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?

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?

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?

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?

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?

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?

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?

The Hoxd9–13 genes are thought to specify digit identity. 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?

The Hoxd9–13 genes are thought to specify digit identity. What would be the consequence of ectopically expressing Hoxd10 throughout the developing mouse limb bud? What about Hoxd11? What about both Hoxd10 and Hoxd11?