Paul has given excellent answers, which I won't repeat. Just a few "sidebar" points:
* Chromosome pairing is only an issue during the production of gametes; its only effect is on fertility. During normal somatic cell division (mitosis), each chromosome duplicates before the cell divides, so it doesn't matter how many sets are present or where they come from. A diploid, even from a wide cross, won't be unhealthy, it will just be infertile.
* Likewise, a wide cross between tetraploids is no easier to make than a wide cross between diploids. Compatibility is mostly about how closely related the species are, rather than their ploidy.
* If an unbalanced tetraploid or a triploid does manage to produce viable gametes, the gametes that have the same makeup as the other parent are the ones most likely to produce viable seed. (This is called selective fertilization.) This accounts for the observations that quarterbreds crossed with halfbreds tend to produce more halfbreds, and that triploids crossed with tetraploids tend to produce more tetraploids.
I have some articles on the theory of diploid, tetraploid, and amphidiploid breeding and fertility on my website, if anyone is looking for some additional reading on the subject. http://www.telp.co/irises/
Regards, Tom
--- In i*@yahoogroups.com, dwiris@... wrote:
>
> Thanks for your explanations.
>
> Dorothy Willott
>
>
>
> In a message dated 3/6/2011 1:33:39 A.M. Eastern Standard Time,
> pharcher@... writes:
>
>
>
>
> I'm asuming you are asking abnout the fertility of the the IB's here. Not
> the actual production of one.
> No one is actually absolutely sure about why there is fertility in the
> IB's from such a cross. Most times the gametes formed are all mixed up. The
> way the chromosomes divide up would depend on how much alike the sets are
> like the other set, the homology. There is no way to tell that just by
> looking at it. That is from experimentation. One plant may tend to produce
> mostly SDB type gametes that have a full set of 8 and a full set of twelve.
> Others may produce gametes composed of all TB chromsomes.
> My experience with the aril quarterbreds has been this. Most of them are
> sterile. They are composed of one aril set, 10 (or 11) chromosomes and 3
> TB sets. So far all I've been able to get off of the few seeds I've gotten
> are halfbreds when crossed to other halfbreds. You would think that you
> would get a few quarterbreds but the seeds either abort or don't survive in
> the garden for me to see them. Most quarterbreds are highly infertile with
> other TB's (Capitola and Elmohr derived quarterbreds seem to be the
> exception based on the Registrations) even though the quarterbreds have three sets
> of TB chromosomes. They seem to favor being balanced toward their overall
> genetic composition of half aril and half TB (i.e. losing a whole set of
> aril chromosomes seems to be a big deal).
> Producing one is simple. An SDB contributes a set of 8 and a set of 12.
> The TB contributes two sets of 12. Said and done. You have an IB. That
> is the foundation of the Class itself.
>
>
> -----Original Message-----
> From: dwiris@...
> Sent: Mar 6, 2011 12:12 AM
> To: i*@yahoogroups.com
> Subject: Re: [iris-species] I. sibirica X I. pseudacorus
>
> <ZZZ>
>
> <ZZZ>
>
> I am interested in knowing what happens when you cross the amphidiploid
> SDB with a TB and get an IB?
>
> Dorothy Willott
>
>
>
> In a message dated 3/5/2011 11:26:18 P.M. Eastern Standard Time,
> pharcher@... writes:
>
> Example 2: SDB's are amphidiploids that combine the tetraploid chromosomes
> of MDB's (32) from pure I. pumila backgrounds and the 48 chromosomes of
> the TB's. Each gamete will get one set of two sets chromosomes and two sets
> of twelve for a total of 40 chromosomes. When these plants form gametes
> the chromosomes divide the set of 16 I. pumila chromosomes into two sets of 8
> and divide the 24 TB chromosomes into two sets of twelve. The set of 8
> chromosomes pair up and the set of twelve pair up. The cell divides and each
> gamete will have one set of eight and one set of twelve for a total of 20
> chromosomes.
>
>
> <ZZZ>
>
