Problems and Discussion Questions

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Chapter 20

This activity contains 30 questions.

Carefully distinguish between the terms differentiation and determination. Which phenomenon occurs initially during development?

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Nuclei from almost any source may be injected into Xenopus oocytes. Studies have shown that these nuclei remain active in transcription and translation. How can such an experimental system be useful in developmental genetic studies?

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The homunculus doctrine postulated that miniature adult entities are contained within the egg and merely unfold and grow to give rise to a mature organism. What sorts of isolated evidence presented in this chapter might have led to this doctrine? Why is the epigenetic theory held as correct today?

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(a) What are the imaginal disks of Drosophila? (b) When do they form, how many are there, and what structures do they form in the adult?

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Distinguish between the syncytial blastoderm stage and the cellular blastoderm stage in Drosophila embryogenesis.

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(a) What are maternal-effect genes? (b) When are gene products from these genes made, and where are they located? (c) What aspects of development do maternal-effect genes control? (d) What is the phenotype of maternal-effect mutations?

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Suppose you initiate a screen for maternal-effect mutations in Drosophila affecting external structures of the embryo and your screen identifies over 100 mutations that affect external structures. Weischaus and Schupbach estimated from their screening that there are about 40 maternal-effect genes. How do you reconcile these different results?

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(a) What are zygotic genes, and when are their gene products made? (b) What is the phenotype associated with zygotic gene mutations? Does the maternal genotype contain zygotic genes?

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List the main classes of zygotic genes. What is the function of each class of these genes?

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Experiments have shown that any nuclei placed in the polar cytoplasm at the posterior pole of the Drosophila egg will differentiate into germ cells. If polar cytoplasm is transplanted into the anterior end of the egg just after fertilization, what will happen to nuclei that migrate into this cytoplasm at the anterior pole?

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In the sea urchin, early development up to gastrulation may occur even in the presence of actinomycin D, which inhibits RNA synthesis. However, if actinomycin D is present early in development but removed at the end of blastula formation, gastrulation does not proceed. In fact, if actinomycin D is present only between the sixth and eleventh hours of development, gastrulation (normally occurring at the fifteenth hour) is arrested. What conclusions can be drawn concerning the role of gene transcription between hours six and 15?

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How can you determine whether a particular gene is being transcribed in different cell types?

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You observe that a particular gene is being transcribed during development, how can you tell whether the expression of this gene is under transcriptional or translational control?

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Define what is meant by a homeotic mutant. If it were possible to introduce one of the homeotic genes from Drosophila into an Arabidopsis embryo that was homozygous for a homeotic flowering gene, would you expect any of the Drosophila genes to negate (rescue) the Arabidopsis mutant phenotype? Why or why not?

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What are Hox genes? What properties do they have in common? Are all homeotic genes Hox genes?

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The homeotic mutation Antennapedia causes mutant Drosophila to have legs in place of antennae and is a dominant gain-of-function mutation. What are the properties of such mutations? How does the Antennapedia gene change antennae into legs?

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The Drosophila homeotic mutation, spineless aristapedia (ss^a) results in the formation of a miniature tarsal structure (normally part of the leg) on the end of the antenna. From your knowledge of imaginal discs, what insight is provided by ss^a concerning the role of genes during determination?

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Embryogenesis and oncogenesis (generation of cancer) share a number of features including cell proliferation, apoptosis (cell death), cell migration and invasion, formation of new blood vessels, and differential gene activity. Embryonic cells are relatively undifferentiated and cancer cells appear to be undifferentiated or dedifferentiated. Homeotic gene expression directs early development, and mutant expression leads to loss of the differentiated state or an alternative cell identity. Lewis (2000. Breast Can. Res. 2: 158-69.) suggested that breast cancer may be caused by the altered expression of homeotic genes. When he examined 11 such genes in cancers, 8 were underexpressed while 3 were overexpressed compared with controls. Given what you know about homeotic genes, what is the likelihood that they are involved in oncogenesis?

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In Drosophila, both fushi tarazu (ftz) and engrailed encode homeobox transcription factors and are capable of eliciting the expression of other genes. Both genes work at about the same time during development and in the same region to specify cell fate in body segments. To discover if ftz regulates the expression of engrailed, engrailed regulates ftz; or if both are regulated by another gene, you perform a mutant analysis. In ftz^- embryos (ftz/ftz) engrailed protein is absent; in engrailed^- embryos (eng/eng), ftz expression is normal. What does this tell you about the regulation of these two genes—does the engrailed gene regulate ftz, or does the ftz gene regulate engrailed?

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Early development depends on the temporal and spatial interplay between maternally supplied material and mRNA and the onset of zygotic gene expression. Maternally encoded mRNAs must be produced, positioned, and degraded (Surdej and Jacobs-Lorena, 1998. Mol. Cell Biol. 18: 2892-2900.). For example, transcription of the bicoid gene that determines anterior-posterior polarity in Drosophila is maternal. The mRNA is synthesized in the ovary by nurse cells and then transported to the oocyte, where it localizes to the anterior ends of oocytes. After egg deposition, bicoid mRNA is translated and unstable bicoid protein forms a decreasing concentration gradient from the anterior end of the embryo, where gap genes along the anterior half of the embryo are activated. At the start of gastrulation, bicoid mRNA has been degraded. Consider two models to explain the degradation of bicoid mRNA: (1)Degradation may result from signals within the mRNA (intrinsic model), or (2)degradation may result from the mRNA' s position within the egg (extrinsic model). Experimentally, how could one distinguish between these two models?

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Formation of germ cells in Drosophila and many other embryos is dependent on their position in the embryo and their exposure to localized cytoplasmic determinants. Nuclei exposed to cytoplasm in the posterior end of Drosophila eggs (the pole plasma) form cells that develop into germ cells under the direction of maternally derived components. Amikura et al. (2001. Proc. Nat. Acad. Sci. (USA) 98: 9133-38.) consistently found mitochondria-type ribosomes outside mitochondria in the germ plasma of Drosophila embryos and postulated that they are intimately related to germ-cell specification. If you were studying this phenomenon, what would you want to know about the activity of these ribosomes?

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One of the most interesting aspects of early development is the remodeling of the cell cycle from rapid cell divisions, apparently void of G1 and G2 phases, to slower cell cycles with measurable G1 and G2 phases and checkpoints. During this remodeling, maternal mRNAs that specify cyclins are deadenylated and zygotic genes are activated to produce cyclins. Audic et al. (2001. Mol. and Cell. Biol. 21: 1662-71.) suggest that deadenylation requires transcription of zygotic genes. Present a diagram that captures the significant features of these findings.

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In studying gene action during development, it is desirable to be able to position genes in a hierarchy or pathway of action to establish which genes are primary and in what order genes act. There are several ways of doing this. One is to make double mutants and study the outcome. The gene fushi-tarazu (ftz) is expressed in early embryos at the seven-stripe stage. All of the genes involved in forming the anterior-posterior pattern affect the expression of this gene, as do the gap genes. However, expression of segment-polarity genes is affected by ftz. What is the location of ftz in this hierarchy?

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A number of genes that control expression of Hox genes in Drosophila have been identified. One of these homozygous mutants is extra sex combs, where some of the head and all of the thorax and abdominal segments develop as the last abdominal segment. In other words, all affected segments develop as posterior segments. What does this phenotype tell you about which set of Hox genes is controlled by the extra sex combs gene?

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The apterous gene in Drosophila encodes a protein required for wing patterning and growth. It is also known to function in nerve development, fertility, and viability. When human and mouse genes whose protein products closely resemble apterous were used to generate transgenic Drosophila (Rincon-Limas et al. 1999. Proc. Nat. Acad. Sci. (USA) 96: 2165-70.), the apterous mutant phenotype was rescued. In addition, the whole-body expression patterns in the transgenic Drosophila were similar to normal apterous. (a) What is meant by the term rescued in this context? (b) What do these results indicate about the molecular nature of development?

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In Arabidopsis, flower development is controlled by sets of homeotic genes. How many classes of these genes are there, and what structures are formed by their individual and combined expression?

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The floral homeotic genes of Arabidopsis are MADS-box proteins, while in Drosophila, they are Hox genes, belonging to the homeobox gene family. In both Arabidopsis and Drosophila, members of the Polycomb gene family control expression of these divergent homeotic genes. How do Polycomb genes control expression of two very different sets of homeotic genes?

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Vulval development in C. elegans is initiated when two neighboring cells (Z1.ppp and Z4.aaa) interact with each other by cell-cell signaling. This signaling involves two components: a membrane-bound signal molecule and a membrane-bound receptor. Initially the cells are developmentally equivalent and produce low levels of both signal and receptor. By chance, the cell that produces more signal causes its neighbor to produce more receptor. The cell producing more signal adopts one developmental fate (anchor cell); the cell producing more receptor adopts another fate (uterine precursor). This form of cell-cell interaction is called the Notch/Delta signaling system and is widely used in metazoan organisms in blood cell development, neurogenesis, retinal development, and other pathways of differentiation. Although it is a widely used signaling mechanism, this pathway works only in adjacent cells. Why is this so, and what are the advantages and disadvantages of such a system?

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The identification and characterization of genes that control sex determination has been another focus of investigators working with C. elegans. As with Drosophila, sex in this organism is determined by the ratio of X chromosomes to sets of autosomes. A diploid wild-type male has one X chromosome, and a diploid wild-type hermaphrodite has two X chromosomes. Many different mutations have been identified that affect sex determination. Loss-of-function mutations in a gene called her-1 cause an XO nematode to develop into a hermaphrodite and have no effect on XX development. (That is, XX nematodes are normal hermaphrodites.) In contrast, loss-of-function mutations in a gene called tra-1 cause an XX nematode to develop into a male. Deduce the roles of these genes in wild-type sex determination from this information.

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Based on the information in Problem 29 and the analysis of the phenotypes of single- and double-mutant strains, a model for sex determination in C. elegans has been generated. This model proposes that the her-1 gene controls sex determination by establishing the level of activity of the tra-1 gene, which in turn, controls the expression of genes involved in generating the various sexually dimorphic tissues. Given this information, (a) does the her-1 gene product have a negative or a positive effect on the activity of the tra-1 gene? (b) What would be the phenotype of a tra-1, her-1 double mutant?

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