14. Genetic Control of Development

Developmental Patterning Genes

Open Question
The pair-rule gene fushi tarazu is expressed in the seven even-numbered parasegments during Drosophila embryogenesis. In contrast, the segment polarity gene engrailed is expressed in the anterior part of each of the 14 parasegments. Since both genes are active at similar times and places during development, it is possible that the expression of one gene is required for the expression of the other. This can be tested by examining expression of the genes in a mutant background—for example, looking at fushi tarazu expression in an engrailed mutant background, and vice versa.Given the hierarchy of gene action during Drosophila embryogenesis, what might you predict to be the result of these experiments?
Open Question
The pair-rule gene fushi tarazu is expressed in the seven even-numbered parasegments during Drosophila embryogenesis. In contrast, the segment polarity gene engrailed is expressed in the anterior part of each of the 14 parasegments. Since both genes are active at similar times and places during development, it is possible that the expression of one gene is required for the expression of the other. This can be tested by examining expression of the genes in a mutant background—for example, looking at fushi tarazu expression in an engrailed mutant background, and vice versa.Based on your prediction, can you predict the phenotype of the fushi tarazu and engrailed double mutant?
Open Question
In contrast to Drosophila, some insects (e.g., centipedes) have legs on almost every segment posterior to the head. Based on your knowledge of Drosophila, propose a genetic explanation for this phenotype, and describe the expected expression patterns of genes of the Antennapedia and bithorax complexes.
Open Question
One of the most interesting aspects of early development is the remodeling of the cell cycle from rapid cell divisions, apparently lacking 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. 21:1662–1671] suggest that deadenylation requires transcription of zygotic genes. Present a diagram that captures the significant features of these findings.
Open Question
The bristles that develop from the epidermis in Drosophila are evenly spaced, so that two bristles never occur immediately adjacent to each other. How might this pattern be established during development?
Open Question
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?
Open Question
You are traveling in the Netherlands and overhear a tulip breeder describe a puzzling event. Tulips normally have two outer whorls of brightly colored petal-like organs, a third whorl of stamens, and an inner (fourth) whorl of carpels. However, the breeder found a recessive mutant in his field in which the outer two whorls were green and sepal-like, whereas the third and fourth whorls both contained carpels. What can you speculate about the nature of the gene that was mutated?
Open Question
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) 96:2165–2170], the apterous mutant phenotype was rescued. In addition, the whole-body expression patterns in the transgenic Drosophila were similar to normal apterous.

What do these results indicate about the molecular nature of development?
Open Question
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) 96:2165–2170], the apterous mutant phenotype was rescued. In addition, the whole-body expression patterns in the transgenic Drosophila were similar to normal apterous.

What is meant by the term rescued in this context?
Open Question
A powerful approach to identifying genes of a developmental pathway is to screen for mutations that suppress or enhance the phenotype of interest. This approach was undertaken to elucidate the genetic pathway controlling C. elegans vulval development.In a complementary experiment, a gain-of-function let-23 mutant with a multi-vulva phenotype was also mutagenized. What types of mutations will suppress the multi-vulva phenotype?
Open Question
A powerful approach to identifying genes of a developmental pathway is to screen for mutations that suppress or enhance the phenotype of interest. This approach was undertaken to elucidate the genetic pathway controlling C. elegans vulval development.A lin-3 loss-of-function mutant with a vulva-less phenotype was mutagenized. Based on your knowledge of the genetic pathway, what types of mutations will suppress the vulva-less phenotype?
Open Question
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?
Open Question
The Hoxd9–13 genes are thought to specify digit identity (see Figure 18.18).You wish to examine the effect of loss-of-function alleles in developing limbs. How would you construct a mouse in which the function of Hoxd9–13 is retained during anterior–posterior embryonic patterning but is absent from developing limbs?
Open Question
The Hoxd9–13 genes are thought to specify digit identity (see Figure 18.18).What would be the consequence of ectopically expressing Hoxd10 throughout the developing mouse limb bud? What about Hoxd11? What about both Hoxd10 and Hoxd11?
Open Question
The floral homeotic genes of Arabidopsis belong to the MADS-box gene family, while in Drosophila, homeotic genes belong 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?