The scent of alarm: ontogenetic and genetic variation in the osmeterial gland chemistry of Papilio glaucus (Papilionidae) caterpillars

Abstract

Within a species, ontogenetic and genetic variation in defensive chemistry can provide the basis for natural selection from different predator types. The osmeterial chemistry of fifth (last) instar Papilio glaucus caterpillars is known to differ qualitatively from the composition of early instar caterpillars. However, the osmeterial chemistry of early instar caterpillars has not been thoroughly characterized and may change as the caterpillars undergo their first three molts. We have used GC/MS to identify a suite of about 50 different terpene compounds in the osmeterial secretions of P. glaucus caterpillars, and found the relative amounts of these compounds changed significantly with each molt. These quantitative changes preceded the more dramatic qualitative switch to the production of 2-methylbutyric and isobutyric acids after the molt to the fifth instar. We also examined the effects of diet and genetic background on the relative quantities of 15 terpenes present in the secretions of third instar caterpillars. Parentage was found to affect the percentages of many more of the individual components than did diet, although both exerted an effect. The ontogenetic and genetic variations in the composition of the osmeterial secretions appear to have an effect on would-be predators. In the laboratory, terpene secretion was found to discourage attack by ants, whereas the switch from terpene to acid production rendered the caterpillars less palatable to a larger predator, the green anole. In the field, the presence of functional osmeteria did not seem to dramatically increase survival in a field study, and only a small, non-significant advantage was seen. Similarly, field data was suggestive that parentage might affect the likelihood of survival in a natural setting, but the stage of the caterpillar and the field site significantly affected survivorship. Further studies with greater replicates will be needed to determine whether and to what extent chemical differences in osmeterial components as well as behavior contribute to differences in outcomes in the field.

Patterns of divergence across the geographic and genomic landscape of a butterfly hybrid zone associated with a climatic gradient

“…While the chemistry of this gland is known to be affected by genetic background and varies substantially through larval development, it is not known to what extent the chemical composition of this gland differs between P. glaucus and P. canadensis (Frankfater, Tellez, & Slattery, 2009). We identified at least two FPPS genes in close proximity (<2 kb) to loci under divergent selection as well as another FPPS gene identified as a divergence hot spot by Cong et al (2015).…”

“…Interestingly, a number of candidate genes for farnesyl pyrophosphate synthase (FPPS) homologues hypothesized to synthesize terpenes for a predator defence gland (osmeterium) unique to Papilionidae (Cong et al., ) were found on chromosome 19. While the chemistry of this gland is known to be affected by genetic background and varies substantially through larval development, it is not known to what extent the chemical composition of this gland differs between P. glaucus and P. canadensis (Frankfater, Tellez, & Slattery, ). We identified at least two FPPS genes in close proximity (<2 kb) to loci under divergent selection as well as another FPPS gene identified as a divergence hot spot by Cong et al.…”

Impacts of climate warming on hybrid zone movement: Geographically diffuse and biologically porous “species borders”

“…When 70 species of Lepidoptera were offered to predatory ants, the prey chemistry was the best predictor of rejection by ants (most noxious chemistry was rejected) and the second best predictor of rejection was diet breadth of the caterpillars; specialists were rejected more frequently than generalists (Dyer, 1995), but instars can differ in defenses (Frankfater et al ., 2009). These findings are in line with the “plant apparency” concept of Feeny (1976), where specialists are able to detoxify and/or sequester nasty chemicals from their host plant leaves, perhaps better that generalists (see also Sime & Brower, 1998).…”

Integrating ancient patterns and current dynamics of insect–plant interactions: Taxonomic and geographic variation in herbivore specialization

“…The Aristolochia feeding Battus philenor is chemically protected by both behavioral deterrents and physiological barriers from a parasite, Trogus pennator (Ichneumonidae) that does attack nine other Papilio species feeding on nine other plant families (Sime, 2002). Of course, chemical defenses of herbivores themselves vary among populations, families, individuals and even instars within individuals (Eisner et al , 2005; Frankfater et al , 2009).…”