Intraspecific variability of the midgut protease complement in Colorado potato beetle larvae fed different plant diets

The relative abundance of midgut (digestive) proteases in Colorado potato beetle (Leptinotarsa decemlineata Say) is strongly influenced by the plant diet, but little is known about the intraspecific, inter-population variability of the protease complement in larvae ingesting leaves of different plants. Here we compared cysteine (Cys) and aspartate (Asp) protease activities in midgut extracts of potato (Solanum tuberosum L.)-, tomato (Solanum lycopersicum L.)- and eggplant- (Solanum melongena L.) fed larvae collected near Québec City (Canada), to corresponding activities in extracts from larvae collected in Maryland (U.S.A.). In vitro protease assays with fluorigenic peptide substrates were carried out to compare midgut Z-Phe-Arg-methylcoumarin (MCA)-hydrolyzing (cathepsin L-like), Z-Arg-Arg-MCA- (cathepsin B-like) and MOCAc-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys(Dnp)-D-Arg-NH2 (cathepsin D-like) hydrolyzing activities in potato-reared 4th-instar larvae maintained for three more days on potato, or transferred for three days on tomato or eggplant leaves. Potato-fed larvae of both Québec and Maryland showed a rapid, linear growth rate of ~35 mg fresh weight/day over 72 h, associated with a sharp decrease of midgut cathepsin L-like activity. By comparison, tomato- and eggplant-fed larvae of both origins showed negligible growth rates over 24 or 72 h, but a diverging adjustment of the midgut protease complement. Cathepsin L-like activity, in particular, was strongly decreased in Québec-collected larvae as also observed with the potato diet, in sharp contrast with cathepsin L activity in larvae from Maryland showing a two- to threefold increase after 72 h. These observations suggest a population-dependent adaptation of the digestive protease complement in potato beetle larvae responding to the plant diet.