IB breeding (was: Triploids)

I wrote
:
:>IBs do show some fertility, a phenomenon that seems to be
:>increasingly common in recent decades. This is useful, for example, in
:>trying to transfer a particular IB trait into SDB or TB lines. But we still
:>can't effectively line breed IBs by crossing IB x IB and selecting the best
:>seedlings.

and Ellen asked
:
:	Ada Godfrey of Hermit Medlars Walk gardens, lists 45 IBs in their
:	'97 catalog and has indicated the ones that are fertile.  There are
:	only 11 out of 45. Is this about average for IBs?
:
:	If two IBs are fertile, couldn't they be used for hybridizing? Tom's
:	last sentence in the quoted paragraph I took as a general statement
:	since most IBs are sterile.

I'll elaborate a little bit. I'm not sure if 1 out of 4 is a typical
proportion of fertile IBs, but it wouldn't surprise me. Also, it's not so
much the case that most are sterile and some are fertile, but rather that
most have very limited fertility (hard to get crosses to take, hard to get
many viable seeds when they do take).

So, suppose with an ordinary fertile TB or SDB I can make 10 crosses and
count on, say, 200 seeds. With a limitedly fertile IB as one of the
parents, I might expect 20 seeds from the 10 crosses. Using limitedly
fertile IBs as *both* parents would cut the expected seed count down to 2!
It's not always that bad, of course--you might cross those two IBs once and
get a full pod of 40 seeds.

But I think you can see that the standard method of breeding TBs (cross two
good TBs, raise a few hundred seedlings, keep the best) is less practical
for IBs. There's a good chance you won't get any seeds from the IBs you
think are best, so you'll be using ones that you like less just because
they're somewhat more fertile, and even then probably not ending up with
enough seedlings to make a proper selections. I'm not saying that the laws
of nature absolutely exclude the possibility of breeding good IBs by
intercrossing other IBs, just that it's such a struggle that the folks who
are doing the traditional thing and crossing TBs with SDBs will be way
ahead of you.

It's also important to point out that when an IB is fertile, it is likely
to produce either TB-type gametes or SDB-type gametes. So if I did succeed
in making an IB x IB cross, I'd expect to get roughly 1/2 IBs, 1/4 SDBs,
and 1/4 TBs. It's an interesting line of work for experimenters, because
you can learn just how much mixing of TB and pumila chromosomes is going
on. IBs also offer an interesting opportunity to smuggle some pumila genes
into the TBs!

Most hybridizers line breed to some degree. What this means is that after
selecting the parents and making the initial cross, you get some seedlings
you like but that are not good enough to introduce. Then what? Well, you
use *those* seedlings in breeding, perhaps even crossing siblings or
backcrossing to one of the parents to emphasize the traits you think the
seedling lacks. Maybe you need another generation. For this kind of work to
succeed, you need to count on reliable fertility in the line. You want to
use the seedlings that have the best characteristics. If only 1 out of 4 is
(limitedly) fertile, you're unlikely to be lucky enough to be able to use
the best ones when breeding the subsequent generations.

Look at the pedigrees of Hager's SDBs and MTBs, for example. You'll see how
much back-and-forth goes on in shaping a "line" that meets the hybridizer's
goals. The same can be said for Keppel's plics, Gatty's pinks, Schreiner's
blues, etc. (although in some cases the line breeding is camouflaged
because some of the irises in the line were named and introduced, so their
ancestry is not given explicitly in the pedigree).

All this is to explain why I said "can't effectively line breed IBs by
crossing IB x IB"; this does not mean you can't cross two IBs and raise
some seedlings that might be attractive or useful in some other sort of
breeding.

Fertile families (such SDBs, TBs, and arilbred halfbreds) lend themselves
to continual refinement and development through breeding lines. Groups with
limited fertility (such IBs, arilbred quarterbreds, arilbred medians, and
triploids) are not readily improved by such techniques. Rather, breeders
must routinely go back to the basic cross that produces the type (for IBs,
this is the TB x SDB cross), and improvement comes only as the parent types
(in this case SDBs and TBs) improve.

Happy irising, Tom
(whose very first seedpod ever was on an IB--because I didn't know enough
not to try!)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tom Tadfor Little                   telp@Rt66.com
Santa Fe, New Mexico (USA)
Telperion Productions  http://www.rt66.com/~telp/
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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