Underground symbiosis between a desert plant, weevil beetles and their bacteria

Symbiotic relationships may vary on the scale from mutualism to parasitism. Moreover, the outcome of a certain relationship may differ depending on the life stage of the organisms, the environmental conditions and the occurrence of additional species involved in the interaction. We studied the underground interaction between Salsola desert plants, weevil beetles that develop within a mud chamber affixed to their roots, and the gut-bacteria of the weevils. Plants with naturally occurring weevils in their roots were found to have more nitrogen in their tissues and to produce larger seeds, suggesting that they acquire nitrogen from the weevils. Molecular analyses of the weevil gut-content confirmed the occurrence of potential nitrogen fixing bacteria in weevils of different populations in the Negev. However direct tests for the occurrence of active nitrogen fixation inside the weevils were inconclusive. Experimental comparisons of plants with and without weevils suggested a complex relationship: weevils negatively affected plant biomass, but the magnitude of the effect depended on the weevil life stage. Moreover, the weevils positively affected the nitrogen content of the plants and altered their N isotopic signature. The results highlight the complexity of plant-animal-bacteria symbiotic interactions in desert environments and call for additional investigations.