Optimal defence, kin conflict and the distribution of furanocoumarins among offspring of wild parsnip |
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Authors: | Arthur R Zangerl James K Nitao |
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Institution: | (1) Department of Entomology, University of Illinois, 320 Morrill Hall, 505 S. Goodwin, Urbana, IL 61801, USA;(2) USDA-ARS Nematology Laboratory, Building 011A, Room 165B, BARC-West, Beltsville, MD 20705-2350, USA |
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Abstract: | The factors influencing the allocation of chemical defences to plant offspring have largely been unexplored, conceptually
and experimentally. Because evolutionary interactions between maternal plants and their progeny can affect resource allocation
patterns among sibling offspring, we suggest that kin conflict as well as herbivore–plant interaction theories need to be
considered to predict chemical defence allocation patterns. Optimal defence theory predicts that maternal plants should defend
more heavily those offspring in which resources have been disproportionately invested. In contrast, kin conflict theory predicts
that natural selection will favour genotypes that can compete successfully for maternal defences irrespective of their quality,
even at the expense of the fitness of siblings and the maternal plant. Evidence for these defence patterns were evaluated
by examining the allocation of furanocoumarins to seeds of the wild parsnip (Pastinaca sativa, Apiaceae). Furanocoumarins
are toxins that are localized within the oil tubes of the maternal tissues of seeds. We evaluated the role of offspring investment
(endosperm mass) and seed genotype on furanocoumarin allocation by mating an array of pollen donors with pollen recipients.
Furanocoumarins were found to be positively correlated with endosperm mass on one side of the seed, a result consistent with
optimal defence theory; however, on the other side of the seed, furanocoumarin content was influenced by seed genotype and
was unrelated to endosperm mass. These effects varied with maternal plant. Further experiments demonstrated that nearly 80%
of furanocoumarin production occurs after pollination, when fertilization products are active. Although the amount of furanocoumarin
influenced by the seed genotype is small relative to the total quantity in the seed, these furanocoumarins are the first line
of defence against important predators, such as the parsnip webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae).
We found that parsnip webworm larvae were able to discriminate among genotypes within an inflorescence. In line with previous
studies, these results suggest that a genotype's ability to influence furanocoumarin defence may affect its probability of
survival. We conclude that the distribution of defences among plant offspring in wild parsnip is probably influenced by competition
among seed genotypes that conflicts with maternal optimal defence.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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Keywords: | defence endosperm furanocoumarins kin conflict Pastinaca sativa seeds wild parsnip |
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