Chapter 05: Review questions
Chapter 5: Development of the Drosophila body plan (Modified from Dr Charles
Sackerson, Department of Biology, Iona College; source: Wolpert text webpage).
Overview: The early development of Drosophila is probably the best understood
developmental system of all animals, at the molecular level. This is due largely to a
saturation mutagenesis screen for embryonic pattern defects carried out in the 1970s by
C. Nusslein-Volhard and E. Wieschaus (see Box 2B on page 62). Approximately 100
genes were identified which can account for most of the pattern formation and
morphogenetic events of the early embryo. This set of genes complement a different set
of genes studied by E. Lewis, whose activities were involved in specifying the identity of
segments observable in the adult fly. Together, these genes became the Rosetta stones
that allowed fly development to be deciphered. At the time, no one suspected that they
would provide the keys to understanding development in all animals, yet that is what has
occurred; even humans use the genetic networks first discovered in flies, albeit in
sometimes very different ways. For this reason, Drosophila development deserves special
attention in studies in developmental biology.
Study tip: Our study of Drosophila embryonic development will cover three processes:
the establishment of segments along the anterior-posterior axis, the establishment of the
dorsal-ventral axis, and the final specification of the identity of the body parts by the Hox
genes. Organize your studies around these three processes. In addition, pay special
attention to the intercellular signaling that occurs during segmentation, and the role of the
Hox genes, for here you will learn about the phenomena that led to the discovery of many
of the genes you have been hearing about in the previous chapters on vertebrates.
Also, see the table on page 188 of the text book for a list of the genes: not all of the
genes in this table were covered.
Keywords: Briefly compare and contrast the following pairs of terms. Check your
answer by using the text and the glossary.
syncitial blastoderm / cellular blastoderm
maternal gene / zygotic gene
enhancer/promoter
gap gene / pair-rule gene
segmentation gene / homeotic gene
Factual recall questions:
1. The portion of the Drosophila body plan which will produce the wing is called
a. telson
b. dorsal
c. A1
2.
3.
4.
5.
6.
7.
d. T1
e. T2
Which of the following genes is never transcribed in the embryo itself?
a. bicoid
b. hunchback
c. even-skipped
d. engrailed
e. abdominal-B
Which would lead to a ventralized embryo?
a. dorsal mutant
b. cactus mutant
c. Toll mutant
d. spätzle mutant
e. bicoid mutant
Which statement describes the role of dorsal protein in D-V axis formation?
a. a gradient of nuclear localization of dorsal sets the ventral side at the
position of highest nuclear dorsal concentration
b. a gradient of nuclear localization of dorsal sets the dorsal side at the
position of highest nuclear dorsal concentration
c. transcription of the dorsal gene is greatest on the dorsal side of the
embryo, in response to spätzle signaling
d. transcription of the dorsal gene is greatest on the ventral side of the
embryo, in response to spätzle signaling
e. high dorsal concentrations specify the dorsal side of the embryo, whereas
high decapentaplegic concentrations specify the ventral side of the embryo
A gap gene mutation would cause which of the following defects in the
embryonic body plan?
a. every other segment would be missing, resulting in T1, T3, A2, A4, etc.
but no T2, A1, A3, and so on.
b. segments A2 through A6 would be missing, but the rest of the pattern is
essentially normal
c. no segmentation would be evident
d. patterning within each segment would be abnormal, causing for example
denticle belts to form across the entire segment
e. the identity of one or more segments would be transformed to that of a
different segment, such that the T3 leg would transformed to a T2 leg
Segments are first positioned at the cell-by-cell level by the
a. maternal genes
b. gap genes
c. pair-rule genes
d. segment polarity genes
e. homeotic genes
Engrailed sets up compartment boundaries by initiating a signaling pathway
involving
a. bicoid and caudal
b. decapentaplegic and short gastrulation
c. even-skipped and fushi tarazu
d. wingless and hedgehog
e. antennapedia and bithorax
Concept questions:
1. Fly development illustrates three important principles in development: (a) the use
of "morphogenetic gradients", (b) hierarchical "genetic cascades", and (c) the
"progressive specification" of body parts. Describe what is meant by each of these
three terms.
2. Working with maternal-effect genes requires a special mind-set. Diagram a
genetic cross describing how you get "bicoid-mutant" embryos that would display
the bicoid phenotype, as in Figure 5.3. Start by designating the genotype of the
bicoid mutant stock "Aa", where "a" is the mutant bicoid gene and "A" is the
wild-type bicoid gene. Follow a mating of these "Aa" flies through as many
generations required to produce the "bicoid-mutant" embryo, indicating the
genotype of the mother of the "bicoid-mutant" embryo and all the possible
genotypes of the "bicoid-mutant" embryo itself. Describe in words how the
genetics of bicoid revealed the role of the mother in anterior-posterior patterning.
3. Review the evidence that bicoid acts as a morphogen: a gene whose concentration
is interpreted as positional information. Include the phenotype of "bicoid-mutant"
embryos, the pattern of mRNA, the pattern of protein expression, and experiments
involving the injection of wild-type cytoplasm in "bicoid-mutant" embryos.
4. Most of the genes involved in patterning the Drosophila embryo are transcription
factors. How does the fact that the early embryo is a syncitium allow Drosophila
to make use of transcription factors for early patterning? Contrast this with the
situation in frogs: what types of molecules are used for patterning, and why?
5. The ability of bicoid to activate hunchback with a sharp posterior boundary
involves both the strength of the binding sites for bicoid in the hunchback
regulatory region, and the cooperative binding of bicoid to the DNA. Cooperative
binding is outside the scope of this course, but let's think about the effect of
binding site "strength". If you were to use genetic engineering techniques to
design a gene to contain several very strong binding sites for bicoid protein,
where would you predict this gene to become expressed in the embryo? Now
picture a gene with only one, very weak binding site: what might its expression
pattern be? Review the gradients of bicoid and dorsal proteins in the early
embryo; all are transcription factors. How might you design a gene to be
expressed at the anterior pole, broadly in the posterior of the embryo, or on the
ventral surface in a narrow region along the midline?
6. Specification of the dorsal side of the oocyte blocks the production of the spätzle
signal; its production occurs therefore only on the ventral side. Describe the
mechanism by which spätzle signaling leads to the establishment of a
morphogenetic dorsal-ventral gradient. Include Toll, dorsal, cactus, and "nuclear
localization" in your answer. Describe how these factors are involved in
vertebrate immune cell differentiation.
7. Describe two kinds of experimental evidence that indicate that hunchback is
activated by bicoid.
8. The strategy of segmentation is not unique to insects; describe the features of the
vertebrate body plan in which segmentation is evident.
9. Review the model for control of even-skipped stripe 2 positioning. What is the
evidence that the eve2 stripe results from an independent regulatory element in
the eve gene? What activators and repressors influence the eve2 enhancer?
Describe how you would predict stripe 2 to be affected in the following cases:
giant mutant embryos, Krüppel mutant embryos, embryos from bicoid mutant
mothers, embryos from mothers with one normal and one mutant copy of the
bicoid gene, embryos from mothers with 6 copies of the bicoid gene.
10. Engrailed is special among the segmentation genes, in that it was originally
classified as a homeotic selector gene, due to its role in specifying a posterior fate
to the posterior compartment of each segment. Examine Figures 5.26 and 5.29
and describe in words the "homeotic transformation" seen at the segment level in
engrailed mutants.
11. Review the main segment polarity genes, and the function of the intercellular
signaling circuit. List the names and functions of mammalian homologues to a
few of these genes.
12. Describe how a smooth hunchback gradient can result in a broad band of
expression of a gap gene. Use a graph, and repression and activation thresholds.
13. Describe how transgenes can be used in promoter analysis to identify the
functions of regulatory sequences. Also use of transgenes in mis-expressing
genes (gain-of-function experiments).
14. Define homeosis. Define posterior prevalence, and anteriorization. List the four
main homeotic genes discussed in the lecture. Graph their segmental pattern of
expression. Describe the segmental phenotypes that result from loss or gain of
function for each.
Developmental genes: Study questions.
Define, and give an example for each term:
Genetic dissection
Mutant
Mutation
Wildtype allele
Diploid
Recessive allele
Dominant allele
Conditional allele
Show a Punnett square analysis for a semidominant mutation, for a recessive mutation,
and for a maternal effect mutation.
Compare and contrast: genetic screen vs. selection.
Explain the genetic risks of marrying a related person.
Limb development: Study questions.
Keywords: Briefly compare and contrast the following pairs of terms. Check your
answer by using the glossary and the text.
apical ridge / progress zone
polarizing zone / morphogen gradient
mesenchyme / epithelium
 Muscles of the vertebrate limb form from
a.
b.
c.
d.
e.
the ectodermal epithelium of the limb bud
the mesodermal mesenchyme of the limb bud, derived from lateral plate ectoderm
mesodermal cells that migrate into the limb bud from the somites
the progress zone
the polarizing region
 Removal of the apical ridge leads to
a. regeneration of the entire limb bud from underlying mesoderm
b. formation of structures proximal to the apical ridge, but no formation of new
distal structures
c. degeneration of the limb bud
d. regeneration of a new apical ridge from adjacent epidermal tissue
e. continued development in the progress zone, once the apical ridge has induced
progress zone formation
 The grafting of a second polarizing region into the anterior of a limb bud results in
a.
b.
c.
d.
e.
degeneration of the bud
formation of a second bud at the site of the graft
formation of extra copies of the anterior-most digit, digit 2
formation of extra copies of the posterior-most digit, digit 4
mirror duplication of the axis, and the formation of a second full set of digits
 Define organogenesis. Where does organogenesis fit into the overall scheme of
development?
 Define the three developmental axes of the vertebrate limb. Which of these three axes
is represented in the human hand by the line running from thumb to little finger?
 Draw a schematic of a limb bud, labeling the apical ridge, the progress zone, and the
polarizing zone. Also indicate the three axes.
 Use Figures 10.5 and 10.6 as models to draw a timeline of bone formation in the
developing chick limb. Indicate the progress zone on your drawings. Also indicate the
points of expression of Wnt-14 and "growth and differentiation factor" -5 (GDF-5).
 What is the evidence from mouse knockouts that Hox genes specify the position of
limb bud formation along the antero-posterior axis of the lateral plate mesoderm? Once
Hox genes have specified their positions, what key signaling molecules initiate limb bud
formation? These factors do their job by triggering the formation of two organizers; what
are these two organizers? How do their roles differ? How do both differ from the
progress zone?
 What family of signaling molecules appears to be key at this point in carrying out the
activity of the apical ridge? Describe an experiment that demonstrates this.
 Describe an experiment that demonstrates the organizer activity of the polarizing
region. What aspects of the experiment are crucial to the interpretation of the results in
terms of a concentration gradient of a morphogen? Of what diffusible signaling molecule
might this gradient be composed? What is the evidence for, and against, this possibility?
 Speculate on a molecular mechanism by which time in the progress zone may be
measured and converted to a position along the proximo-distal axis. Consider Hox gene
expression in the limb bud (see Figure 10.16) and the organization of the Hox complex
on the chromosome (see Chapter 4).
 Combine the bottom panel of Figure 10.16 with the experiment shown in the center
panel of Figure 10.11 to predict the pattern of Hox gene expression in a limb bud with an
extra polarizing region grafted to its anterior edge.
 What is the role of programmed cell death in limb formation?
o How many Hox genes are expressed in the limb bud? What role might this play
in digit specification? Describe two genetic causes for polydactyly.
o Describe the human limb defects caused by thalidomide. What is the mechanism
of action by this drug? What species are affected?
o Describe the role of limb identity genes. What experiment showed how Tbx5
functions in forelimb development?