Same Host-Plant,Different Sterols: Variation in Sterol Metabolism in an Insect Herbivore Community |
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Authors: | Eric M Janson Robert J Grebenok Spencer T Behmer Patrick Abbot |
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Affiliation: | (1) Department of Biological Sciences, Vanderbilt University, VU Station B #351634, Nashville, TN 37235-1634, USA;(2) Department of Biology, Canisius College, 2001 Main St., Buffalo, NY 14208-1098, USA;(3) Department of Entomology, Texas A&M University, Minnie Belle Heep Building, Room 509, College Station, TX 77843-2475, USA |
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Abstract: | Insects lack the ability to synthesize sterols de novo, which are required as cell membrane inserts and as precursors for steroid hormones. Herbivorous insects typically utilize
cholesterol as their primary sterol. However, plants rarely contain cholesterol, and herbivorous insects must, therefore,
produce cholesterol by metabolizing plant sterols. Previous studies have shown that insects generally display diversity in
phytosterol metabolism. Despite the biological importance of sterols, there has been no investigation of their metabolism
in a naturally occurring herbivorous insect community. Therefore, we determined the neutral sterol profile of Solidago altissima L., six taxonomically and ecologically diverse herbivorous insect associates, and the fungal symbiont of one herbivore. Our
results demonstrated that S. altissima contained Δ7-sterols (spinasterol, 22-dihydrospinasterol, avenasterol, and 24-epifungisterol), and that 85% of the sterol pool existed
in a conjugated form. Despite feeding on a shared host plant, we observed significant variation among herbivores in terms
of their qualitative tissue sterol profiles and significant variation in the cholesterol content. Cholesterol was absent in
two dipteran gall-formers and present at extremely low levels in a beetle. Cholesterol content was highly variable in three
hemipteran phloem feeders; even species of the same genus showed substantial differences in their cholesterol contents. The
fungal ectosymbiont of a dipteran gall former contained primarily ergosterol and two ergosterol precursors. The larvae and
pupae of the symbiotic gall-former lacked phytosterols, phytosterol metabolites, or cholesterol, instead containing an ergosterol
metabolite in addition to unmetabolized ergosterol and erogsterol precursors, thus demonstrating the crucial role that a fungal
symbiont plays in their nutritional ecology. These data are discussed in the context of sterol physiology and metabolism in
insects, and the potential ecological and evolutionary implications. |
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