Xylem consumption impact on fecundity and obligate symbionts in Macrosiphum euphorbiae aphids

Aphids (Hemiptera: Aphididae) are phloem feeders that occasionally ingest xylem sap. Phloem is an unbalanced diet: low concentration or absence of essential amino acids, and high concentration of sucrose. Essential amino acids are supplemented by Buchnera bacterial endosymbionts, and are limiting for nymph production. High osmotic pressure of the ingested phloem sap, mainly caused by sucrose concentration, leads to aphid dehydration if not regulated down to haemolymph osmotic pressure. Osmoregulation is achieved by sucrose assimilation (through nymph production and Buchnera metabolism) and synthesis of polysaccharides in the stomach. Honeydew excreted is iso-osmotic with haemolymph. Xylem sap being a dilute fluid and having its consumption related to dehydration and fecundity, we hypothesized that xylem consumption is an additional mean to regulate osmotic pressure in haemolymph. Three predictions were tested on the Macrosiphum euphorbiae (Thomas) - Solanum tuberosum L. system. First, we observed an inverse relation between fecundity and the proportion of xylem consumption duration over total sap intake [xylem consumption / (xylem + phloem consumptions)] across alate aphid life. Second, we observed an increase of the proportion of xylem consumption in Buchnera-free aphids, obtained after antibiotic treatment. Third, after feeding aphids on different sucrose-concentration diets, we observed a positive relation between sucrose concentration in the diet and the proportion of xylem consumption when feeding on a subsequent plant. These results demonstrated that xylem consumption compensates for reduced assimilation and is a response to increasing sucrose concentration, thus supporting our hypothesis.