Insect Nutrition

We are investigating how certain insects utilize phloem sap of plants.  Phloem sap is an ‘extreme diet’ that is used by very few animals: various insects of the order Hemiptera and no other animals use it as their sole food source throughout the life cycle.

There are two nutritional barriers to animal utilization of plant phloem sap

The high sugar (often sucrose) concentrations and associated high osmotic pressure, and 
The unbalanced amino acid composition
, with a deficit of essential amino acids (i.e. amino acids that contribute to protein but cannot be synthesized de novo by animals).
Figure.
Concentration of sucrose and amino acids (mean + s.e., 10 replicates) in the phloem sap of potato plants.  The phloem sap was collected by severing the stylets of aphids Aulacorthum solani as they fed on the plant.  The essential amino acids account for <10% of the total amino acid content.

 

We are investigating how aphids overcome these nutritional barriers

1. The high sugar barrier

The osmotic pressure of plant phloem sap is up to 4-5 times higher than that of aphid body fluids. Aphids would be expected to lose water to the gut and suffer osmotic collapse, i.e. shrivel as they feed.

Aphids avoid osmotic collapse partly because the dietary sucrose is modified in the gut.


We have shown that the pea aphid has a gut sucrase and transglucosidase that transform the glucose moiety of the sucrose into long-chain oligosaccharides, so depressing the osmotic pressure of the gut contents.

Figure. Size (number of hexose units, up to 18 shown) of oligosaccharides in honeydew of pea aphids Acyrthosiphon pisum, determined by MALDI-TOF.  The aphids were reared on diets containing 0.75 M sucrose.

 

We have identified the aphid gut sucrase, and localized the transcript and protein to the intestine, i.e. distal to the stomach.

Figure. Transcript of the pea aphid gut sucrase (ACYPI000002) localized to the intestine, distal to the stomach (blue). 
[Price et al. 2007 Insect Biochemistry and Molecular Biology 37, 307-317]



We have demonstrated that aphid osmoregulation requires the functional gut sucrase.  Acarbose is a potent inhibitor of the aphid gut sucrase in vitro and in vivo.  When pea aphids are fed on diets with 5 µM acarbose, the osmotic pressure of their hemolymph (blood) rises significantly and the aphids die in 2-3 days.
Figure.
Impact of 5 µM acarbose on the hemolymph osmotic pressure of pea aphids fed for 2 days on diets containing 0.75 M sucrose. [Karley et al. 2005 Journal of Insect Physiology 51, 1313-1319]




Aphid osmoregulation is also dependent on the function of an aquaporin (water channel).  We have identified the aquaporin gene AQP1 (ACYPI006387) and localized it to the stomach and distal intestine.
Figure. Transcript of the pea aphid gut aquaporin (ACYPI006387) localized to the stomach and distal intestine (blue). [Shakesby et al. 2009 Insect biochemistry and Molecular Biology 39, 1-10]

 

 

We have found that the osmotic pressure of the aphid hemolymph is elevated when expression of the gene AQP1 is depressed by RNAi.
Figure. Impact of RNAi on pea aphids.
A.
Transcript abundance of the aquaporin gene ApAQP1 in aphids administered dsRNA-ApAQP1 relative to those administered dsGFP (data are normalized to the reference gene ßTUB).
B.
Hemolymph osmotic pressure is significantly elevated at 2-3 days after administration of ApAQP1 dsRNA to aphids. RNAi-mediated knock-down of gene expression in aphids has the predicted phenotypic effect, even though the reduction in ApAQP1 expression is incomplete and transient.  [Shakesby et al. 2009 Insect Biochemistry and Molecular Biology 39, 1-10]

 

 

We propose that the aquaporin mediates the flux of water down the osmotic gradient from the distal intestine to stomach, mediating cycling of ingested water.

Figure.
Predicted water relations in the pea aphid gut. Sucrase/transglucosidase activity reduces osmotic pressure of gut contents distal to stomach, with water flow along osmotic gradient from the distal intestine to stomach (triple arrow) promoted by the aquaporin at the zone of contact between distal intestine and stomach.

2. The low essential amino acids barrier

The essential amino acids in plant phloem sap are insufficient to meet the aphid requirement for these nutrients.  Aphids derive supplementary essential amino acids from their symbiotic bacteria Buchnera aphidicola.  We have demonstrated and quantified the transfer of essential amino acids from Buchnera to the aphid host.   For more on this symbiosis see Insect interactions with beneficial microorganisms.

To investigate how the synthesis and release of essential amino acids is regulated, we are reconstructing and analyzing the metabolic network of Buchnera aphidicola and its pea aphid host.  The Buchnera are restricted to insect cells (bacteriocytes) in an organ called the bacteriome.

Figure. Schematic of the aphid-Buchnera model. Block arrows represent fluxes across cellular boundaries. B: biomass production