Environmental impact on host-parasite interactions. A study on the adaptive value of host castration and gigantism when hosts can regain reproduction

The aim of this thesis was to investigate how altering the external environment affects life history strategies of hosts and their parasites, and to gain a better understanding of evolutionary theories on castration and gigantism in parasitized hosts. We found that maintaining Daphnia magna in a high resource growth medium resulted in increased reproduction and decreased survival, suggesting that reproduction is costly for this organism. However, within growth mediums we found a positive association between reproduction and survival, suggesting that high quality individual were able to invest more in both aspects of life history compared to low quality individuals. These findings may seem contradictory, but one would only expect to be able to detect cost of reproduction when the increase in reproductive investment outweighs the differences in individual quality. In addition, we found that increased resources led to reproduction more closely resembling reproduction under natural conditions than under standard laboratory conditions. This illustrates the importance of studying host-parasite interactions under differing resource levels. When infecting the crustacean D. magna with the bacterial parasite Pasteuria ramosa we found that a substantial percentage of infected animals regained reproduction. Such a regain has never been reported before in this host-parasite system. We attribute this finding to the fact that we used a natural source of growth medium in our experiments, while previous experiments have used an artificial growth medium. In addition, our findings suggest that increasing the resources available to the host can be of great benefit to the host, while being of no or only a limited benefit to the parasite. This differs from previous findings, which have shown that both hosts and parasites benefit greatly from increased resources. The findings from the system suggest that the impact of additional resources on the life histories of the host and parasite is dependent upon the resource in question. Castration of parasitized hosts has received substantial attention with respect to its adaptive value and has been suggested to be adaptive to the parasite, the host, both or neither. However, for castration to be a host adaptation the host has to be able to regain the ability to reproduce. Our finding that P. ramosa infected D. magna could regain reproduction enabled us to test for the adaptive value of castration in this system. For two different clones, one sympatric and one allopatric, we found that host castration was only adaptive to the parasite. This supports findings from previous studies, in which the hosts were permanently castrated. In addition, our results support the existence of local adaptation in host-parasite interactions, as the parasite was more successful in the host it was locally adapted to. Gigantism of the host often accompanies castration and has also been suggested to be adaptive to the parasite, the host, both or neither. In the D. magna-P. ramosa system, this phenomenon has been viewed as adaptive to the parasite since the parasite benefits from infecting large host, and there has been found a positive correlation between host size and parasite fitness. However, we found that the hosts which regained reproduction were larger and contained fewer spores than the hosts which were permanently castrated. This finding questions whether host gigantism is adaptive to the parasite when the host can regain reproduction. We also suggest that permanent castration is not the norm under natural conditions in this host-parasite system. It is therefore important to further investigate the adaptive value of host gigantism, especially when hosts can regain reproduction and under natural conditions. Finally, the fact that these studies show that P. ramosa infected D. magna can regain reproduction opens for the possibility that the parasite could be transmitted vertically, which would alter our understanding of this host-parasite system