Genetically controlled food preference: biochemical mechanisms.

Food choice is known to be correlated with genotype in the crustacean Gammarus palustris. Given a choice of Enteromorpha intestinalis (E) over Ulva lactuca (U), individuals homozygous for the Amy II.52 allele have a greater preference for E than do Amy II.55 homozygotes. To account for this correlation, we hypothesized that the proportions of saccharides released by the enzymatic action of Amy II.52 on E or Amy II.55 on U starches differ from and better stimulate feeding than those released by Amy II.52 on U and Amy II.55 on E starches. To test this, the two forms of amylase were purified by glycogen/ethanol precipitation and preparative PAGE. Their product distributions with each of the starches were determined by HPLC. Each amylase/starch combination gave different distributions of the main products: maltose, maltotriose, and maltotetraose. Feeding preference tests using artificial foods containing these sugars showed that the product distributions from Amy II.52/E starch or Amy II.55/U starch were preferred over those from Amy II.52/U or Amy II.55/E. Patterns of preferences for the artificial foods closely matched those observed in earlier experiments in which different genotypes fed on intact algae. Thus, genetic differences in feeding preferences can be understood in terms of variation in biochemical properties of a digestive enzyme. These results highlight a previously unappreciated role for digestive enzymes: in their capacity to modify the chemical nature of environmental stimuli prior to gustation, digestive enzymes can be viewed as having important chemosensory roles.