The course is arriving to an end. The students are working on their own projects, doing experiments and searching scientific literature to develop their proposals. Going around the class and asking them you can see how they are getting it. If you ask, at the beginning they hesitate. If you let them talk, the story starts to make sense. They just need a bit of guidance. Sometimes they make mistakes, who hasn’t! But that’s science: you do it, you do it again, and again and again, and in the end you can do it almost with your eyes closed. Tomorrow they will start the presentations and on Saturday everything will come to an end. And end with promise to be continued: more flies, some molecular biology, plans for the future of East Africa… I can’t believe this is over.
I’ve taken the chance today to play around with the pictures I took over a week ago with the intention of illustrating the work with flies which, in the end, is the reason for us to be here. The pictures are a bit pathetic due to the precarious photographic technique (my small digital camera looking through the ocular of the dissection scope) and the equally precarious shape of the scope (no complains! they were very valuable donations!). Excuse the low quality of the pictures and take the chance, if at all possible, to learn something.
A cornerstone of fly work is differentiating males and females since the arranged crosses are at the base of genetics. Males have their bottom black and they have two small black structures in the anterior pair of legs called sex combs. They use these combs in the sexual courtship of the female.
In order to make sure that we get only the progeny that we want out of each cross, it is fundamental that the females are virgins. Virgin flies are distinguishable from the older ones because their pigmentation is lighter and in their abdomen the meconium is still visible. They are frequently bigger than older flies because they are like inflated.
The light pigmentation of the virgins is obvious even without the scope and it’s easier to recognize when there are older flies around to compare them with. You should be careful, though, when working with flies of different genetic background since there are mutations that make the pigmentation lighter and old flies can look like virgins to the untrained eye.
The flies that have just hatched are very easy to recognize since they still have their wings folded and they are not bloated.
When we need many many flies to set up a cross (because we need a lot of progeny for our next experiment) it is necessary to work with the help of temperature to get the most out of the clock. The optimum temperature for fly development is 25ºC. At this temperature the flies reach sexual maturity in six hours (I could maybe say eight, but I prefer not to risk it). When kept at 19ºC, development slows down and that time doubles. The maths are easy: if you want every fly that comes out of a tube (or bottle), the usual is to collect them early in the morning, after lunch and right before you leave in the evening. You keep them at 25ºC during the day and at 19ºC at night, and you don’t want to leave early to make sure that they are still virgins in the morning. Taking into account that a female can lay around 500 eggs, if you collect all the progeny from a cross with four or five females, with the normal proportion of 1:1 male:females, you can see how in a week you can get a considerable number of flies. Most of the times you don’t need so much hassle.
If we can’t collect virgins so often and we want to increase the number of flies available for the cross we can select females at the late pupal stage. It’s more difficult to see them, but the same sex combs present in the adult male can be seen at this stage. I know the cartoons are pathetic but I already said I am no artist, right? This technique is also useful to select males and females that need to be kept isolated from the very beginning for a behavioral experiment.
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