Question 1

Indicate whether each of the following groups of genes typically creates a transient pattern in the developing embryo (T) or a long-lived pattern that is preserved (L). Your answer would be a five-letter string composed of letters T and L only, e.g. TTTLT.
( )	Egg-polarity genes
( )	Pair-rule genes
( )	Hox genes
( )	Gap genes
( )	Segment-polarity genes

Accepted Answer

Egg-polarity, gap, and pair-rule genes create transient patterns that are remembered by segment-polarity and Hox genes.

Question 2

Indicate true (T) and false (F) statements below regarding animal development. Your answer would be a four-letter string composed of letters T and F only, e.g. FFFF.
( )	A fertilized egg is totipotent.
( )	Differences in their regulatory DNA can largely explain the differences between animal species.
( )	Inductive signaling is mostly mediated through G-protein-coupled receptors.
( )	A cell's response to a signal depends on its exposure to other signals at that present time as well as in the past.

Accepted Answer

A fertilized egg is totipotent, able to give rise to all the different cell types in an organism. Changes in regulatory DNA seem to be mainly responsible for the dramatic differences between one class of animals and another, even though the coding DNA has been, for the most part, highly conserved between the classes. Most known inductive signaling events during animal development are mediated by the transforming growth factor-? (TGF?), Wnt, Hedgehog, and receptor tyrosine kinase (RTK) pathways. During development and in an adult organism, the cellular response to a signal depends not only on the identity of the signal, but also on the other signals that the cell is receiving, as well as on the previous experiences of the cell.

Question 3

In a developing Drosophila melanogaster embryo, a hierarchy of gene regulatory interactions subdivides the embryo to regulate progressively finer details of patterning. For each of the following proteins, indicate which expression pattern (1 to 4) in the schematic drawing below is more appropriate. Your answer would be a four-digit number composed of digits 1 to 4 only, with each digit used once, e.g. 3412.
  
( )	Hunchback (product of a gap gene)
( )	Engrailed (product of a segment-polarity gene)
( )	Bicoid (product of an egg-polarity gene)
( )	Even-skipped (product of a pair-rule gene)

Accepted Answer

In the regulatory hierarchy of egg-polarity and segmentation genes, the gap-gene products provide a further tier of positional signals by controlling the expression of other genes, including the pair-rule genes. The pair-rule genes, in turn, collaborate with one another and with the gap genes to set up a regular, periodic pattern of expression of the segment-polarity genes, which collaborate with one another to define the internal pattern of each individual segment.

Question 4

Indicate true (T) and false (F) statements below regarding Hox genes. Your answer would be a four-letter string composed of letters T and F only, e.g. TTTF.
( )	The order of expression of Hox genes along the body corresponds to their order in the Hox complex.
( )	Generally, the more anterior of the Hox genes dominate (or suppress) posterior Hox genes.
( )	When a posterior Hox gene is artificially expressed in an anterior region of the embryo, the tissue maintains its anterior character.
( )	Hox genes control the A-P axis in both vertebrates and invertebrates.

Accepted Answer

The genomic location of Hox genes (which control the anteroposterior [A-P] axis in both vertebrates and invertebrates) corresponds well with their order of expression along the A-P axis. In other words, Hox genes are expressed according to their order in the Hox complex. Generally, the more posterior of the Hox genes dominate the more anterior ones, so that when both are expressed in a given segment, the segment will assume an identity dictated by the more posterior Hox genes.

Question 5

Imagine a morphogen gradient established from left to right in a field of cells in a developing tissue, as shown in the following schematic diagram. Below a first threshold of morphogen concentration, cells do not respond to the morphogen and express gene "red" by default. Cells exposed to morphogen concentrations above this threshold respond by expressing gene "white" instead, while those exposed to even higher concentrations, above a second threshold, express gene "blue." As shown, the initial pattern resembles a French flag with equally wide blue, white, and red expression domains. With no other change, if the diffusion rate of the morphogen is increased (by a modification that decreases its affinity for heparan sulfate proteoglycans, for example), the gradient profile changes from the gray curve to the black curve, as indicated. Under this new condition, indicate whether each of the following would be expected to increase (I), decrease (D), or remain unchanged (U) in its range. Your answer would be a three-letter string composed of letters I, D, and U only, e.g. UUI.
  
( )	Blue expression domain
( )	White expression domain
( )	Red expression domain

Accepted Answer

I
D
 Changes in parameters such as diffusion rate can affect the patterning of cells by a morphogen. In this example, the more shallow gradient widens the gap between positions corresponding to the first and second thresholds-i.e. the "white" expression domain-at the expense of the other two domains.

Question 6

Sort the following organisms from the least to the most degree of asymmetry in the unfertilized egg (i.e. pre-defined axes of polarization). Your answer would be a three-letter string composed of letters A to C only, e.g. BCA.
(A)	M. musculus
(B) D. melanogaster
(C) X. laevis

Accepted Answer

The mouse egg is almost perfectly symmetrical. In contrast, fly eggs are highly polarized. Many other organisms, such as the frog, lie between these two extremes.

Question 7

Which of the following is correct regarding the regulation and maintenance of Hox gene expression in Drosophila?
A) Proteins of the Polycomb and Trithorax groups maintain inactive and active states of Hox gene expression, respectively.
B) If Polycomb or Trithorax group genes are defective, Hox gene expression patterns are still initiated, but cannot be correctly maintained.
C) The remodeled chromatin at the Hox complex is heritable through cell generations.
D) If all the Hox genes in an embryo are deleted, segmentation still occurs but distinct segment identities are lost.
E) All of the above.

Accepted Answer

Hox proteins give each segment its individuality; without them, segments will lose their individual characters. Trithorax and Polycomb group proteins work together, in opposite ways, to enable the Hox complexes to maintain a permanent record of positional information. Without them, the Hox gene expression pattern is set up correctly, but cannot be maintained.

Question 8

The gene clusters known as the Bithorax complex and the Antennapedia complex contain…
A) a subset of segment-polarity genes.
B) segmentation genes.
C) homeobox-containing genes.
D) genes encoding chromatin repressors.
E) genes encoding chromatin remodelers.

Accepted Answer

Hox genes lie in one or the other of two gene clusters, the Bithorax complex and the Antennapedia complex.

Question 9

In Drosophila melanogaster, the expression of genes Ultrabithorax and Antennapedia can normally be observed in the third thoracic segment which bears a pair of legs as well as a pair of halteres. Homeotic mutations associated with these two genes can give rise to remarkable disturbances in the organization of the adult fly: two pairs of wings in the case of Ultrabithorax, and legs in the place of antennae in the case of Antennapedia. Would you expect these to be gain-of-function or loss-of-function mutations?
A) Loss-of-function for both genes
B) Loss-of-function for Ultrabithorax and gain-of-function for Antennapedia
C) Gain-of-function for Ultrabithorax and loss-of-function for Antennapedia
D) Gain-of-function for both genes

Accepted Answer

Loss of Ultrabithorax transforms the segment into a more anterior identity and gives rise to a pair of wings in this segment. Gain of Antennapedia transforms a head segment such that it bears a leg in the place of an antenna.

Question 10

Cortical rotation following fertilization in X. laevis places the …(1) pole at the point of sperm entry, while Wnt11 mRNA is transported to the …(2) pole.
A) 1: anterior; 2: posterior
B) 1: posterior; 2: anterior
C) 1: animal; 2: vegetal
D) 1: dorsal; 2: ventral
E) 1: ventral; 2: dorsal

Accepted Answer

Fertilization in Xenopus laevis, through a reorganization of the microtubule cytoskeleton, triggers a rotation of the egg cortex. This places the ventral pole at the point of sperm entry. The reorganization also leads to the transport of several cytoplasmic components, including the Wnt 11 mRNA, which becomes concentrated at the dorsal side.

Question 11

Indicate whether each of the following proteins is most concentrated near the dorsal (D) or ventral (V) side of the Drosophila embryo. Your answer would be a four-letter string composed of letters D and V only, e.g. DDDV.
( )	Decapentaplegic
( )	Dorsal (nuclear fraction)
( )	Toll (active form)
( )	Twist

Accepted Answer

Activated Toll receptors on the ventral side of the embryo result in the nuclear localization of Dorsal at this side. The highest nuclear Dorsal concentration activates the expression of Twist, whereas the lowest concentration de-represses Decapentaplegic (Dpp). At intermediate concentrations, Short gastrulation (Sog) expression is induced.

Question 12

The simplified drawing below depicts early stages of animal development. Indicate which letter (A to E) in the drawing corresponds to each of the following terms. Your answer would be a five-letter string composed of letters A to E only, e.g. ECDBA.
  
( )	Blastula
( )	Ectoderm
( )	Endoderm
( )	Mesoderm
( )	Gastrula

Accepted Answer

Please refer to Figure 21-3.

Question 13

Indicate whether each of the following descriptions better applies to patterning by lateral inhibition (L), reaction-diffusion systems (R), or sequential induction (S). Your answer would be a four-letter string composed of letters L, R, and S only, e.g. SSRR.
( )	It does NOT generate asymmetrical patterns from an initial noisy field.
( )	It is based on short-range activation and long-range inhibition.
( )	It can readily generate complex patterns resembling the spots of a leopard or stripes of a zebra.
( )	It is commonly mediated by Notch signaling.

Accepted Answer

Lateral inhibition, commonly mediated by Notch signaling, can generate asymmetry when combined with positive feedback. Reaction-diffusion systems, based on short-range activation and long-range inhibition, are also capable of spontaneous pattern generation through positive feedback, and give rise to complex patterns such as spots and stripes. Sequential induction, on the other hand, is a strategy to refine initial patterns into more complicated ones, and cannot create asymmetrical patterns by itself.

Question 14

Which of the following schematic drawings better depicts the regulatory network that maintains Engrailed (en), Hedgehog (hh), and Wingless (wg) expression following cellularization in a developing Drosophila embryo?

Accepted Answer

In each segment, cells that express the signal protein Wingless induce their neighboring cells to express the transcription regulator Engrailed, which activates the expression of the signal protein Hedgehog in these cells. This in turn activates the expression of Wingless in the first cell, maintaining the fine patterning within the segment.

Question 15

Indicate whether each of the following organs or tissues arises from ectoderm (C), mesoderm (M), or endoderm (N). Your answer would be a four-letter string composed of letters C, M, and N only, e.g. MMCC.
( )	Blood
( )	Liver and pancreas
( )	Brain
( )	Bone and cartilage

Accepted Answer

Ectoderm gives rise to the epidermis and the nervous system; endoderm gives rise to the gut tube and its appendages, such as lung, pancreas, and liver; mesoderm gives rise to muscles, connective tissues (e.g. bone and cartilage), blood, kidney, and various other components.

Question 16

Indicate whether each of the following descriptions better applies to these poles established in an early embryo: anterior (A), posterior (P), dorsal (D), ventral (V), animal (N), or vegetal (G). Your answer would be a six-letter string composed of letters A, D, G, N, P, and V only, e.g. ADGPVN.
( )	It defines the part to become internalized in gastrulation.
( )	It defines the parts to remain external.
( )	It defines the location of the future head.
( )	It defines the location of the future tail.
( )	It defines the location of the future belly.
( ) It defines the location of the future back.

Accepted Answer

Three axes generally have to be established in the early embryo. In most animals, the animal-vegetal (A-V) axis defines which parts are to remain external in gastrulation and which are to become internalized. The anteroposterior (A-P) axis specifies the locations of the future head and tail. The dorsoventral (D-V) axis specifies the future back and belly.

Question 17

Indicate whether each of the following descriptions better applies to egg-polarity genes (E), gap genes (G), pair-rule genes (P), or segment-polarity genes (S). Your answer would be a four-letter string composed of letters E, G, P, and S only, e.g. SSGG.
( )	Mutations in these genes show a maternal effect.
( )	Mutations in these genes leave the embryo with only half the number of normal segments.
( )	Mutations in these genes produce a normal number of segments but with part of each segment replaced by a mirror-image duplicate of other parts of the segment.
( )	Mutations in these genes can eliminate one or two groups of adjacent segments altogether.

Accepted Answer

Egg-polarity genes in Drosophila are maternal-effect genes: their mRNA transcript is deposited by the mother into the egg, and hence it is the mother's genome (rather than the zygote's genome) that is critical. Mutations in gap genes result in the elimination of one or more adjacent segments altogether. Mutations in pair-rule genes cause a series of deletions affecting alternate segments, leaving the embryo with only half as many segments as usual. Finally, mutations in segment-polarity genes produce the right number of segments but with a part of each segment replaced by a mirror-image duplicate of another part of the segment.

Question 18

Plants and animals use different developmental strategies and have very different ways of life. In which of the following fundamental cellular processes during development are plants most different to animals?
A) Cell proliferation
B) Cell-cell interactions
C) Cell specialization
D) Cell movement

Accepted Answer

Plant cells do not actively migrate through the embryo.

Question 19

Which of the following is true regarding maternal-effect genes?
A) Bicoid and Nanos are maternal-effect genes.
B) A female homozygous for a loss-of-function maternal-effect gene mutation can be fully normal, but her offspring will show the phenotype.
C) The offspring of a female homozygous for a loss-of-function maternal-effect gene mutation will show the phenotype regardless of the paternal genotype.
D) The second-generation offspring of a male homozygous for a loss-of-function maternal-effect gene mutation can show the phenotype.
E) All of the above.

Accepted Answer

For maternal-effect genes, it is the mother's genome rather than the zygotic genome that is critical.

Question 20

Sort the following primary axes in the order that they are established during the development of Xenopus laevis. Your answer would be a three-letter string composed of letters A to C only, e.g. ACB.
(A)	A-P
(B)	D-V
(C)	A-V

Accepted Answer

The animal-vegetal (A-V) asymmetry is established before fertilization. The point of sperm entry then helps determine the dorsoventral (D-V) axis. The anteroposterior (A-P) axis is established later.

Quiz 21: Development of Multicellular Organisms

Sort the following organisms from the least to the most degree of asymmetry in the unfertilized egg (i.e. pre-defined axes of polarization). Your answer would be a three-letter string composed of letters A to C only, e.g. BCA. (A) M. musculus (B) D. melanogaster (C) X. laevis

Which of the following is correct regarding the regulation and maintenance of Hox gene expression in Drosophila?

The gene clusters known as the Bithorax complex and the Antennapedia complex contain…

Cortical rotation following fertilization in X. laevis places the …(1) pole at the point of sperm entry, while Wnt11 mRNA is transported to the …(2) pole.

Which of the following schematic drawings better depicts the regulatory network that maintains Engrailed (en), Hedgehog (hh), and Wingless (wg) expression following cellularization in a developing Drosophila embryo?

Indicate whether each of the following organs or tissues arises from ectoderm (C), mesoderm (M), or endoderm (N). Your answer would be a four-letter string composed of letters C, M, and N only, e.g. MMCC. ( ) Blood ( ) Liver and pancreas ( ) Brain ( ) Bone and cartilage

Plants and animals use different developmental strategies and have very different ways of life. In which of the following fundamental cellular processes during development are plants most different to animals?

Which of the following is true regarding maternal-effect genes?

Sort the following primary axes in the order that they are established during the development of Xenopus laevis. Your answer would be a three-letter string composed of letters A to C only, e.g. ACB. (A) A-P (B) D-V (C) A-V