Insulin-like peptide said to regulate ‘weapons’ growth in beetles

Through a new study scientists have determined that a specific insulin-like peptide called ILP2 is said to have the final say when it comes ot regulation of growth and size of “weapons” in Gnatocerus cornutus beetles in different nutritional environments.

Researchers found mandible size when expression of the peptide was suppressed, and that it was specifically expressed in the “fat body”, where beetles store nutrients. This has important implications for understanding how striking growth occurs in different environments for different organisms.

Insulin-like peptides (ILPs) and growth factors (IGFs) are found in a wide range of both invertebrates and vertebrates, like humans, and are thought to play common roles related to growth and metabolism. Recent work into the genetics of insects has revealed a wide variety of ILPs playing different functional roles. Examples include fruit flies which have 8, pea aphids with 10 and silkworms with over 40. Yet, despite some understanding of what ILPs do in model organisms like fruit flies, we know little about their function outside of the lab, in particular, how their molecular function contributes to phenotypic complexity, or the diversification of traits in a particular natural environment.

Tokyo Metropolitan University scientists examined the broad-horned flour beetle, G. cornutus. Found in grain stores and factories, male flour beetles have exaggerated mandibles for male-male combat; large beetles have larger mandibles, and past work has found it is linked to better nutrition as a larva. This makes it ideal for studying the molecular basis for conditional growth. The team examined the beetle’s genome, and found five different ILP genes, labeled GcorILP1 to 5, all encoding for a different ILP (ILP1 to 5). Firstly, they found that sufficiently fed larger larvae showed an elevated expression of ILP2 at the pre-pupa stage, while “poorly” fed smaller larvae did not. They also found that the same peptide was specifically expressed in the fat body; smaller larvae expressed less of the peptide as a consequence of having less fat. Furthermore, by using a technique known as RNA interference to impede the expression of ILP2, they found a direct correlation between so-called GcorILP2 “knock-down” (KD) and diminished mandible size, not to mention less of a correlation with how well the larvae were fed.