Patterns of Phytochemical Variation in Mimulus guttatus (Yellow Monkeyflower)

Abstract

The search for general patterns in the production and allocation of plant defense traits will be facilitated by characterizing multivariate suites of defense, as well as by studying additional plant taxa, particularly those with available genomic resources. Here, we investigated patterns of genetic variation in phytochemical defenses (phenylpropanoid glycosides, PPGs) in Mimulus guttatus (yellow monkeyflower). We grew plants derived from several natural populations, consisting of multiple full-sibling families within each population, in a common greenhouse environment. We found substantial variation in the constitutive multivariate PPG phenotype and in constitutive levels of individual phytochemicals within plants (among leaves of different ages), within populations (among full-sibling families), and among populations. Populations consisting of annual plants generally, but not always, had lower concentrations of phytochemicals than did populations of perennial plants. Populations differed in their plastic response to artificial herbivory, both in the overall multivariate PPG phenotype and in the individual phytochemicals. The relationship between phytochemistry and another defense trait, trichomes, differed among populations. Finally, we demonstrated that one of the PPGs, verbascoside, acts as a feeding stimulant rather than a feeding deterrent for a specialist herbivore of M. guttatus, the buckeye caterpillar (Junonia coenia Nymphalidae). Given its available genetic resources, numerous, easily accessible natural populations, and patterns of genetic variation highlighted in this research, M. guttatus provides an ideal model system in which to test ecological and evolutionary theories of plant-herbivore interactions.

Variation in Volatile Emission in Mimulus guttatus and Implications for Plant-Insect Interactions

“…One of these traits is the presence of glandular trichomes on its leaves. In previous work, higher trichome density did not deter feeding by the generalist xylem-feeding spittlebug, Philaenus spumarius, in a no-choice experiment, and in choice tests, P. spumarius did not discriminate against plants with high trichome density (Holeski 2007a (Medeiros and Moreira 2002), and in glandular form they can also contain volatile and nonvolatile secondary metabolites that are repellant or toxic to herbivores Tumlinson 1997, Laue et al 2000). To our knowledge, no studies have examined effects of M. guttatus trichomes on foliar herbivores.…”

“…In natural populations, M. guttatus experiences herbivory from a variety of generalist herbivores (Holeski 2007a), and M. guttatus exhibits a number of defense traits that could affect interactions with its herbivores. One of these traits is the presence of glandular trichomes on its leaves.…”

“…guttatus also synthesizes a variety of phenylpropanoid glycosides (PPGs) in its foliage (Holeski et al 2013, Keefover-Ring et al 2014, which have been found to act as feeding deterrents to generalist herbivores (Mølgaard 1986). However, one of these PPGs, verbascoside, functions as a feeding stimulant and increases performance of Junonia coenia (Holeski et al 2013), a specialist lepidopteran herbivore that commonly feeds on M. guttatus (Bowers 1984). In previous work, larval mass of J. coenia fed artificial diets that contained verbascoside was four times greater than larval mass of J. coenia reared on diets without verbascoside (Holeski et al 2013).…”

“…As mentioned earlier, inbreeding is known to reduce defensive compounds in S. carolinense (Campbell et al 2012), but inbreeding effects on M. guttatus secondary chemistry are unknown. M. guttatus is known to contain phenylpropanoid glycosides (PPGs; Holeski et al 2013, Keefover-Ring et al 2014, and one of these PPGs, verbascoside, has been shown to act as a feeding stimulant and increase larval mass of J. coenia (Holeski et al 2013). Leaf consumption by J. coenia was also greatest on outbred plants, suggesting that there may be greater levels of feeding stimulant in these plants.…”

“…Junonia coenia is a specialist herbivore that inhabits much of the United States and Mexico. Its larvae commonly feed on M. guttatus (Holeski et al 2013), as well as on plants that contain iridoid glycosides, such as Plantago lanceolata (Bowers 1984). We purchased J. coenia larvae from Shady Oak Butterfly Farm (Brooker, FL) that were reared on P. lanceolata prior to behavioral assays.…”