Reliance on pollinators predicts defensive chemistry across tobacco species

Abstract

Defensive traits are typically studied in the context of avoiding antagonists, but may also mediate key interactions with mutualists. Plant chemical defences occur in flowers, suggesting pollinators may be agents of selection on defence. We hypothesised that floral defences would deter pollinators, and therefore, pollinators would select for lower defences in outcrossing than self‐pollinating species. We measured pollinator reliance and alkaloid levels in 32 greenhouse‐grown Nicotiana species. Using a comparative phylogenetic approach, we found significantly lower nectar, floral and leaf nicotine concentrations in outcrossing than selfing species, with a 15‐fold decrease in leaf nicotine levels. Nicotine concentrations were positively correlated across tissues, suggesting that selection against floral defences could constrain the evolution of leaf defences. Thus, pollinators could shape the evolution not only of floral defences but also of defences in other tissues where herbivores have traditionally been considered the dominant agent of selection.

Resistance of developing honeybee larvae during chronic exposure to dietary nicotine

“…Nicotine can be present in both pollen and nectar of Nicotiana species (Detzel and Wink, 1993;Adler et al, 2012) but bees are more likely to be exposed through its continuing use as a botanical insecticide (Casanova et al, 2002). The lack of consistent, negative effects of nicotine on the survival of developing honeybee larvae supports previous studies showing limited effects of nicotine in sucrose solutions on survival of caged workers (Köhler et al, 2012a) and on hatching success and survival of honeybee larvae in 'minihives' maintained in enclosures (Singaravelan et al, 2006).…”

“…Previously, we have found that survival of caged Apis mellifera scutellata workers was unaffected by 30 µM (5 ppm) nicotine in sucrose diets, and actually improved in the case of weak colonies (Köhler et al, 2012a). At the colony level, Singaravelan et al (2006) fed honeybees nicotine in sucrose solutions for 26 days: hatching success and larval survival were not affected by nicotine levels up to 30 µM, which can occur naturally in floral nectar of Nicotiana species (Adler et al, 2012). …”

Plant mating system transitions drive the macroevolution of defense strategies

“…At an intraspecific scale, antagonists such as herbivores can affect the expression of mating system and reproductive traits (30,31) as well as the magnitude of inbreeding depression (32). Conversely, mating systems (inbreeding, outcrossing) and sexual or pollination systems (e.g., sex) can affect the expression of defense traits (32)(33)(34)(35). Inbreeding effects on defense and genetic diversity (4) could thereby alter selection for defense strategies within populations.…”

Florivory as an Opportunity Benefit of Aposematism

“…Plant defense strategies often involve responding to leaf damage by the production of toxins (Euler and Baldwin 1996;Adler et al 2006;McCall and Karban 2006), which could increase the toxins available to specialized herbivores and so lead to more florivory, provided that the physiological link between leaf and flower toxin concentrations (Adler et al 2012) is broken. The selective impact of herbivores on plant strategies may thus be even more complex than is currently appreciated (van Meijden 1996).…”