Insect Nutrition

Important foci of our research on insect nutrition are the sterol and sugar nutrition of phloem sap feeding insects, especially aphids.  One goal of this research is to identify novel targets that can be developed for improved control of these important crop pests. 

Details of our complementary research on the contributions of microorganisms to insect nutrition is available here.

1. Sterol nutrition of aphids

Insects, unlike mammals cannot synthesize sterols and they absolutely require an exogenoussupply of these compounds, usually from the diet.  Sterols (usually cholesterol) are an essential component of animal membranes and precursors of steroid hormones, e.g. ecdysteroids of insects. We are studying the sterol nutrition of aphids in collaboration with Spence Behmer (Texas A&M) and Bob Grebenok (Canisius College, Buffalo).

Plants produce a side diversity of sterols, and chewing phytophagous insects have various adaptations to utilize these phytosterols. Surprisingly, the sterol profile of the phloem sap in plants we have tested is dominated by cholesterol, and readily utilized by aphids.

We are currently pattern of utilization of cholesterol and other sterols by the peach-potato aphid Myzus persicae and pea aphid Acyrthosiphon pisum.  Of particular interest, aphids perform very poorly on plants with phloem sterol content dominated by atypical steroids, offering the opportunity to control populations of these crop pests through interference with their sterol nutrition.
Figure Tobacco plants one week after infestation with Myzus persicae. A. Control plants bearing population of aphids. B. Essentially aphid-free plant with modified phloem sterol content. C. Stunted growth of control tobacco plant caused by aphid infestation. Data related to this figure available at Behmer ST, Grebenok RJ and Douglas AE, 2010. Plant sterols and host plant suitability for a phloem-feeding insect. Functional Ecology 25: 484-491 Journal link).

2. Sugar nutrition of phloem-feeding insects

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.  We are applying our developing understanding of sugar nutrition and osmoregulation in aphids and other phloem-feeding insects to develop novel control strategy for these insects, using RNAi in collaboration with Georg Jander (Boyce Thompson Institute)

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 Pubmed link]

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 Pubmed link]

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 Pubmed link]

                 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 Pubmed link]

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.

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.