Aversion and attraction to harmful plant secondary compounds jointly shape the foraging ecology of a specialist herbivore.
Most herbivorous insect species are restricted to a narrow taxonomic range of host plant species. Herbivore species that feed on mustard plants and their relatives in the Brassicales have evolved highly efficient detoxification mechanisms that actually prevent toxic mustard oils from forming in the bodies of the animals. However, these mechanisms likely were not present during the initial stages of specialization on mustard plants ~100 million years ago. The herbivorous fly Scaptomyza nigrita (Drosophilidae) is a specialist on a single mustard species, bittercress (Cardamine cordifolia; Brassicaceae) and is in a fly lineage that evolved to feed on mustards only in the past 10-20 million years. In contrast to many mustard specialists, S. nigrita does not prevent formation of toxic breakdown products (mustard oils) arising from glucosinolates (GLS), the primary defensive compounds in mustard plants. Therefore, it is an appealing model for dissecting the early stages of host specialization. Because mustard oils actually form in the bodies of S. nigrita, we hypothesized that in lieu of a specialized detoxification mechanism, S. nigrita may mitigate exposure to high GLS levels within plant tissues using behavioral avoidance. Here, we report that jasmonic acid (JA) treatment increased GLS biosynthesis in bittercress, repelled adult female flies, and reduced larval growth. S. nigrita larval damage also induced foliar GLS, especially in apical leaves, which correspondingly displayed the least S. nigrita damage in controlled feeding trials and field surveys. Paradoxically, flies preferred to feed and oviposit on GLS-producing Arabidopsis thaliana despite larvae performing worse in these plants versus non-GLS-producing mutants. GLS may be feeding cues for S. nigrita despite their deterrent and defensive properties, which underscores the diverse relationship a mustard specialist has with its host when lacking a specialized means of mustard oil detoxification.
Ecology and Evolutionary Biology University of Arizona Tucson Arizona 85721; Rocky Mountain Biological Laboratory Gothic Colorado 81224; Present address: Organismic and Evolutionary Biology Harvard University Cambridge Massachusetts 02138.
Parris T
PT
Humphrey
Ecology and Evolutionary Biology University of Arizona Tucson Arizona 85721; Rocky Mountain Biological Laboratory Gothic Colorado 81224.
Andrew D
AD
Gloss
Ecology and Evolutionary Biology University of Arizona Tucson Arizona 85721; Rocky Mountain Biological Laboratory Gothic Colorado 81224.
Nicolas M
NM
Alexandre
Ecology and Evolutionary Biology University of Arizona Tucson Arizona 85721; Rocky Mountain Biological Laboratory Gothic Colorado 81224.
Martha M
MM
Villalobos
Rocky Mountain Biological Laboratory Gothic Colorado 81224.
Marcella R
MR
Fremgen
Ecology and Evolutionary Biology University of Arizona Tucson Arizona 85721; Rocky Mountain Biological Laboratory Gothic Colorado 81224.
Simon C
SC
Groen
Boyce Thompson Institute for Plant Research Ithaca New York 14853; Present address: USDA-ARS Plant Genetics Research Unit Columbia Missouri 65211.
Lisa N
LN
Meihls
Boyce Thompson Institute for Plant Research Ithaca New York 14853.
Georg
G
Jander
Ecology and Evolutionary Biology University of Arizona Tucson Arizona 85721; Rocky Mountain Biological Laboratory Gothic Colorado 81224; Present address: Department of Integrative Biology University of California Berkeley California 94720.
Noah K
NK
Whiteman
Drosophila; glucosinolate; inducible defense; jasmonic acid; oviposition; preference–performance