Conflict between a parent plant and its offspring?

Examples of parent-offspring conflict in animals are numerous.  However, there is little evidence for such conflicts in plants.  Parent-offspring conflict arises because the genotype of an offspring differs from that of either parent. In simple terms, from the perspective of the mother, fitness is maximized by allocating resources and defenses among her offspring (seeds) so as to maximize the number of surviving offspring. In contrast, the fitness of an offspring is maximized by garnering as many nutrients and defenses as possible, even if this occurs at the expense of siblings.  The allocation of nutrients and defenses to offspring fathered by different pollen donors could take either of the forms shown in the box above.  An equal allocation is favorable if the mother is unable to assess the relative fitness of each of her offspring.   On the other hand, an unequal allocation may be favored if she can detect relative fitness and allocates accordingly (better offspring receive more nutrients and defenses). allocation.jpg (8330 bytes)

Seeds are complex genetic entities.  seed consists of a diploid maternal genotype  derived from the ovary wall (shown in yellow), a diploid embryo (shown as a black dot) with equal genetic contributions from the pollen donor (father or sire) and pollen recipient (mother or dam), and a triploid endosperm (shown in tan), in which the maternal genetic contribution is twice that of the paternal parent.  In wild parsnip, the embryo is immature, meaning it is small and poorly developed.  The defense chemicals, called furanocoumarins, are located in oil tubes (shown as two red bands on the outside (non-commissural) side of the seed and as four bands on the other commissural side.

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When three parsnip "mothers" were crossed with seven "fathers", we found that the identity of the father had a significant effect on the furanocoumarin concentration in the non-commissural oil tubes.  This influence did not extend to the amount of endosperm or to the amount of furanocoumarins in the commissural oil tubes.In other words, allocation of a portion of the defense among offspring differed from the allocation of nutrition (endosperm) among offspring.  From the perspective of the maternal plant, this does not appear to be an optimal allocation.



Why is there a pollen donor effect on noncommissural defense?

The seeds of the wild parsnip are born in fruits on the maternal plant.  Each fruit is a schizocarp that splits at maturity into two mericarps, each of which contains a seed.   While still part of the fruit, each seed has its noncommissural face exposed on the outside of the fruit.  Parsnip webworms include these unripe fruits in their diet, but prior to consuming any one fruit, the caterpillars scrape the surface of the fruit with their mouthparts and only then "decide" whether to consume the fruit.  Thus, the quantity of non-commissural furanocoumarins may determine the fate of an offspring.  Under such circumstances, natural selection will favor offspring that are able to defend themselves more effectively.


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Can parsnip webworms distinguish between offspring sired by different pollen parents?

The answer is that they can.  Given their choice of fruits, all of which came from a single inflorescence but were sired by different pollen donors, larvae showed a clear preference for fruits sired by plant E compared to fruits sired by plants A or B.

Pollen parents can also influence photosynthesis of fruits

last updated 1/11/2000