SOLVED PROBLEM 1. The Bicoid gene (bcd) is a maternal-effect gene required for the development of the Drosophila anterior region. A mother heterozygous for a bcd deletion has only one copy of the bcd gene. With the use of P elements to insert copies of the cloned bcd+ gene into the genome by transformation, it is possible to produce mothers with extra copies of the gene. The early Drosophila embryo develops an indentation called the cephalic furrow that is more or less perpendicular to the longitudinal, anteroposterior (A–P) body axis. In the progeny of mothers with only a single copy of bcd+, this furrow is very close to the anterior tip, lying at a position one-sixth of the distance from the anterior to the posterior tip. In the progeny of standard wild-type diploids (having two copies of bcd+), the cephalic furrow arises more posteriorly, at a position one-fifth of the distance from the anterior to the posterior tip of the embryo. In the progeny of mothers with three copies of bcd+, it is even more posterior. As additional gene doses are added, the cephalic furrow moves more and more posteriorly, until, in the progeny of mothers with six copies of bcd+, it is midway along the A–P axis of the embryo. Explain the gene-dosage effect of bcd+ on the formation of the cephalic furrow in light of the contribution that bcd makes to A–P pattern formation.
The determination of anterior–posterior parts of the embryo is governed by a concentration gradient of Bicoid protein. The furrow develops at a critical concentration of bcd. As bcd+ gene dosage (and, therefore, Bicoid protein concentration) decreases, the furrow shifts anteriorly; as the gene dosage increases, the furrow shifts posteriorly.