Re: RE: Hyb: Tet vs Diploid
Just some possibly relevant thoughts here.
Based on chromosome numbers alone, there is nothing that should be
difficult about a tetraploid crossed with a diploid. What is probably
more important is how closely related are they (ie. how long have they
been evolving away from one another, and how different have the
chromosomes become). Very close relatives should produce a first
generation without much trouble (in theory, but not always in practice, as
other factors such as flower size, pollen tube length, or chemical
inhibitors, etc. might be issues). I try to put pollen from the larger
(or longer) flower on the smaller (or shorter) flower, as the physical
limitations are less likely to exist. A short pollen tube may not reach
the ovules below a long style.
In the first generation you should get mostly triploids though, and these
should all be infertile or of reduced fertility do to the odd chromosome
compliment. Odds of a triploid producing pollen or ovules with a "proper"
full chromosome compliment (either one set or two) are low. The trick is
getting proper diploid (almost impossible) or tetraploid (unlikely)
offspring so that you can continue the breeding lines. In some plants
this rule of thumb doesn't apply (probably groups with lots of duplicate
chromosomes in their makeup), but in Iris it does.
I suspect that the difficulty with I. aphylla crossed with diploids may
have more to do with a rather distant relationship or perhaps with length
of pollen tubes than with actual chromosome numbers.
I. pallida is apparently very closely related to the TB Near Eastern
tetraploids, and it makes sense that those crosses might be easier. Most
of the early successes of diploid x tetraploid in the early TB lines were
with I. pallida or an I. pallida x I. variegata hybrid crossed with a Near
Eastern tetraploid TB species. Closely related species (those with a more
recent common ancestry) will have more genetic material that is the same,
and pairing of chromosomes in such crosses is more likely to be
successful. With higher the chromosome numbers (such as with
tetraploids), likelihood of getting hybrid offspring from more distantly
related species seems somewhat higher (more material present to mix and
match I suppose), so tetraploids are often somewhat easier to cross with
one another than are otherwise equivalent diploids. However, even at
tetraploid or higher levels, the more distant the relationship, the less
likely success will be.
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