Question

A) How are the gradients of maternal and zygotic proteins established during early embryogenesis in Drosophila?

B) Explain how these genes control the expression of GAP genes along the AP axis of Drosophila.

Answer 1

A ) Maternal genes establish the body axes.
Maternal gene products, mRNAs, and proteins are expressed in the ovary.
Zygotic genes are expressed by an embryo.
About fifty maternal genes set up the A/P and D/V axes: the framework of positional information (spatial distributions of RNA and proteins).
Zygotic genes respond to maternal gene expression.
First broad regions are established, then smaller domains (with a unique set of zygotic gene activities) in a hierarchy of gene activity.

B) Three classes of maternal genes set up the A/P axis
Maternally expressed genes distinguish the anterior from the posterior.
Maternal effect mutants result in females that can not produce normal progeny.
Three mutant classes are 1) anterior, 2) posterior and 3) terminal classes.
Anterior class: loss of head and thorax (sometimes replaced with posterior).
Posterior class: loss abdominal segments.

The other important function of the gradients of Bicoid, Hunchback, and Caudal proteins is in the transcriptional regulation of other zygotically expressed proteins. Many of these are the protein products derived from members of the "gap" family of developmental control genes. giant, huckebein, hunchback, knirps, Krüppel and tailless are all gap genes
Terminal class: missing Acron and telson.
bicoid, hunchback, Nanos, and caudal are key to A/P axis.bicoid is sequestered in the oocyte during oogenesis.
bicoid sets an A/P morphogenic gradient and controls the first steps in embryo development and, thus, is essential to the developing organism.
bicoid mRNA is localized to the anterior end of the unfertilized egg.
After fertilization, the mRNA is translated and a concentration gradient is formed along the A/P axis.
bicoid was the first evidence of a morphogen gradient.