HYB: chromosome notation and sorting out the little guys
- To: Multiple recipients of list <i*@rt66.com>
- Subject: HYB: chromosome notation and sorting out the little guys
- From: T* T* L* <t*@rt66.com>
- Date: Wed, 26 Mar 1997 16:46:34 -0700 (MST)
This is a follow-up to my posting about a new notation for chromosome stuff.
The notation gives us a "breeder's classification" for irises in the
bearded and aril groups. It also corresponds roughly with the horticultural
classification scheme for bearded irises used by the AIS. There are some
key differences, though. Most of the difference seem to come about because
a set of "T" chromosomes may include genes from very different kinds of
bearded species: true tall beardeds like pallida, cypriana, and
mesopotamica; median-sized species like variegata and aphylla; and even
true dwarf species like reichenbachii and suaveolens. When aphylla is used
deliberately to produce medians, small plants are obviously the intended
result. I have a hunch that small irises that appear unexpectedly in
breeding (such as BBs from TB lines or MDBs from SDB lines) may be the
result of height-controlling genes from smaller bearded species being
smuggled along for generations in the chromosomes of TBs, waiting to come
together in the right combination to produce small-sized plants.
Be that as it may, we can recognize that an iris with a "T" set may be
either large or small compared with others of the same type, depending on
what genes are carried in that set. If we want a horticultural
classification system, we need to respect the size variation present in
irises with similar genetic make-up. Let's do it this way:
chromosomes large size small size
TTTT, TT TB BB, MTB
TTTP IB ? SDB
TTPP SDB MDB
TPPP MDB MDB
PPPP MDB -
AAAA, AA AR -
AAT AB+ ? ABM+
AATT AB ABM (arilbred median)
ATTT AB- ? ABM-
AAPP ABD (arilbred dwarf) -
APTT ABM ? ABD
APT ABD ? ABD
Some relatively uncommon chromosome combinations have been omitted for the
sake of clarity. A "-" means one would expect no small segregates, because
"T" chromosomes are not present (one could make a case for distinguishing
tall arils from small ones, but I won't bother). A "?" means I'm not aware
of any small irises in commerce with this chromosome breakdown, but I would
expect them to be categorized as shown.
The terms "arilbred dwarf" and "arilbred median" are not official
classifications, and require some explanation. I use "arilbred dwarf" to
refer to an arilbred that has dwarf bearded ancestry and grows in the MDB
or SDB height range. I use "arilbred median" to refer to an arilbred that
has dwarf bearded ancestry (often mixed with a lot of TB ancestry) and
grows in the IB or SDB height range. I allow some overlap, the distinction
being whether dwarf characteristics seem to predominate over TB
characteristics in the plant.
One could, in principle, establish a separate "class" for each entry in
this table. Many entries are conveniently and naturally lumped
together--the differences between the different kinds of MDBs are not very
obvious to the average gardener, for example. Likewise, there is no real
need to subdivide the ABMs and ABDs any further.
The table raises an old question: do we need three median classes in the
IB/BB/MTB height range? Originally, each class represented a particular
type of breeding: TTTP/TTTT/TT. The pumila genes made IBs bloom early, and
tetraploidy made the BBs larger and coarser than the MTBs. Separate classes
were erected to encourage work in each of these different families. With
increasing numbers of IBs, BBs, and MTBs from aphylla breeding (all TTTT),
the boundaries are being pushed. If the TTTT type ever comes to dominate
all three classes (as it may), it may become appropriate to merge them into
a single median class.
It also raises a new question: why are there no arilbred dwarf and arilbred
median classes? I addressed this subject in an earlier post and will not
repeat it here. But I think an ABM class is needed now, and an ABD class
will be needed before many years have passed.
Other questions for the aril lovers on the list: do we need separate
classes for irises (aril and arilbred) from predominantly regelia breeding?
Should AB+, AB, and AB- be kept separate, or merged?
Currently, AB- irises are separated out by the AIS awards system: they
compete for the William Mohr Medal. AB, AB+, and AR all compete for the
C.G.White Medal.
As long as we have two classes anyway, I think a superior breakdown for
gardeners would be
AB (to include AB-, AB, and AB+) vs. ABM (to include ABM and ABD)
Pure arils are not widely gardenable enough to require an AIS award
category, in my opinion. In fact, no pure aril has ever been in the running
for the C.G.White Medal (or C.G.White Award).
If I were God (or the AIS Board of Directors, which is nearly the same
thing), the chart would look something like this:
predominantly bearded mixed predominantly aril
< 8" MDB ABD AR
8"-12" SDB ABD AR
12"-15" SDB ABM AR
15"-22" median ABM AR
22"-26" median AB AR
>27" TB AB AR
I would put most AB- quarterbreds in the "predominantly bearded" column,
and some AB+ 3/4-breds (the ones Sharon often calls "near arils") in the
"predominantly aril" column.
My opinion is that this the minimum number of classes that communicate to
gardeners what to expect when they buy a plant. Height is very important in
garden design and planning, and also (within the bearded group) is a good
indication of bloom season and need for winter cold. Aril content is very
important to the form and color pattern of the bloom, and also is a good
indication of bloom season and the need for dry summers.
A classification system that does not make these basic distictions (such as
the current system for arilbreds) does a disservice to gardeners.
Some of these classes could be further divided, however, to encourage
particular lines of breeding within them that might be otherwise ignored.
There is some merit in dividing the "median" class for this reason, and
also in dividing the "predominantly aril" classes to encourage those
working with regelias or pure oncocyclus hybrids.
Well, I ended up writing more than I intended. It seems more like an
article than a posting. Perhaps, at least, it will encourage some to think
of classification systems in terms of the logic behind them and what they
are used for, rather than simply as unquestioned conventions.
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Tom Tadfor Little telp@Rt66.com
Iris-L list owner * USDA zone 5/6 * AIS region 23
Santa Fe, New Mexico (USA)
Telperion Productions http://www.rt66.com/~telp/
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