Effects of altitude of origin on trichome-mediated anti-herbivore resistance in wild Andean potatoes

The diversity and abundance of herbivores often decreases with increasing altitude. Plants are expected to respond to reduced herbivore pressure by allocating fewer resources to anti-herbivore resistance at higher altitudes. This study uses a greenhouse experiment and parallel bioassay to examine population variability in trichome-mediated defenses across a range of accessions of Solanum berthaultii Hawkes and S. tarijense Hawkes that originated from different altitudes. S. berthaultii bears two types of glandular trichomes, type A and type B, whereas S. tarijense has type A trichomes only, and hairs resembling type B trichomes that are eglandular. Both type A and type B glandular trichomes on S. berthaultii deterred ovipositing female Phthorimaea operculella (Zell.) (Lepidoptera: Gelechiidae). The density of type A, but not type B trichomes decreased with increasing altitude of origin in S. berthaultii populations. The ratio of type A to type B trichomes on the abaxial surface of S. berthaultii negatively affected oviposition and was inversely related to altitude of origin. In S. tarijense, type A trichomes deterred, but eglandular leaf hairs stimulated oviposition. Consequently, the ratio of type A trichomes to eglandular leaf hairs was negatively related to the number of eggs laid. The total numbers of trichomes per leaf generally increased with increasing altitude of origin in S. tarijense whereas the ratio of type A trichomes to eglandular hairs declined. In the oviposition bioassay, we found no direct relationship between resistance and altitude of origin, which may have been due to differences in leaf area at the time of the bioassay. Nevertheless, the results suggest that populations of both plant species that originated from higher altitudes were generally more susceptible to ovipositing P. operculella.