AP axis by bicoid and nanos

Question 1

How AP axis genes identified?

Answer 1

Maternal genes responsible for establishment of Drosophila axes were identified using mutants by Nusslein-Volhard and Weishaus. Found anterior, posterior and terminal mutants and hence were able to identify the affected genes.

Question 2

What were the defects of anterior mutants?

Answer 2

Had tip of the head but not the whole head

Question 3

What are the anterior genes?

Answer 3

Bicoid, hunchback, exuperantia, swallow, staufen

Question 4

What were the defects of posterior mutants?

Answer 4

Missing large parts of abdomen

Question 5

What are the posterior genes?

Answer 5

Nanos, tudor, oskar, vasa, valois, pumilo, caudal

Question 6

What were the defects of terminal mutants?

Answer 6

Lack most of anterior and posterior structures

Question 7

What are the terminal genes?

Answer 7

Torso and trunk

Question 8

Which of these genes are most critical for formation of AP axis?

Answer 8

Bicoid and nanos

Question 9

Where is bicoid mRNA located? Why?

Answer 9

At the anterior tip of the unfertilised egg due mRNA complex attaching to dynein proteins once in oocyte that are maintained at MTOC at anterior end.

Question 10

Where is nanos mRNA located? Why?

Answer 10

Trapped at posterior end of oocyte by passive diffusion, binds to cytoskeleton through 3’ UTR.

Question 11

What happens to bicoid and nanos mRNA?

Answer 11

Bicoid and nanos mRNAs get translated into protein in the egg. The proteins acts as morphogens - diffuse in the syncitial blastoderm forming opposing gradients that are critical for AP patterning.

Question 12

How do we know Bicoid is related to AP patterning?

Answer 12

• Bicoid gradient is highest at the anterior - suggest anterior development
• Embryos lacking bicoid have no head.
• Embryos injected with bicoid mRNA get ectopic head at site of injection. (Area around injection becomes thorax, supports concentration dependent signal).

Question 13

How do we know Nanos is related to AP patterning?

Answer 13

• Highest at posterior end
• If no nanos, no abdomen.

Also represses translation of uniformly distributed maternal gene hunchback and so hunchback is anteriorly expressed.

Question 14

Summarise

Answer 14

Nanos and Bicoid are maternal mRNA distributed at opposite ends of the oocyte, therefore when translated, the proteins produced act as morphogens. They create opposing gradients, the concentrate of which determine AP polarity.

They target zygotic segmentation genes, therefore differing concentrations of the proteins determine expression of different zygotic genes. The zygotic genes are responsible for segmental identity.