Basically, we had to collect virgin flies of different stocks, and cross them with males of a different stock to see expression of various genes, the aim being to study the effect of one gene on another. The first question that almost everyone would ask me and that came to my mind almost at once too was: how do we tell the virgins apart? Turns out it wasn’t so difficult - virgins have a swollen abdomen and pigmented spots. What’s more intriguing is that flies don’t mate 7-8 hours after hatching, thus the best time to collect virgins would be right after they hatch. Early morning and evening are the times when the eggs hatch most and virgin flies are likely to be available. Why virgins? In order to study the effect of two traits, our offspring needs to be only from the cross we plan on studying, any non-virgin may give an offspring that doesn’t result from our cross and only obscures our results.
Telling the males and females apart wasn’t so difficult either. Males have a darkened tip of the abdomen whereas females have dark stripes on their abdomen. Moreover, males have black spots on their front most legs whereas no such spots exist in the males. What was sad was that the naughty females (non-virgins) were killed by throwing them in a grave (a bottle of some liquid that killed the flies), since they weren’t needed for any study. What was amusing was the way they would become anesthetized using CO2 and then whatever work had to be done could be done on them easily. However, too much carbon dioxide could be fatal to them and hence had to be administered carefully. They quickly gained consciousness and had to be given CO2 in small bursts. Once when I let a whole tube of flies escape I learned there was no point in trying to kill the escaped flies since they wouldn’t be able to survive for more than two hours in the Lahore heat – ordinarily, they live and survive at 25°C and hence, are stored and kept in an incubator all the time.
Then, there’s the question of how do they do it? They do it like all other creatures do it! One curious friend was very sympathetic to the flies because he thought we forced the flies to mate. One doesn’t force them to mate; one only gives them access to limited kinds of themselves i.e. the particular stocks we need to cross. Whatever they do, they do of their own accord.
The most important question I was asked was: why study flies? What good would that do? I’d like to answer that question by quoting Ryder and Russell from their paper published in Briefings in Functional Genomics and Proteomics in 2003 entitled Transposable Elements as tools for Genomics and Genetics in Drosophila:
“With over 70 % of the 1378 human disease genes defined by Online Mendelian Inheritance in Man (OMIM) as having one or more orthologues/paralogues in Drosophila genome, it is hoped that studies in Drosophila can act as a Rosetta Stone for unlocking the secrets of how human embryonic development progresses and how with the use of fly models, diseases such as Alzheimer’s and Parkinson’s are caused.”