Question

A Drosophila transgene has coding sequences for GFP positioned downstream of a 7-kb DNA fragment taken from the segment-polarity gene wingless (wg). In Drosophila embryos, the transgene expresses GFP fluorescence in exactly the same pattern as Wg. Crosses are used to place the transgene in different mutant backgrounds.

a. Describe, with words and diagrams, the pattern of GFP fluorescence you expect to see in each of the following types of embryos if they are examined at germ-band extension.

i. wild-type

ii. embryos laid by a mother who lacks the function of the maternal-effect gene bicoid

iii. embryos that lack the function of the pair-rule gene even-skipped

iv. embryos that lack the function of the segment-polarity gene engrailed

v. embryos having a mutation in the Hox gene Ultrabithorax that leads to a homeotic transformation of T3, the third thoracic segment which normally has a haltere, to T2, the second thoracic segment which normally has a wing.

Answer 1

i)

• In wild type, germ band extension in wingless develops certain patterns.
• Mesoderemal invagination takes place.
• Both Mesoderm invagination and germ band extension are independent process.
• But both processes overlaps.
• Cell shape changes and invagination of ventral groove takes place.

ii)

• In these embryos no germ band extension takes place.
• Here no posterior midgut invagination takes place.as the posterior pole is blocked in these embryos.
• The anterior posterior pattern is defective and so the directionality is also defective

iii)

• when pair rule genes like hairy is defective , patterning will also be defective .
• There wont be cell intercalation .

iv)

• Segment polarity genes shows defective patterning in neural tissues.

v)

• Hox gene mutation causes improper orientation of body segments.
• For eg . GFP shows limb proteins are localised at different positions.