Why
Parthenocarpy? |
| The question perplexed even Darwin. Parthenocarpic
fruits are devoid of embryo and endosperm. Several edible fruits, such as seedless
grapes, have been bred to afford the taste of the fruit without the seed. That such
fruits should occur with frequency in nature has been a mystery, because they do not yield
viable offspring and thus do not contribute directly to fitness. Wild parsnips
produce these fruits in remarkable abundance. As much as twenty percent of the fruit
crop can be parthenocarpic. |
 |
A back-lighted parthenocarpic fruit is easily visible to the naked eye as
a green structure, similar in size to normal fruits, but without the large endosperm that
creates a shadow in the normal fruit. |
|
Watch as the lighting shifts between front- and back-lighting;
parthenocarpic fruits are indistinguishable from normal fruits under front-lighting. Only
when viewed with lighting from the back, do parthenocarpic fruit reveal their true nature. |
|
| A role in defense
The persistence of parthenocarpic fruits in parsnips appears
to be related to their defensive value against their most destructive enemy the parsnip
webworm. Given a choice between parthenocarpic and normal fruit, webworms
overwhelmingly prefer those that are parthenocarpic. This preference is unaffected
by the relative frequency of normal and parthenocarpic fruit; even when rare, the
parthenocarpic fruits are preferentially eaten |
 |
 |
This preference is all the more remarkable because the parthenocarpic
fruits are inferior food. Larvae that are fed only partheno- carpic fruit eat less
of them per unit time, convert less of what they eat to body mass, and, consequently, grow
more slowly than larvae fed normal fruits. |
 |
A case of junk food webworms. That webworms grow poorly on
parthenocarpic fruits is not surprising. These fruits contain far less
nitrogen and calories than normal fruit. However, they also contain only half
as much of the toxic furanocoumarins . Furanocoumarins are known to enhance
resistance of parsnips to webworms. A speculative explanation of this phenomenon is
that the lower furanocoumarin concentration of parthenocarpic lures the webworm into
feeding on those fruits. Since furanocoumarin concentrations vary widely from plant
to plant (see parent-offspring conflict), it probably is beneficial, on average, for the
insect to avoid high furanocoumarin concentration tissues. The benefit of such
fruits to the parsnip are clear--parthenocarpic fruit, in which few resources have been
invested, are sacrificed for fruits containing viable offspring. |
 |
All of the work on parthenocarpic fruit was conducted in the
laboratory. Does this phenomenon occur in the field? In a field population
infested with webworms, we counted the proportions of parthenocarpic fruits on plants that
had webworms and plants without webworms. Plants with webworms had proportionately
fewer parthenocarpic fruits than plants without webworms, a result that is consistent with
the behavior that webworms prefer parthenocarpic fruit. |
| |
|
