I was speaking about cultivated plants where triploidy is used to
prevent seed formation, such as in triploid watermelons, triploid
marigolds and a hypothetical triploid grape. Even such triploids may
rarely produce a seed but they are seedless for practical purposes.
Technically, meiosis requires an even chromosome number. A triploid
dandelion producing a mix of 1n to 2n gametes doesn't really fit the
definition of meiosis. Meiosis is defined as a cell division that
produces four cells, each with half the chromosome number of the
beginning cell.
I don't know how the idea that 'Thompson Seedless' was triploid got
started. Assuming a seedless fruit is triploid is not a good bet. Most
commercial seedless fruit are not triploid. They are seedless for
other reasons such as lack of pollination or lack of compatible
pollen.
The assumption that 'Lexia' grape is triploid and parthenocarpic
because most 'Lexia' raisins don't have seeds may be a bad assumption.
'Lexia' is one of many synonyms for 'Muscat of Alexandria' or
'Muscatel' grape:
http://www.psgrill.net/Wine/Varietal...er/muscat.html
Raisins made from Muscat grapes often have their seeds removed during
processing:
http://www.internationalrecipesonlin...ionary.pl?5637
http://www.ams.usda.gov/standards/raisin.pdf
Therefore, 'Lexia' may not be a seedless grape. Occasional seeds in
'Lexia' raisins may just be seeds not removed during processing.
David R. Hershey
(Graeme Hirst) wrote in message ...
David Hershey wrote:
It is a widespread misconception that 'Thompson Seedless' grape is
triploid. I have even seen that misconception in biology textbooks.
Several university websites also say 'Thompson Seedless' grape is
triploid:
http://www.biology.arizona.edu/cell_...iosis/04q.html
http://www.bol.ucla.edu/~bredelin/RDGenes/week3.html
http://fybio.bio.usyd.edu.au/vle/L1/...fTestAnswers/L
+ivingSystems/Unit09.htm
However, they are in error.
The University of California seedless grape review link I posted
earlier mentions that seedlessness in grapes in caused mainly by
stenospermocarpy. It does not even mention the term triploid:
Table Grape Berry Growth and Development: A Review:
http://cetulare.ucdavis.edu/pub/Grape0302.PDF
'Thompson Seedless' grape cannot be triploid (3n) because it has
been used as a parent in breeding of other seedless grape cultivars.
Triploids cannot produce viable pollen or eggs because meiosis
requires an even ploidy number such as 2n or 4n.
I do not know what the situation is in this grape cultivar, but it is
not generally true that triploid plants are incapable of parenthood.
In fact, there are biologists who study the so-called 'triploid
bridge' that enables (a low rate of) gene transfer in wild
populations between diploid and tetraploid 'species'.
The following sort of situation has been observed (for example in
dandelions). During meiosis in the triploid, two of the three
complete sets of chromosomes pair with each other, and the remaining
set of chromosomes end up in varying numbers in each of the two
products of that first meiotic division. In effect, the gametes from
the triploid contain numbers of chromosomes that cover the entire
range of numbers between 1n (half the diploid number) and 2n (half
the tetraploid number). It is thus possible for a triploid to produce
a very small number of offspring either from crossing with a diploid
or from crossing with a tetraploid, or even (a much lower number of
offspring) from crossing with a similar triploid (if pollen
self-incompatibility in the stigma and style don't prevent this).
My favourite 'Lexia' raisins occasionally contain a seed, and I have
even managed to germinate these, but have not yet produced a strong
plant that doesn't get mildew. I had assumed that 'Lexia' was a
triploid grape that is also parthenocarpic (since the fruit don't
abort even if there are no mature seeds), but I don't actually know
if this is the case.
==
Nadia Talent / .
Department of Botany / University of Toronto
and Centre for Biodiversity and Conservation Biology / Royal Ontario Museum