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.

Describe 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. embryosthatlackthefunctionofthepair-rulegeneeven-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

Green florescent protein (GFP) is used to monitor gene expression and protein localization in living cells,particularly in developing embryos from a variety of species.GFP expression has been used extensively to study developmental processes in zebrafish,chickens,Drosophila,mic,ascidians andetc.GFP first describe by Prasher and his collagues(1992).Germ band extension in Drosophila embryos has served as a paradigm for body axis elongation.Drosophila embryonic germ band extension is an important morphogenetic process during embryogenesis,by which the lenght of the germ band is more than doubled along the anterior posterior axis.Mesodermal invagination is the first morphogenetic movement during Drosophila gastrulation.it is initiated by apical cell contraction of mesodermal precursor cells at ventral side of embryo.Mesodermal invagination,cephalic furrow formation,endoderm invagination and germ band extension ocvur concomitantly.it is however,well knowm thst germ band extension is independent of mesoderm invagination,since it timely proceeds in absence of medoder invagination.(1)..In wild type embryos,germ band extension over with mesoderm internalization for several minutesThe displacement of germ band cells in first few minutes proceed as in wild type embryos.Thus ,no obvious influence of mesoderm movements on germ band extension has been observed.Some signs of germ band extension can be observed in absence of cell intercalation in mutants with impaired planar polarity,such as mutants affecting the gap gene Kruppel,the pair rule genes,and the polarity gene Bazooka/Par3(2)...Bicoid and Hunchback are the matenal effect genes that are most important for patterning of anterior parts(head&thorax)of Drosophila embryos.In embryos from bicoid mutant mothers,the heaf and thoracic structure are converted to the abdomen making embryo with posterior structure on the both ends,a lethal phenotype.The Bicoid ,Huchback, and caudal proteins are transcription factors.The Bicoid is morphogen as well Bicoid has a DNA binding homeodomin that binds both DNA and the nanos mRNA.Bicoid binds specific RNA sequence in the 3' untranslated region,called the Bicoid3'UTR regulatory element,of caudal mRNA and blocks translation(3)..The pair rule gene even skipped is required for initiation of metameric pattern in Drosophila. But Drosophila segmentation is evolutionary derived & is not representative of most insects.Expression and function of the homolog of Drosophila even skipped from intermediate germband insects Oncopeltus fasciatus.In Oncopeltus,even skipped striped expression initiates in a segmental and not pair rule pattern.Weak RNAi suppression of Oncopeltus even skipped shows no apparent pair rule like phenotype,while strong RNAi suppression shows deletion of nearly entire body.These result suggests that in Oncopeltus,is not acting as pair rule gene.The engrailed gene is important for segmentation of Drosophila embryo.