Parasites often cause changes in the growth and reproduction of their hosts. For example, during parasitic infections, tiny algae-eating crustaceans known as water fleas sometimes become sterile and swell to twice their normal size. Researchers investigating this host-parasite system found that the parasite benefit from stealing the host’s resources allocated to growth. The work was published in Proceedings of the Royal Society B this week. 

Like many parasites, the bacterium Pasteuria ramosa castrates its host before killing it. Several days after the freshwater flea Daphnia magna is exposed to the parasite’s spores -- usually by drinking in spores released into the water after older hosts have died -- the host reproduction stops and growth accelerates. But just before the castration, infected hosts often have larger clutches compared with uninfected individuals; this is a host adaptation called fecundity compensation. And the timing of castration is key: The longer it takes the parasite to castrate its host, the higher the host reproduction and the lower the parasite spore production. 

Scientists know that this infection-induced castration and gigantism is caused by a redirection of within-host resources away from reproduction and towards growth, but because these changes can affect both host and parasite success, it can be difficult to determine who (if anyone) benefits. 

In order to closely follow the energy budget within a host, a team led by Clayton Cressler from Queen's University raised their own water fleas from Daphnia collected from a pond at Kaimes Farm, Leitholm, Scottish Borders. The water fleas were exposed to Pasteuria spores and then fed algae in intervals ranging from one to six days. Increasing the interval between feedings increases the amount of starvation stress. Each of the six groups continued on their feeding schedule for about a month and a half, and then their Pasteuria spore loads were counted. 

Starvation primarily affected investment in reproduction. The various feeding treatments had minimal impact on growth trajectories: Starved fleas grew almost as fast as the frequently fed ones, but they definitely spent less energy reproducing. In the 4-day feeding interval treatment, the fleas were 7 percent smaller, but reproduced 48 percent less on average. The age at castration, they found, was the same across the treatments. Increasing starvation stress reduces the flea’s gigantism as well as the parasite’s fitness (spore production, in this case), without affecting castration.

By castrating its host as early as possible, Science explains, Pasteuria forces the flea to concentrate on growing -- which benefits the parasite since it steals resources allocated for growth to fuel its own replication.

Image: Hajime Watanabe, PLoS Genetics 2011 via Wikimedia