arilbred chromosome mysteries revealed (was: OGB+ in NY)
- Subject: arilbred chromosome mysteries revealed (was: OGB+ in NY)
- From: t*@Lanl.GOV (Tom Tadfor Little)
- Date: Tue, 5 Mar 96 10:51:13 MST
Linda writes
:good grief - i just read what the world of iris has to say about arilbreds -
:what a genetic mess! i was going to go see if i could figure out what OGB+
:is without having to ask someone to repeat the definition, but now my head is
:spinning.
:
:what is ogb+? and how many chromosomes does it/them have?
An OGB+ is any arilbred with more than 1/2 aril complement.
*Most* of these irises are produced by crossing the amphidiploid
halfbreds (OGB) with diploid pure arils (RC, OG, or OH).
The amphidiploid halfbreds usually have two sets of aril
chromosomes (10 each) and two sets of TB chromosomes (12 each),
for a total of 44.
The diploid arils usually have two sets of 10 each; total 20.
(Regelias have 11 chromosomes, so some diploid arils have 21
or 22 chromosomes.)
So a typical OGB+ gets 10+12=22 chromosomes from the arilbred
parent and 10 from the aril parent, for a total of 32.
But of course halfbred x diploid aril is not the only cross that
produces OGB+, it is just the most common. The aril parent could
be a tetraploid, for example.
:
:sharon - are hybridizers still adding tall bearded genes to the arilbreds?
: you mentioned tetraploids - do all current arilbred tetraploids commonly
:referred to have the same chromosome number and what is it? are they fertile
:with tbs?
I'll butt in and answer some of the questions for Sharon too. :)
Tall bearded genes can be added to the arilbreds by basically two types
of crosses: TB x arilbred and TB x aril.
The first type of cross gives mostly sterile quarterbreds (OGB-),
so you add the TB genes to the seedlings from the cross, but that's
basically the end of the line, because the seedlings are not fertile. The
fertile
arilbreds (OGB) we have today are mostly descended from crosses C.G.White
made in the 1940s and 1950s, so the TB genes they carry are pretty old.
The TB x aril crosses also tend to produce sterile seedlings, if the
aril parent is a diploid. The first arilbreds were produced this way
early in the century, but it is a strategy that's not been used much
in recent decades, now that we have the fertile halfbreds.
HOWEVER, we now have tetraploid arils to breed with. Crossing a TB
with a tetraploid aril produces a fertile halfbred (OGB). So today
there is, at long last, a way to introduce modern TB genes into the
arilbred gene pool: crossing modern TBs with tetraploid arils.
Here's a quick chromosome count summary of the different arilbred
classes. "A" represents a set of 10-11 aril chromosomes, "B" represents
a set of 12 TB chromosomes.
PURE ARILS--diploid: AA = 20, 21, or 22 *fertile*.
PURE ARILS--tetraploid: AAAA = 40-44 *fertile*.
C.G.WHITE-TYPE HALFBREDS (OGB)--amphidiploid:
AABB = 44-46 (usually 44) *fertile*.
OGB+ --triploid: AAB = 32-34 (usually 32) *sterile*.
OGB+ --tetraploid: AAAB = 42-45 *sterile*.
OGB- --tetraploid: ABBB = 46-47 (usually 46) *sterile*.
Keep in mind that these are just the most common chromosome makeup
of the different classes. Many combinations are possible. Also,
"fertile" and "sterile" are generalizations; there are quite a
few exceptions.
The tetraploid arils are a very exciting development, because
now we can create *new* *fertile* arilbreds by crossing arils with
TBs. Previously, we had been limited to line-breeding the small
group of fertile arilbreds from C.G.White and their kin.
The ASI Yearbook for 1995, which should be out in a couple weeks,
has several feature articles on tetraploid arils and their breeding
potential.
Happy irising, Tom.
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Tom Tadfor Little tlittle@lanl.gov -or- telp@Rt66.com
technical writer/editor Los Alamos National Laboratory
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Telperion Productions http://www.rt66.com/~telp/
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