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Modern Genetic Analysis Chapter 16. The Genetic Basis of Development
Solved Problem

1. | In developmental pathways, the crucial events seems to be the activation of master switches that set in motion a programmed cascade of regulatory responses. Identify the master switches and explain how they operate in the two examples of sex determination and differentiation discussed in this chapter: (a) sex determination in Drosophila and (b) sex determination in mammals. See answer

Solution

a.In Drosophila sex determination, the master switch is the transcriptional regulation of the Sxl gene during early embryogenesis. The properties of the SXL protein and the existence of a constitutive late promoter for Sxl ensure that, when active SXL protein is produced, an autoregulatory loop will continue to maintain SXL protein activity in the cell. SXL protein will also initiate a downstream set of regulated RNA splicing events, culminating in the production of dsx-F mRNA. In the absence of SXL, the default RNA splicing machinery will produce dsx-M mRNAs. The key then is whether or not the Sxl switch is flipped on or stays off. This is controlled by the level of active numerator-encoded transcription factors. The higher level of these transcription factors conferred by an X:A ratio of 1 is necessary to activate early Sxl transcription and set the female developmental pathway in motion. In the absence of these higher levels of numerator-encoded transcription factors, development continues along the default pathway leading to male development.

b.In mammalian sex determination, the master switch is the presence or absence of the SRY~Sry gene, which is ordinarily located on the Y chromosome. In the presence of the protein product of this gene, which acts as a DNA-binding protein, certain cells of the gonad (Leydig cells) synthesize androgens, male-inducing steroid hormones. These hormones are secreted into the bloodstream and act on target tissues to induce the transcriptionfactor activity of the androgen receptors. In the absence of androgen-receptor activation, development proceeds along the default pathway leading to female development. The factors that activate SRY~Sry expression in the testis are not understood. Because the master switch here is the actual presence or absence of the SRY~Sry gene itself, it is likely that the regulatory molecules that activate SRY~Sry are present in the indifferent gonad early in development.

See question

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