Re: RE: HYB SDB SA
I don't remember the exact quote, but there was recently a comment about
waiting a long time to run across the rare unreduced diploid gamete in a
diploid to get pairing with a diploid gamete from a tetraploid.
This is just a suspicion, but I do suspect that one would not have to wait
long at all, in fact I suspect it would be a rather common event.
Plants produce lots of gametes (especially pollen, in the case of Iris).
Unreduced versions shouldn't be rare at all, accidents in cell division
happen all the time. So, when pairing gametes from a diploid with those of
a tetraploid, it would seem to me that it would be almost inevitable that
some small percentage would come out as fertile tetraploids on a regular
basis (assuming the base compliments are all compatible, as with a diploid
base 12 x a tetraploid base 12).
This would seem to be backed by the fact that in early crosses of I.
pallida, (and fewer, but still notable numbers of I. (pallida x variegata),
and I. variegata) with tetraploid species, there were an amazingly (in my
mind) high number of fertile tetraploid clones produced. The hybridizers
may have gone through hundreds of progeny, perhaps thousands, to find them,
but none-the-less, there were quite a few of them produced, and they were
the foundations of the TB strains that we grow now.
Yes it is rather rare to have a tetraploid in a diploid wild population,
and rare to have a hexaploid or octaploid in a tetraploid population, but
it is not unheard of. I think the higher the ploidy, the more rare it is.
However, unreduced gametes are probably the main (only?) way that wild
tetraploids came to be. It is rare that unreduced gamete would meet
unreduced gamete in the wild, but it does occur. More often an unreduced
gamete would pair with a normal gamete to produce a sterile (or nearly
sterile) triploid, which would be a dead end and produce few further
progeny, so the diploid ploidy level tends to be maintained. Even if a
fertile polyploid occurs, it would have to occur at the same time and the
same place as another to start producing fertile offspring of a higher
ploidy. So, usually ploidies tend to stay the same in wild populations
(not always, but usually). I suspect that unreduced gametes actually occur
relatively frequently in wild plants, but the odds are against them leading
to anything. We have the advantage of being able to put unlike with unlike
and then to search for the fertile ones. So, they all have a better chance
of getting together.
Nature has a way of getting around blockades such as ploidy levels though.
Occasionally the odds are indeed beaten. I have seen many wild triploid
hybrids produce fertile seed (mostly cacti, not Iris, but the principal is
the same), even though they aren't supposed to. In most cases this seems
to be achieved by the production of unreduced gametes that pair with
gametes of the appropriate number from another plant (often yet another
species). The offspring from these seeds are often totally fertile with
normal ploidies (often they are not).
Well, enough of that.
I guess the bottom line is don't think of it as "hopeless", but rather as
very "possible".
Dave
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