The two Daphnia magna shown above are genetically identical sisters, born at the same time and raised in identical environments. The daphnid on the left is healthy, carrying a large clutch of eggs in her brood pouch. The daphnid on the right, on the other hand, is barren of eggs, nearly twice its normal biomass, and filled with the spores of the bacterial parasite Pasteuria ramosa. While it has long been suspected that Pasteuria benefits from inducing this castration and gigantism, the reasons why were unclear. The answer, it turns out, has to do with the flow of energy in the host. Pasteuria gets its energy from resources the host allocates to growth: by castrating the host and forcing it to put all of that energy into growth instead, Pasteuria increases its access to resources. Gigantism is an indirect effect of this process that may have important value to the parasite as well. The paper reporting these results, published Sept. 2014 in Proceedings B, is the second in a series, following on an Ecology Letters study published earlier this year, exploring the role of energy allocation in disease processes. For more information, click on links to the Science News piece on the paper or the links to either open access publication below.
University of Calgary press release: http://www.ucalgary.ca/utoday/issue/2014-08-29/exploring-role-energy-allocation-host-pathogen-dynamics
Proceedings of the Royal Society of London B: http://rspb.royalsocietypublishing.org/content/281/1792/20141087.abstract
Ecology Letters: http://onlinelibrary.wiley.com/doi/10.1111/ele.12229/abstract