OPTIMAL DEFENSE, KIN CONFLICT AND THE DISTRIBUTION OF FURANOCOUMARINSAMONG OFFSPRING OF WILD PARSNIP

The factors influencing the allocation of chemical defences to plant o ffspring have largely been unexplored, conceptually and experimentally . Because evolutionary interactions between maternal plants and their progeny can affect resource allocation patterns among sibling offsprin g, 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 disp roportionately invested. In contrast, kin conflict theory predicts tha t natural selection will favour genotypes that can compete successfull y for maternal defences irrespective of their quality, even at the exp ense of the fitness of siblings and the maternal plant. Evidence for t hese defence patterns were evaluated by examining the allocation of fu ranocoumarins 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 i nvestment (endosperm mass) and seed genotype on furanocoumarin allocat ion by mating an array of pollen donors with pollen recipients. Furano coumarins were found to be positively correlated with endosperm mass o n one side of the seed, a result consistent with optimal defence theor y; 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 furano coumarin influenced by the seed genotype is small relative to the tota l quantity in the seed, these furanocoumarins are the first line of de fence against important predators, such as the parsnip webworm, Depres saria pastinacella (Lepidoptera: Oecophoridae). We found that parsnip webworm larvae were able to discriminate among genotypes within an inf lorescence. In line with previous studies, these results suggest that a genotype's ability to influence furanocoumarin defence may a?ect its probability of survival. We conclude that the distribution of defence s among plant offspring in wild parsnip is probably influenced by comp etition among seed genotypes that conflicts with maternal optimal defe nce.