Bruce writes:

> Wirt Atmar writes:
>
>  >Seedless
>  >varieties are almost always produced by hybridization of two fully sexual,
>  >normally genetically isolated species to produce a sterile hybrid so that
>  >no further floral fertilization (a pollen grain meets up with an ovule) is
>  >required for fruit set.
>
>  One exception is the navel orange. All of today's navel orange trees are
>  the result of sequential grafts of a single mutant orange tree found in
>  Brazil in the 1850s (not sure of the exact date).

Two things: (i) this seems to be my day to respond, so after this reply, I'll
shut up and sit down, and (ii) what follows is more than any sane person would
want to know.

I did not mean to imply that parthenocarpy occurs only due to human influence.
Sterile hybrids are a common and completely natural result of closely related
species cross-pollinating one another, in both plants and animals. Sterile
hybrids, of which parthenocarpous varieties are a common result, frequently
occur in natural situations. In the genus Citrus, bees are the primary
pollinators. Because the genus is speciose, accidental hybridizations are
relatively common in Citrus. If the same bee should travel between similar
species, sterile hybrids often result.

Nor is parthenocarpy relegated only to angiosperms (flowering plants). It also
occurs in wind pollinated plants such as the gymnosperms (pine trees), but
there the name is changed a bit and the "seedless" hybrid is called
parthenoconus (parthenocony).

The sterile line clearly doesn't survive long. Its propagation can only be
vegetative, a form of clonal reproduction. Ultimately it dies due to the
accumulation of somatic mutations (copying errors).

However, to emphasize Bruce's point, parthenocarpy is much more nowadays than
it is in nature; it is, after all, a "deadend" out there in the wild. But,
because seedless fruits have enhanced agronomic value and consumers are
willing to pay more for the fruit of these particular varieties, people have
learned how to induce parthenocarpy in a great number of plants, not only by
hybridization, but also through the application of plant hormones, such as
auxins, at the appropriate time in the plants' development.

Wirt Atmar