.
d. Pax6 is required for eyespot formation in planaria.
The correct answer is d—
A. Answer a is incorrect. Pax6 is a mouse gene that can initiate eye development
when it is introduced into a fly.
The correct answer is d—
B. Answer b is incorrect. When the head of a ribbon worm is removed, it regenerates.
Pax6 genes are expressed in the region of the regenerating head where the new
eyespots will form.
The correct answer is d—
C. Answer c is incorrect. Pax6 is the mouse gene that is functionally equivalent to
the eyeless gene in flies. When the fly has a mutation in eyeless (Pax6), no eyes
form.
The correct answer is d—Pax6 is required for eyespot formation in planaria.
D. Answer d is correct. Planaria can regenerate eyespots whether or not the Pax6
gene is expressed. This result shows Pax6 is not required for eyespot formation in
planaria.
10. Which of the following statements about Tbx5 is true?
a. Tbx5 is found only in tetrapods.
b. Tbx5 is involved in limb development in vertebrates.
c. Tbx5 is only found in the ancestors of tetrapods.
d. Tbx5 interacts with the same set of genes across different tetrapod species.
The correct answer is b—
A. Answer a is incorrect. Tbx5 occurs in tetrapod ancestors as well.
The correct answer is b—Tbx5 is involved in limb development in vertebrates.
B. Answer b is correct. Tbx5 is a transcription factor gene that interacts with
different sets of genes among different species, resulting in different patterns of
limb development.
The correct answer is b—
C. Answer c is incorrect. Tbx5 is found in tetrapods as well
The correct answer is b—
D. Answer b is incorrect. Variation in the gene Tbx5 interacts with is apparently part
of the source of variation in limb development among different tetrapods.
11. Homeosis—
a. refers to a maintained and unchanging genetic environment
b. is a temporal change in gene expression
c. is a spatial change in gene expression
d. is not an important genetic mechanism in development
The correct answer is c—
A. Answer a is incorrect. Homeosis is not a process of maintaining the genetic
environment.
The correct answer is c—
B. Answer b is incorrect. A temporal change is heterochrony.
The correct answer is c—is a spatial change in gene expression
C. Answer c is correct. When the spatial pattern of gene expression changes, it is
referred to homeosis.
The correct answer is c—
D. Answer d is incorrect. Homeosis is a central mechanism of development.
12. Transcription factors—
a. are genes
b. are sequences of RNA
c. are proteins that affect the expression of genes
d. none of the above
The correct answer is c—
A. Answer a is incorrect. Transcription factors are themselves coded for by genes,
but they are not genes.
The correct answer is c—
B. Answer b is incorrect. Transcription factors may interact with RNA during
transcription but they are not RNA.
The correct answer is c—are proteins that affect the expression of genes
C. Answer c is correct. Transcription factors have been identified as a major source
of gene expression variation, especially that underlying developmental shifts.
The correct answer is c—
D. Answer d is incorrect. One of the answers is correct, answer d.
13. Independently derived mutations of the CYC gene in plants—
a. suggests that bilateral floral symmetry among all plants is homologous
b. establishes that radial floral symmetry is preferred by pollinators
c. establishes that radial floral symmetry is derived for all plants
d. none of the above
The correct answer is d—
A. Answer a is incorrect. Since the mutations arose independently, that suggests
convergence, not homology.
The correct answer is d—
B. Answer b is incorrect. Radial symmetry is not preferred by pollinators, bilateral
symmetry is.
The correct answer is d—
C. Answer c is incorrect. Radial floral symmetry is the ancestral state.
The correct answer is d—none of the above
D. Answer d is correct. None of the previous answers is correct.
Challenge Questions
1. Several examples of genes being co-opted for new functions were given in the
chapter. Cite two and explain what is meant by co-opted.
Answer—Brachyury, Tbx5, and others, are examples of genes that have a defined
function in a basal set of taxa, and in more derived taxa the function switches. In the case
of these two genes, the new function is central to defining what is novel or derived in the
new taxa. Co-opting is the use of an existing structure for a new purpose. In the case of
development, co-opting appears to be a very common mechanism associated with major
shifts in developmental programs
2. From the chapter on evolution of development it would seem that the generation
of new developmental patterns would be fairly easy and fast, leading to the ability
of organisms to adapt quickly to environmental changes. Why then does it take
millions of years, typically, for many of the traits examined to evolve?
Answer—Development is a highly conserved and constrained process; small
perturbations can have drastic consequences, and most of these are negative. Given the
thousands or hundreds of thousands of variables that can change in even a simple
developmental pathway, most perturbations lead to negative outcomes. Over millions of
years, some of these changes will arise under the right circumstances to produce a
benefit. In this way, developmental perturbations are not different from what we know
about mutations in general. Beneficial mutations are rare, but with enough time they will
emerge and spread under specific circumstances.
3. There are several ways in which phenotypic diversity among major groups of
organisms can be explained. On one end of the spectrum, such differences could
arise out of differences in many genes that control development. On the other
end, small sets of genes might differ in how they regulate the expression of
various parts of the genome. Which view represents our current understanding?
Explain.
Answer— The latter view represents our current understanding. There are many
examples of small gene families (such as, Hox, MADS) whose apparent role in generating
phenotypic diversity among major groupings of organisms is in altering the expression of
other genes. Alterations in timing (heterochrony) or spatial pattern of expression
(homeosis) can lead to shifts in developmental events, giving rise to new phenotypes.
Many examples are presented in the chapter, such as the developmental variants of two
species of sea urchins, one with a normal larval phase, and another with direct
development. In this case the two species do not have different sets of developmental
genes, rather the expression of those genes differ. Another example that makes the same
point is the evolution of an image forming eye. Recent studies suggest, in contrast to the
view that eyes across the animal kingdom evolved independently multiple times, that
image-forming eyes from very distantly related taxa (such as, insects and vertebrates)
may trace back to the common origin of the Pax6 gene. If that view is correct, then genes
controlling major developmental patterns would seem to be highly conserved across long
periods of time, with expression being the major form of variation.