How nutritionally imbalanced is phloem sap for aphids?

Abstract

Aphids harbour intracellular symbionts (Buchnera) that provide their host with amino acids present in low amounts in their diet, phloem sap. To find out the extent to which aphids depend on their symbionts for synthesis of individual essential amino acids, we have evaluated how well phloem sap amino acid composition matches the aphids' needs. Amino acid compositions of the ingested phloem sap were compared to amino acids in aphid body proteins and also to available information about optimal diet composition for other plant feeding insects. Phloem sap data from severed stylets of two aphid species, Rhopalosiphum padi (L.) (Homoptera: Aphididae) feeding on wheat, and Uroleucon sonchi (L.) (Homoptera: Aphididae) feeding on Sonchus oleraceus (L.), together with published information on phloem sap compositions from other plant species were used.Phloem sap has in general only around 20% essential amino acids, with a range from 15-48%. Aphid body proteins and optimal diets for two other plant feeding insects have around 50%. The phloem sap of early flowering S. oleraceus ingested by U. sonchi contained 48%, which seems to be exceptional. Aphids feeding on different plants appear to be very differently dependent on their symbionts for their overall essential amino acid synthesis, due to the large variation in proportion of essential amino acids in phloem sap from different plants.The profile of the essential amino acids in phloem sap from different plant species corresponds rather well to profiles of both aphid body proteins and optimal diets determined for plant feeding insects. However, methionine and leucine in phloem sap are in general low in these comparisons, suggesting a higher dependence on the symbiont for synthesis of these amino acids. Concentrations of several essential amino acids in phloem from different plant species seem to vary together, suggesting that levels of symbiont provisioning of different amino acids are adjusted in parallel.

Effect of Variegation on Stem Exudates of Coleus and Life History Characteristics of Citrus Mealybug (Hemiptera: Pseudococcidae)

“…In general, carbohydrate concentrations in phloem are high and not limiting for development of phloemfeeding insects (Rhodes et al 1997). Other references, however, suggest that the concentration of free amino acids in phloem sap is generally regarded as a limiting for sucking insects (Cole 1997, Sandströ m andMoran, 1999).…”

Isotopic enrichment in a phloem-feeding insect: influences of nutrient and water availability

“…As a result, aphids can synthesize all protein amino acids using carbons from sugars (Febváy et al 1999). Further, the suite of amino acids synthesized is variable because the endosymbiont maintains an optimal amino acid composition regardless of diet quality (Douglas 1998;Sandström and Moran 1999). Bacterially synthesized amino acids show large variation in carbon isotope ratios (Abelson and Hoering 1961;Macko et al 1987) indicating that biosynthetic pathways for diVerent amino acids fractionate to diVerent extents (Gannes et al 1997).…”

Effects of elevated atmospheric CO2concentrations on phloem sap composition of spring crops and aphid performance

“…According to Dadd (1985), essential amino acids for aphids are histidine (His), threonine (Thr), tryptophan (Trp), methionine (Met), valine (Val), phenylalanine (Phe), isoleucine (Ile) and lysine (Lys). However, the demand of individual amino acids differs with the aphid species (Emden & Bashford 1971;Sandstro¨m & Moran 1999, 2001. While Myzus persicae Sulz.…”