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.

Dave

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