The Nutritional Mechanism Of Symbiotic Fungus Adapting To The Leaf Rolling Weevil

Mutualistic symbiosis between insects and fungi is of great significance in biology and evolution.Like domesticating and growing crops,the attelabid weevil Europs chinensis developed the capacity to specifically cultivate fungus Penicillium herquei as a food source.Previous studies have demonstrated how the attelabid weevil cultivates and manages fungal crop,how the symbiotic fungus provides chemical to protect the fungal garden of the attelabid weevil,and is vertically transmitted by weevil through generations.Although it has been observed that weevil larvae feed on complex of fungus and leaf in leaf roll,it is still unclear what kind of nutrition the symbiotic fungus provided for the attelabid weevil.Based on the phenomena that there is a little feces of weevil larvae in the leaf-roll and the abundance of carbohydrates of the leave,we hypothesize that the symbiotic fungus P.herquei provides nitrogen source for weevil development.Therefore,we focus on what kinds of nutrition are provided by the symbiotic fungus and how the symbiotic fungus adapts to provide nutrition to the leaf rolling weevil.After observation of P.herquei hyphae under miscroscope,we found there are a large number of the nodular protrusions coupled with clustered microfiber networks on the hyphal surface on the cultures of the fungus.In the meanwhile,the protrusions look like the secreted materials other than the simple bulges of the hypha cell wall.The protrusions and microfiber networks are more abundant on the hyphae of leaf rolls.In order to analyse the potential nutrition of the symbiotic fungus provided for the weevil larvae,we selected 3 symbiotic isolates and 3 free-living isolates from soils of P.herquei and compared their components,especially nitrogen.The results showed that the nitrogen content of the symbiotic isolates were 45%higher than that of wild strains and the total amino acid content were 57%higher than that of wild strains,especially the content of arginine,which is essential in the fungi and insects association,increased 95%compared with the wild strains.Those results suggested that the symbiotic fungus provides the nitrogen resource for weevil larva development.Furthermore,we investigated the nutritional requirements of carbon and nitrogen sources for the growth of symbiotic fungus.The results showed that the symbiotic strains have lower efficiency to degrade macromolecular polysaccharides and organic complex nitrogen sources on solid media,while can effectively utilize some soluble polysaccharides and amino acids,indicated that the symbiotic fungal strains have been adapted to be mutualistic association with weevil other than saprophytism.Moreover,it can effectively utilize the non-essential amino acids of plant leaves.We also found that the symbiotic fungal strains can grow on nitrogen-free solid and liquid media.Although the investigation using nitrogen-fixing approaches such as nitrogenase gene Nif detection and N-fixing bioassay with acetylene reduction method and 15N natural abundance method has not demonstrated that the ability of the symbiotic stains fix nitrogen,the growth on nitrogen-free media and the higher nitrogen content of the symbiotic strains still remain to be a quite interesting topic for future research.