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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.
   parthen.JPG (64688 bytes) 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

parthsurv.JPG (13233 bytes)
webwrom_parth.JPG (49533 bytes) 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.
char_parth.JPG (12360 bytes) 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.  
percparth.JPG (52624 bytes) 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. 
   


last updated 3/8/2000