I sent David's posting to my favorite grape geneticist and got this
reply (posted with her permission):

In article ,
David Hershey wrote:
Seedlessness in grapes is not due to triploidy although that seems to
be a common misconception. Most seedless grape cultivars are not even
parthenocarpic, as are most other species or cultivars that produce
seedless fruit. Parthenocarpic species or cultivars produce fruit
without fertilization so the fruit are truly seedless. 'Black
Corinth'is considered a parthenocarpic grape.

Most seedless grapes are stenospermocarpic, which means the seeds
start to develop after normal pollination and fertilization but abort
before maturity. The remains of the aborted seed(s), called seed
traces, can be seen in the "seedless" fruit.

There are apparently two stenospermocarpic mutations which account for
almost all seedless grape cultivars. There is no reason to believe that
this exhausts the possibilities, since the cultivars in the two groups are
either clones of each other or in some cases parent/offspring pairs. Many
of these clones have been propagated so long that enough somatic mutations
have accumulated to consider them unique individuals, not just sports.

For more information about these groups, see "Simple Sequence Repeat
Analysis of a Clonally Propogated Species: A Tool for Managing a Grape
Germplasm Collection", by GS Dangl, ML Mendum, BH Prins, MA Walker, CP
Meredith, and CJ Simon, GENOME 44: 432-438 2001. Don't know if it's
available on the web or not.


Grape breeders now use tissue culture to prevent the tiny embryos from
aborting. This technique is called embryo rescue. It allows two
seedless grapes to be crossed and produce viable offspring. Embryo
rescue had been a major reason why there are so many new seedless
grape cultivars.

The other reason is the institution of several breeding programs, at UCD
and among some commercial companies. It helps that DNA fingerprinting
(available through the Meredith lab previously, shortly to be offered
through the Foundation Plant Material Service when Bud finishes making the
transition) can now positively identify patented individuals, making it
much harder to steal the fruits of a breeding program, if you'll parden the
pun.

Previously, grape varieties were identified through ampelography: sight
identification through leaf and cluster characteristics, mostly.
Unfortunately, grapes show a wide variation in these characteristics
depending on climate and other conditions, and there are darned few really
good ampelographers out there. (The aforementioned Andy Walker is one, and
there's a guy in France with an unspellable name, Jean-Micheaux Borsechot,
or something similar.)

The DNA fingerprinting is a lot cheaper, a lot surer, and much, much easier
to learn.

One nice thing about this thread is that I learned two new words. Since
my spouse also likes new words, I thought I'd try them out on him:

Me: Here's a new word for you: ampelography.
Him: Hmm. "Enough writing"?
Me: Bah! How about stenospermocarpic then?
Him: [long pause] "Male note taker with sore wrists"?