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

2.

In Drosophila, you have identified a new mutation, mll (malelike), that causes flies with an X:A ratio of 1 to develop as phenotypic males. You want to understand how the product of the mll gene acts in the developmental pathway for Drosophila sex determination and differentiation. You measure the presence of functional SXL, TRA, DSX-F, and DSX-M protein in animals with an X:A ratio of 1. In each of the following cases, propose a role for MLL protein in the sex determination and differentiation pathway.
a. You observe that functional early SXL, late SXL, TRA, and DSX-M proteins are produced.
b. You observe that functional early SXL and DSX-M are produced. No functional late SXL or TRA is produced.
c. The only functional protein that you observe is DSX-M.

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Solution

It is actually through experiments like these that the sequential pathway of sex determination and differentiation in Drosophila was elucidated. More generally, it is through observations like these, where the effects of a mutant in one gene on the mRNA or protein expression of another gene are studied, that developmental pathways are pieced together.

a.Given that SXL and TRA proteins are still operating normally in the context of an X:A ratio of 1, whereas the alternative splicing of dsx mRNA is leading to the male-specific form of the protein, it is likely that mll contributes to dsx RNA-splicing regulation.

b.Here, the block seems to be between early and late SXL protein production. This finding may indicate that mll plays a role in the autoregulation of late Sxl alternative splicing.

c.In this example, the block is before Sxl expression in the early embryo. One possible explanation is that MLL protein contributes to the numerator function in interpreting the X:A ratio.

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