Pineapple medium or orange medium, which medium do I choose? Both of it were her favourite, zooming past the other flies, flapping her wings vigorously, Mrs. Drosophila landed on the pineapple medium. She was soon followed by Mr. Drosophila. Little did they both know that the pineapple haven which they chose to breed on had something totally amiss and that they were going to learn and remember this flavour for long time, and also pass on the experience of an bitter episode to their kids in the near future .In this blog post I have made an earnest attempt to understand views, experimental data involved in the Experimental Evolution of learning ability in fruit flies, a study conducted by Frederic Mery and Tadeusz J. Kawecki.
Learning rate and memory are two important aspects very much vital for survival and being part of an environment which is constantly at change it is necessary for any system to learn, adapt, remember and finally pass on the information to successive generations. Here in today’s post let’s discuss about the improved learning ability and better memory seen in Drosophila melanogaster when exposed to favor associative learning with regard to oviposition substrate choice. In short the experiment consisted of three phases; phase 1 known as the training period, phase2&3 called the test period respectively. Each phase was of a 3 hr duration wherein the flies were offered choice between an orange medium and a pineapple medium. The experimental population flies were supplied with both mediums, one of which contained the chemical cue quinine hydrochloride in addition to the tasty fruit pulp. From the beginning it was seen that the experimental population flies strongly avoided the quinine medium and showed drastic decrease in egg laying. The training period was meant for the flies to associate with the medium containing quinine in it and hence to learn and remember to avoid it from next generations. The 2&3rd phase known as test period was where there was no chemical cue added to either of the medium, this phase was just to determine the level of conditioning that had occurred in phase 1. For further studies, next generation population was bred from eggs laid in period 3 on the medium that had not contained the chemical cue, it was made sure that the larvae was always reared on the same medium, which precluded any kind of preference induced by larval medium. The control flies on the other hand were treated equally except they were never given quinine containing medium.
Fig1:Design of the experimental evolution:selection Regime in the experimental lines at even and odd numbered generations. Only eggs laid in medium 3 on one medium, i.e., orange in odd and Pineapple in even were used to breed next generation.
The experimental flies from period 1, which had tasted the bitter medium would have to learn to remember and thus avoid choosing the same medium without the chemical cue in period 3. Within 20 generations of selection, there was significant evolutionary changes, wherein the flies evolved and learnt to avoid the medium with quinine.
Fig 2: ‘‘Conditioned to avoid pineapple”means that quinine was present in the pineapple medium offered in period 1. The proportion of eggs laid on the orange medium was averaged over period 2 and 3.
Fig3: Comparison of rate of learning. The response of experimental and control populations to conditioning time. Solid lines, flies conditioned to avoid orange, dashed lines, flies conditioned to avoid pineapple.
The improvement in the flies with regard to avoiding the quinine containing medium, was attributed towards faster learning and longer memory. To test the decay of the conditioning response, i.e., to test the memory of the flies, the authors also conducted an Decay of conditioned Response test wherein they observed the decline of the effect of conditioning on oviposition as the time elapsed. With respect to experimental flies it was observed that the flies laid a smaller proportion of eggs than the control flies on the medium they were conditioned to avoid.
With this study the authors concluded that the flies were able to learn and avoid the bitter medium with the chemical cue and it also commented on their memory playing vital role.
In another study conducted by the same team of researchers, Olfactory Memory in the flies was investigated, wherein the flies were conditioned with an airborne odour with mechanical shock and then tested for odour choice. With olfactory shock task, it was pointed that a genetic variation was present which underlined the experimental evolution of learning performance which affected several phases of memory formation in olfactory aversive learning. Another aspect to look into was, which form of memory is involved in course of the experimental evolution. It has already been demonstrated that Drosophila’s memory works basically, with four distinct forms of olfactory memory. They are Short term memory (STM) which forms within seconds and decays within less than an hour, Middle term memory (MTM) which arises within minutes, reaches a peak at about an hour, and decays within several hours. Anesthesia-resistant memory (ARM) begins to form within 30min and can last for 24 hours and finally Long term memory (LTM) lasts for several days.
It was established by F. Mery and T.J. Kawecki and the team, that fruit flies have ability to learn, remember and transmit the acquired knowledge to the next generation when faced with variations, be it in any context, such as a particular stimuli as in orange and pineapple medium or particular context as in oviposition in cages or particular behaviour as in oviposition. This experiment established the fact that learning and memory gives rise to change, which in turn results in improvement and is beneficial to the species, all of these ultimately gives way to Evolution.
‘’Change is inevitable’’ every system strives for its own growth and development. The demonstration by the fruit fly to adapt and memorise opened up new prospects in the field of experimental Biology, putting the modest fruit fly on an apostle, making it an intelligent star in the insect world, and further pulling it up on top of the model organism chart. Three cheers for our dear fruit fly…
- Mery F. and T.J. Kawecki. 2002. Experimental evolution of learning ability in fruit flies. Proc Natl Acad Sci USA 99: 14274–14279
- Mery F. and T.J. Kawecki. 2003. A fitness cost of learning ability in Drosophila melanogaster. Proc R Soc B 270:2465–2469.
- Mery F. and T.J. Kawecki. 2005. A cost of long-term memory in Drosophila. Sci-ence 308:1148.