Transgenesis in Drosophila melanogaster

The wide array of genetic tools available for Drosophila makes it one of popular eukaryotic model system to study numerous biological questions concerning development, genetics,evolution, cell biology, neuroscience and disease. We so far talked about UAS GAL4 system and P element mutagenesis and today we look into another very pivotal genetic tool used to study gene function. After the genome sequence of Drosophila was released in 2000, it has been realized that only 20% out of total (~13600 genes) could be identified by available techniques.The aim of present era of genomics is to identify and characterize all the genes relevant to specific biological process.

The fly’s genome permits the most sophisticated manipulations of any of the known eukaryotes. Drosophila, with its powerful repertoire of genetic tools has contributed a lot in understanding of gene function in various biological pathways. One being the method to identify gene of interest is the way of chemical mutagenesis screen and other fruitful approach was use of P-element-mediated germ-line transformation. (It can be combined with UAS GAL4 system also).

Drosophila transgenesis mainly relies on P-element which was for the first time used by Gerry Rubin and Alan Spradling in 1982. The use of p element for transformation was a major breakthrough in the germline transgenesis in Drosophila.

Technique:

The transgene (e.g. UAS gene X) needed to be transformed is first cloned in P element vector. This vector contains a marker (mini white gene) which will act as indicator when the transgene gets integrated in fly genome.

The cloned vector (plasmid) along with Helper plasmid , which is the source for transposase are injected in pole cells (posterior end) of less than one hour old Drosophila embryo.These embryos will be from a wild type fly except for the eye colour, meaning this fly will have white eyes instead of red.

When the pole cells take up the transgene one could see a fly with Red eyes in the next or F1 generation (because the plasmid injected gets integrated into the germ cells, which eventually forms gametes. So the fly which hatch after injection of embryos, will have transgene integrated in their gametes and when this fly is mated with wild type white eyed flies then some of progeny show red eyes because of integration of transgene (F1 generation).

Drosophila research has been highly dependent on P element-mediated transgenesis, even though it has two major drawbacks: the size of the DNA that can be integrated is limited and the insertion sites cannot be controlled.

Reference:

Rubin, G. M. and Spradling, A. C. (1982). Genetic transformation of Drosophila
with transposable element vectors. Science 218, 348-353.