Trichoplusia Ni Transcriptomic Responses to the Phytosaponin Aglycone Hederagenin: Sex-related Differences

Abstract

Many plant species, particularly legumes, protect themselves with saponins. Previously, a correlation was observed between levels of oleanolic acid-derived saponins, such as hederagenin, in the legume Medicago truncatula and caterpillar deterrence. Using concentrations that reflect the foliar levels, hederagenin did not have a toxic effect on 4th instar caterpillars of the cabbage looper Trichoplusia ni nor did it act as a feeding deterrent. However, female caterpillars consumed more diet than males, presumably to obtain the additional nutrients required for oogenesis, and are, thus, exposed to higher hederagenin levels. When fed diet with lower hederagenin levels, male caterpillars expressed more genes encoding trypsin-like (LOC113500509, LOC113501951, LOC113501953, LOC113501966, LOC113501965, LOC113499659, LOC113501950, LOC113501948, LOC113501957, LOC113501962, LOC113497819, LOC113501946, LOC113503910) and the cytochrome P450 6B2-like (LOC113493761). When fed the high hederagenin, male caterpillars expressed stress-responsive genes (LOC113503484, LOC113505107). In contrast, female caterpillars expressed high levels of the cytochrome P450 6B7-like (LOC113492289). Bioinformatic tools predict that cytochrome P450s could catalyze the oxygenation of hederagenin which would increase the hydrophilicity of the compound. Expression of the MFS transporter (LOC113492899) showed a hederagenin dose-dependent increase in gene expression suggesting that this transporter may be involved in saponin efflux. These sex-related differences in feeding and detoxification should be taken into consideration in insecticide evaluations to minimize pesticide resistance.