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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). |
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| 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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Crosses
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.
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Pollen parents can also influence
photosynthesis of fruits
last updated 1/11/2000
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