Sterol nutrition of insects

 

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

We have shown that the fecundity of the pea aphid Acyrthosiphon pisum is promoted by dietary cholesterol or sitosterol, but no other sterol tested.  Cholesterol and sitosterol are major sterols in the pea aphid host plant, Vicia faba (faba bean).  Cholesterol is also a dominant sterol of phloem sap in other dicot plants we have tested, even though phytosterols dominate the bulk sterol content of these plants.  The implication is that the sterols ingested by sap-feeding and chewing insects on the same plant can be very different. For further details, see Behmer et al. 2011 and Bouvaine et al. 2012, and figure below. 

Figure  Sterol composition of phloem sap and leaf tissue in Vicia faba, host plant of the pea aphid. Data from Bouvaine et al. 2012.

RNAseq analysis of pea aphids reared on plants and diets with different sterols (Figure below) confirmed the expression of all predicted genes with function in sterol acquisition and metabolism.  The multiple statistically-significant differences in transcript abundance among aphids reared on the different diets, but they did not included genes with annotated function related to sterol acquisition or metabolism and could not be linked to the identity or concentration of the dietary sterol.  Aphid metabolic response to variation in the dietary content or composition of dietary sterols may be dominated by processes operating at the level of enzyme activity or pool size of specific metabolites.

Figure Hierarchical cluster analysis of gene expression profiles of 2281 genes differentially expressed between pea aphids reared on different diets. The color scheme bar represents values of log2 (RPKM). For further details see Bouvaine et al. 2012.