Hi, my name’s Daisy the Drosophila.
Some people may know us as fruit flies, but personally, I prefer vegetables.
Today, I want to tell you about how scientists uncovered tools which would later help them
understand more about my genome.
Gene transfer has been at the heart of genetic research for numerous decades where the discovery
of transposons in maize by Barbara McClintock in the mid 20th century led to the discovery
of transposons in prokaryotes in the 1970s, paving the way for the development of tools
to transfer genetic information between prokaryotes.
Before we get ahead of ourselves, transposons are DNA sequences flanked by terminal inverted
repeats.
The enzyme, transposase, is responsible for cutting the transposon out of one section
at the terminal inverted repeats and pasting it into another section.
This gives transposons the ability to move from one location of a genome to another.
You might think, “Seems easy enough, if transposable elements exist in prokaryotes,
scientists just have to search for them in eukaryotes”, right?
Well, it wasn’t as easy as it took almost ten more years until eukaryote research caught
up when scientists noticed a strange syndrome in Drosophila.
They noticed that when males carrying a DNA sequence called a P element were crossed with
females lacking P elements, their offspring displayed abnormal genetic traits such as
sterility, mutations and chromosomal rearrangements.
This syndrome was termed ‘hybrid dysgenesis’ and the culprit was believed to be P elements.
They suspected that when P elements were introduced into germ line cells from the male fly, this
caused transposition of a smaller P element, disrupting other genes in the genome and inducing
mutations.
They finally discovered transposons in eukaryotes and it was all because of flies like me!
Oops!
But why should we even care about transposons?
Scientists believed that if they could identify a natural vehicle for gene transfer like the
P element, they could use these as vectors to transfer genes of interest into Drosophila.
Scientists Gerald M. Rubin and Allan C. Spradling did just this, endeavouring to prove the efficiency
and stability of their gene transfer method.
It involved creating a transposon with a gene, flanked by P element terminal inverted repeats
and co-injecting this with a fully functional P element into Drosophila embryos whick lacked
P elements.
To test the success of their gene transfer system, the rosy gene was used as a genetic
marker and only flies homozygous for the mutant rosy allele were injected.
If successful gene transfer occurred, the progeny of the injected flies would have restoration
of their wildtype red eye colour.
If gene transfer had not occurred, the flies would still have the abnormal brown eye colour,
indicating they were homozygous for the mutant rosy allele.
Of the 40 fertile adults derived from the injected embryos, 18 gave progeny with red
eyes, showing that successful transfer of the gene had occurred.
This confirmed their hypothesis that elements code for the enzyme transposase, acting at
the P element terminal inverted repeats to facilitate the transposition of the gene in
between.
They then generalised this method by proving any gene of interest could be transferred
by co-injecting a transposon containing the DNA of interest along with a rosy transposon
as a phenotypic screen alongside the P element.
This method of gene transfer has further been developed into a tool for generating different
types of mutations by using specifically designed P elements.
This proved influential to the field of genomics with the completion of the Drosophila melanogaster
sequence in 2000 using this method to insert mutations to tag thousands of genes, with
scientists discovering that P elements could mutate a quarter of the vital Drosophila genes.
And due to the homology between some Drosophila and human genes, this proved to be a stepping
stone to the completion of the human genome sequence in 2003!
Thanks for listening today and I hope you learnt about how Drosophila like me have single-handedly
saved the world!