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HYB: Tetraploids


From: Sharon McAllister <73372.1745@compuserve.com>

Message text written by Cindy Rust:

>Sterling, the only way I know that an iris can be called tetraploid is: 1.
if it is from two tetraploid parents, 2. it is tested and proven to be a
tetraploid  3.  if it crosses readily and gives fertile seeds with
tetraploids, some people will assume that it too, is a tetraploid.<

Yes, these are the simplified rules usually applied to autotetraploids --
which also work for those allotetetraploids that have been derived from
highly homologous species.  

At this point in the discussion, someone new to the topic almost always
makes the mistake of equating tetraploidy with fertility.  The
above-mentioned rules are certainly valuable to those working within a
fertile family.  In this case, the three rules can be restated:
1.  Crossing two compatible autotetraploids produces seedlings of the same
type.
2.  "Proven to be tetraploid" means that the chromosomes have been not only
counted as divisible by four, but can actually be separated into matched
sets.
3.  A cultivar whose breeding behavior matches that of known members of the
fertile family is often assumed to also be a member of that family.

BUT, the world of unbalanced tetraploids has its own set of rules, which
restate the above-mentioned ones as:
1.  Crossing two nonhomologous autotetraploids or two allotetraploids
produces seedlings that are tetraploid -- ranging from fully fertile to
essentially sterile, depending upon the relationship between their
ancestral lines.
2.  "Tetraploid" means simply that it has four sets of chromosomes, tells
us nothing about fertility.
3.  Breeding behavior plays a significant role in seedling selection and
evaluation.

If you want to delve into this further, there's a lot of material on this
in the Archives -- especially in discussions about hybridizing arilbreds.  

The recent thread on I. balkana is a good example.  Although our modern TBs
have been derived from a number of ancestral species, they have evolved
into a family of artificial autotetraploids.  I. balkana is also
tetraploid, but not completely homologous, so it's not surprising that the
first generation cross could produce functional amphidiploids.   Crossing
one of those back to either parental line would, however, likely result in
offspring that are unbalanced tetraploids.  These tend to exhibit just the
lack of fertility that's been described, but are sometimes fertile enough
to produce breakthrough breeders that contributed significantly to the gene
pool.

That's what makes work with wide crosses so much fun.

Sharon McAllister
73372.1745@compuserve.com

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