The Drosophila homeotic mutation spineless aristapedia (ssᵃ) results in the formation of a miniature tarsal structure (normally part of the leg) on the end of the antenna. What insight is provided by (ssᵃ) concerning the role of genes during determination?

Embryogenesis and oncogenesis (generation of cancer) share a number of features including cell proliferation, apoptosis, 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. M. T. Lewis [(2000). Breast Can. Res. 2:158–169] 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, could they be involved in oncogenesis?

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 plasm) form cells that develop into germ cells under the direction of maternally derived components. R. Amikura et al. [(2001). Proc. Nat. Acad. Sci. (USA) 98:9133–9138] 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?

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?

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?

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. 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?

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