Temperature, the cellular immune response of the host and host selection by a parasitoid


Insects can defend themselves against pathogens and parasites by a cellular immune response. Recognition of a foreign body in the hemolymph results in aggregation of certain types of blood cells around the foreign body. These cells form a dense capsule in which melanine is secreted and the foreign body is killed either by asphyxiation or by poisoning.

Endoparasitoids often place their eggs in the body cavity of insect larvae and are vulnerable to the cellular immune response of the host. Endo-parasitoids have evolved a number of mechanisms to escape encapsulation. In several species the egg is placed inside the brain, or under the cells lining of the gut and escape in such a manner exposure to the host's immune response. Other parasitoids produce viruslike particles, which are injected into the host at oviposition. The viruslike particles are instrumental in killing blood cells and preventing their multiplication. Yet other parasitoids have a protein coating on the outside of the egg, which makes that the egg is not recognized as foreign. Finally, some parasitoids have a coating of sticky proteins on the egg, which make the egg adhere to gut and fat tissue of the host. Blood cells can no longer reach the parts of the egg covered by fat tissue or gut, and encapsulation of the egg is no longer possible.

To escape encapsulation for parasitoids is often a race against the clock and the outcome of the battle between the immune system of the host and the ploys of the parasitoid to escape encapsulation may therefore depend on the temperature. In the parasitoid-host relation between Drosophila melanogaster and Asobara tabida, an increase in rearing temperature from 20°C to 25°C resulted in a significant increase in encapsulation of the parasitoid's eggs (van Alphen & Drijver, 1982) In western Europe, the preferred host of Asobara tabida is D.subobscura. This species lacks the ability to encapsulate parasitoid eggs and is therefore a high quality host for

A.tabida. The cosmopolitan D.melanogaster, of African origin, is also used as host. In northern Europe, this host is only accepted when D.subobscura is rare. In southern Europe, D.subobscura is often absent during the warm summer months, and D.melanogaster is more often used as host. Southern populations of A.tabida are better in evading the immune response of D.melanogaster, but still may suffer significant egg mortality by encapsulation of the eggs (Kraaijeveld & van Alphen, 198?).


In this project we want to investigate the potential effects of a warmer climate on the rate of encapsulation of A.tabida eggs by Drosophila melanogaster. We predict that encapsulation rate will increase with temperature, as found before. If indeed we find that encapsulation of A.tabida eggs varies with temperature, we can use the finding that the profitability of D.melanogaster as host varies with temperature to predict host species selection. A.tabida females are most often time limited (Ellers et al., 200?) hence they can maximize their fitness by maximizing the number of offspring per unit of time. We can now calculate whether an A.tabida female should accept or reject D.melanogaster larvae when foraging on a patch containing a mixture of both species of larvae. Optimal host species selection depends on the encounter rates with both D.subobscura larvae and D.melanogaster larvae, on oviposition and recognition times and on host profitability. To test the predictions of this simple host selection model We measure profitability of D.subobscura larvae and D. melanogaster larvae at 18°C, 21°C and 24°C for the Sospel strain of A.tabida. Next we allow A.tabida females of the Sospel strain to search at 21°C for D.subobscura and/or D.melanogaster larvae at different densities and measure s and m. Similar experiments at 18°C and 24°C will show if encounter rates increase with temperature or not. Using the optimal host selection model, we then calculate at which density they should begin to accept D.melanogaster larvae.

Next we do host selection experiments offering equal numbers of D.melanogaster and D.subobscura at the density where they should switch. Stochastic variation in encounter rates with both species will cause partial acceptance and partial rejection of D.melanogaster larvae in this experiment. We then repeat the experiments at 18°C and 24°C. The predicted outcome

is that rejection rates will be higher at 24°C and lower at 18°C. This tells us that A.tabida responds or not to temperature with a switch in host selection behaviour. Although finding a response would be spectacular, establishing that such a response is lacking is also useful to predict the change in impact of A.tabida on D.melanogaster populations when the climate becomes warmer.


Contact: Jacques van Alphen


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This page contains a single entry by administrator published on April 11, 2008 2:58 PM.

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