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Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Forming Complex Pattern: Establishing Positional Information
Genetics In Process 16-4: Studying the BCD gradient

How do we know that molecules such as BCD and HB-M contribute AP positional information? Let's consider the example of BCD in detail. First, genetic changes in the bcd gene alter anterior fates. Embryos derived from bcd homozygous null mutant mothers lack anterior segments (see the adjoining micrograph). If, on the other hand, we overexpress bcd in the mother, by increasing the number of copies of the bcd+ gene from the normal two to three, four, or more, we "push" fates that ordinarily appear in anterior positions into increasingly posterior zones of the resulting embryos (as the first diagram shows). These observations suggest that BCD protein exerts global control of anterior positional information.

Second, bcd mRNA can completely substitute for the anterior determinant activity of anterior cytoplasm (see the second diagram). If one removes the anterior cytoplasm from a punctured syncitial embryo, anterior segments (head and thorax) are lost. Injection of anterior cytoplasm from another embryo into the anterior region of the anterior cytoplasm-depleted embryo restores normal anterior segment formation, and a normal larva is produced. Similarly, synthetic bcd mRNA can be made in a test tube and injected into the anterior region of an anterior cytoplasm-depleted embryo. Again, a normal larva is produced. Unlike that of anterior cytoplasm, transplantation of cytoplasm from middle or posterior regions of a syncitial embryo does not restore normal anterior formation. Thus, the anterior determinant should be located only at the anterior end of the egg. As already discussed, this is exactly where bcd mRNA is found.

Third, also as already discussed, the BCD protein shows the predicted asymmetric and graded distribution to fulfill its role of establishing positional information.

© 1999 by W. H. Freeman and Company. All rights reserved.