Chapter 19, Problem 35P

In Drosophila with loss-of-function mutations affecting the Ubx gene, transformations of body segments are always in the anterior direction. That is, in bx mutants, the anterior compartment of T3 is transformed into the anterior compartment of T2, whereas in pbx mutants, the posterior compartment of T3 is transformed into the posterior compartment of T2 (Fig. 19.26). In wild type, the Ubx gene itself is expressed in posterior T2–anterior A7 (PS5–PS12) and most strongly in posterior T3–anterior A1 (PS6). (See Fig. 19.29.)

a.	The Abd-B gene is transcribed in the abdominal parasegments 10–13. Assuming the mode of function of Abd-B is the same as that of Ubx, what is the likely consequence of homozygosity for a null allele of Abd-B (that is, what segmental or parasegmental transformations would you expect to see)?
b.	Because Abd-A is expressed in parasegments 7–12, all three genes of the BX-C (Ubx, Abd-A, and Abd-B) are transcribed in parasegments 10–12 (see Fig. 19.29). Why then are the abdominal parasegments 10, 11, and 12 morphologically distinguishable?
c.	What parasegment transformations would you expect to see in an animal deleted for all three genes of the BX-C (Ubx, Abd-A, and Abd-B)?
d.	Certain Contrabithorax mutations in the BX-C cause transformations of wing to haltere. Propose an explanation for this phenotype based on the transcription of the Ubx gene in particular parasegments. Do you anticipate that Contrabithorax mutations would be dominant or recessive to wild type? Explain.