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Re: Tetraploid Arilbreds

Lloyd Zurbrigg wrote:

:   but the aril-breds are notoriously difficult. Now that there are a few
:   fertile tetraploids among the aril-breds, the difficulty should ease up.

Anticipating the next question, I'll go ahead and expand on this....

Arilbreds may be diploid, triploid, tetraploid, and once-in-a-while even
pentaploid.  I like to think that there's no such thing as a completely sterile
iris -- just ones who haven't yet found the proper mate -- so I work with all

That said, Lloyd is absolutely right about the value of the fertile tetraploids.
But there's a catch:  you can NOT assume that an arilbred is fertile just
because it is tetraploid.  There are four types of tetraploid arilbreds:

Tetraploid 1/4-breds have only one set of aril chromosomes and three sets of
nonaril chromosomes, so they're called "unbalanced".   Example: the classic
10:12:12:12 oncobred.  Although they are "supposed" to be sterile, I've found
that many do exhibit some partial fertility with the right partner.  Their main
role in arilbred hybridizing has been to discourage beginners who start
spreading pollen around without first learning something about the genetics of
arilbreds, but they've also produced some nice things in the hands of experts
like Les Peterson.  

Unbalanced tetraploid 1/2-breds have two sets of aril chromosomes and two sets
of nonaril chromosomes, but have limited fertility because the two sets of
nonaril chromosomes are different.  For example, a 10:11:8:12 arilbredmedian.
These are relatively rare and those who work with them are either masochists or
seeking out a real challenge.  (I do NOT consider myself a masochist -- I no
longer try to grow TB seedlings in this climate!)
Balanced tetraploid 1/2-breds have two sets of aril chromosomes and two sets of
matching nonaril chromosomes.  Some of these are 10:11:12:12, but even among
those with some regelia heritage most are 10:10:12:12 because the chance that
the regelia metacentric gets lost increases with each generation.  The technical
term for this is AMPHIDIPLOIDLIKE HYBRIDS.  More important than the name, though
is the fact that their balanced nature makes them fertile.  This is the type
Lloyd referred to.  They constitute a fertile family, just as tetraploid TBs are
a fertile family.  Select breeding stock from this group, and you can daub
pollen around among them with the same liberty you would take working
exclusively with TBs.  And probably better odds of getting something worthwhile
because there's much less competition.

Tetraploid 3/4-breds have three sets of aril chromosomes and only one set of
nonaril chromosomes, so they are also unbalanced and their fertility is
typically as limited as that of the unbalanced 1/4-breds.  There are a few
exceptions that exhibit enough fertility to be useful in advanced programs --
but I wouldn't include them in the group of  "fertile tetraploids" that Lloyd
wrote about.  

More than you wanted to know?  Not enough?  The floor is open to questions....

Sharon McAllister (73372.1745@compuserve.com)
Hybridizing in Southern New Mexico for over 20 years

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