|Uninfected (top right) and
infected waterflea (lower left)
Photo by Meghan Duffy
Under such circumstances, will the presence of M. bicuspidata exacerbate the existing arms race between Daphnia and P. ramosa, or will it simply get in the way? If resistance for P. ramosa is also associated with resistance to M. bicuspidata, then it means Daphnia has a general mechanism for resisting both parasites. This scenario will simply select for general parasite resistance in the Daphnia population, reducing the level of genetic variation in the population (the raw material for ongoing Red Queen-style evolutionary arms race). On another hand, if Daphnia resistant to P. ramosa are negatively associated with resistance to M. bicuspidata, then it means resistance for one parasite will come at the cost to another - this trade-off in defending against two different parasites sets up an additional selective pressure that can potentially accelerate the arms race.
There are a number of key differences between the two parasites. While P. ramosa reduces the reproductive capacity of the host more than M. bicuspidata, the latter kills the host quicker. Metschnikowia bicuspidata is extremely lethal, killing infected waterfleas within 2-3 weeks of infection (whereas waterfleas can live up to 5-7 weeks after being infected by P. ramosa). The fungus releases its spores after the waterflea dies, and those infective spores can even survive passage through a fish's gut if their host Daphnia is eaten.
And unlike P. ramosa, infection success of M. bicuspidata depends not so much on encountering a host with the right genes, but through sheer persistence - the more often a waterflea is exposed to M. bicuspidata spores, the more likely that they become infected. This difference also manifests in the nature of outbreaks caused by the two parasites. Outbreaks of P. ramosa tend to be rarer and more limited, especially in genetically diverse populations, whereas M. biscuspidata is more prone to massive outbreaks that spread widely across the whole population. Even though it is not as discriminate about host genotype as P. ramosa, it is not as if M. bicuspidata does not influence the evolution of its host. But the way it affects host evolution is different to that of P. ramosa - instead of selecting for specific genotypes, it influences how much the waterfleas allocate their resources into either reproduction or parasite resistance.
In this study, the researchers found that different genetic lines of waterfleas varied considerably in their resistance to M. bicuspidata, but a waterflea's resistance to the fungal parasite did not in turn predict how well it also resisted P. ramosa. Instead, as found in previous studies, infection success of different P. ramosa strains depended upon the specific combination of host genotype and parasite genotype. This indicates that waterfleas have very different ways of resisting the two parasites, and that resistance to one does not lend protection to the other, but at the same time, nor does protection against one parasite increases a waterflea's vulnerability to the other.
Therefore, as far as the Red Queen arms race between waterfleas and P.ramosa is concerned, even though M. bicuspidata looms as a significant threat to the waterflea population, it is unlikely to significantly alter the coevolutionary dynamics between Daphnia and P. ramosa.
Auld SKJR, Hall SR, Duffy MA (2012) Epidemiology of a Daphnia-Multiparasite System and Its Implications for the Red Queen. PLoS ONE 7(6): e39564. doi:10.1371/journal.pone.0039564