Parasitism induces negative effects of physiological integration in a clonal plant

Clonal integration often increases fitness of clonal plants, but it may decrease it when some but not all connected plants (ramets) within a clone are parasitized. This hypothesis was synthesized in a conceptual model and tested by growing pairs of connected ramets of two congeneric clonal plants,Sphagneticola trilobataandSphagneticola calendulacea, with and without parasitizing one ramet withCuscuta australisand with and without severing the connection (allowing or preventing integration). Consistent with the model, integration inS. calendulaceadid not affect biomass of the parasitized ramet, decreased biomass of its connected, unparasitized ramet by 60% and of the clone by 40%, and increased biomass of the parasite by 50%. By contrast, integration inS. trilobatadid not affect biomass of the clone or the parasite. The parasite increased export of nitrogen-15 from the connected, unparasitized ramet seven-fold inS. calendulaceabut did not affect export inS. trilobata. Parasitism can cause clonal integration to negatively affect fitness in clonal plants because parasites can import resources from connected, unparasitized ramets, possibly partly through signaling. This is the first experimental demonstration that clonal integration can decrease fitness in plants induced by parasitism and may help explain community-level effects of parasites.