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Re: Re: Broken Colour

  • Subject: Re: Re: Broken Colour
  • From: "Kelly D. Norris" <kellydn@frontiernet.net>
  • Date: Fri, 26 Dec 2008 20:09:02 -0600

This is a good subject for "kitchen table debates" or "on-screen debates" at
this time of year.  The only other thing worth doing this time of year in
the Midwest is a) travel to another climate or b) go birding and I'm doing
that tomorrow.
Chuck wrote:  "If one of the anthocyanin producing genes has a functioning
transposon, then production of anthocyanin will be sometimes turned on and
sometimes turned off. This will produce spots if turning off and on is
rapid, or streaks if turning off and on is slow."
With regards to transposons and broken color irises, I agree with nearly
everything Chuck said.  That's a much more salient and cursory explanation
than I could've managed!  But with regards to "forms" of broken colors
(streaks versus splash versus dotting, like that which is seen in 'Minnesota
Mixed-Up Kid'), the deviation is caused when transposition occurs at
different points during cellular division and the duration of those
transposed states.  For example, colorless sectors of petals (in otherwise
purple or colored petals) reveal that the transposon has completely turned
off the pigmentation gene.  Speckled or dotted petal segments indicate
transposition has occurred relatively late in cellular division (ie-fewer
cells have been affected and few mitotic clones have been produced) and
could represent (as it does in corn, the model system for transposons) the
movement of the gene segment out of the original site it "jumped" into.
Streaking indicates (as Fred alluded to in his email) transposition
relatively early in cellular division, hence resulting in large swatches of
identical cells formed after continued rounds of mitosis (preceded first of
course by a meiotic event).  As a sidebar, transpositions are often not
associated with dominant inheritance patterns though a few cases have been
noted in corn suggesting other mechanisms working in concert with the
transposons.  The danger with any discussion of transposons is the inference
of their movements.  While likely not entirely random, it's easy to fit the
ends to the means.
But transposition may not entirely explain broken colored irises (though I'm
of the opinion that it's the best explanation).  It certainly isn't the only
way that a broken pattern could result.  I've observed similar patterns that
one might call broken coloredness in irises damaged by herbicide.  The
immediate implication of course is mutagenic activity.  But what's peculiar
is that a similar phenotype can be produced in a very different way than a
transposition event.  Here's two other BC possibilites:
1.   Heteroplastic segregation-This is a maternally inherited condition.
The banner child for this sort of appearance is many of the variegated
poinsettias.  This condition is caused by differing cytoplasmic gene
products (gene products of plastids (like chloroplasts and chromoplasts) and
mitochondrial DNA) and again would only be inherited through the pod parent.
This system is very complex and isn't well understood outside of model
systems like poinsettia.  At least 20 different "types" of mutations have
been recorded in poinsettia and some can occur at the same time, leading to
some of the exciting patterns observed in these holiday favorites.  Does
anyone know of broken color lines that are only maternally inherited?
2.  Post-transcriptional gene silencing- This is a fun one.  Basically what
all that alphabet soup means is that the RNA transcript of the original DNA
is destroyed or interfered with.  Essentially, a plant can have a normal
gene that isn't able to code for a normal protein because of some level of
interference in the cellular machinery.  This has been shown in petunias
where researchers artificially "gave" a plant more copies of a gene than it
normally possessed.  The results where opposite of what the researchers
expected (they were trying to improve flower coloration).  Instead the
"overdose" shut off both the added and normal copies of the gene and
resulted in a white flower (with a few purple sectors where this shut down
effect didn't occur).  So what if a mutation in irises occurred that
resulted in replicate copies of the plicata locus (or any other locus for
that matter)?  Could gene silencing lead to mosaic patterns?  Again, cutting
edge stuff that isn't impossible to think about in other realms of the plant
Keep in mind this is just me noodling on the couch after a supper of too
much chili.  While all of these, from transposons to the last ones I
discussed, are completely plausible in theory, they are absurd without
evidence.  It can be difficult to isolate the effects of any one of these
with crosses along as well.  The Puccini/Expose/ "distalata" pattern also
leaves one to wander about the effects of transposons in these cultivars.
Wow that got long!  Let me know if I need to clarify my position.
Kelly D. Norris
Farm Manager, Rainbow Iris Farm
Co-Editor, Bulletin of the American Iris Society
Bedford & Ames, IA 
Zone 4b/5a
 <http://www.kellydnorris.com/> www.kellydnorris.com

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