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Re: example of mitotic recombination in Hosta


Jim Hawes wrote:
RE:>>So not all yellows come from nuclear mutations. Many are derived from
mutant plastids according to authorities cited.
-------------------

Hi Jim, Ben, Joe, Bill, Dan, Preston, Bill N., and anyone else that has
some interest in this genetics discussion re: Hosta Sports,

I gave both of these postings (RE: mitotic recombinations) a quick scan
and, like Paul's comment on Sue's references and mine on copyrights, "maybe
you're both right" but further investigation may produce a different
opinion.  Unfortunately, I didn't know up from down on this subject a year
ago, so the ol' learning curve has been pretty steep.   I've spent some
time investigating this phenomena from the inside out, starting from the
perspective of the molecular biologist with the DNA molecules in the
nucleus and working out to the plastids.  I'm still in a quandary but
occasionally feel like progress is being made.

If the pollen parent transfers a haploid set of chromosomes to the pod
parent, and the pod parent's strands of DNA are already held tightly
together by multiple polypetide chains, the DNA from the pollen parent will
be much more likely to be transposed than the pod parent.  This helps to
explain why the coloration in the leaves is controlled by the pod parent.
Coloration, is due primarily to the chloroplasts, chromoplasts, and
amyloplasts and mom has a lot more to say about this than dad.  The
yellow/orange portions of the leaf have a high content of chromoplasts with
an abundance of carotenoids but no chlorophylls.  The chloroplasts are
abundant with chlorophylls but the carotenoids are less abundant (but
visible in the fall when the chlorophylls die) while the amyloplasts have
no pigments (white leaf tissues?)

In looking at Arabidopsis Thaliana, researchers discovered that only 35% of
the genes were unique, while gene families containing >5 members was
37.4%.  Thus, a goodly portion of the genome is segmentally duplicated
across the five chromosomes.  There were quite a number of genes that were
simply reversed in orientation.  Since one amino acid being incorrect can
cause as serious a disease as sickle-cell anemia, doesn't it seem feasible
that a single nucleotide being incorrectly transcribed from the nuclear DNA
to the DNA of the plastids could cause significant differences in the
pigmentation of the leaf?  (I'm confused about this plastid DNA business
still, but I'm gaining on it).

In Nature, Dec 14, 2000, pg. 810, the authors state, "of the seven subunits
of the cytochrome B6F complex (PetA-D, PetG, PetL, PetM), only one (PetC)
was found in the nuclear genome, whereas the remainder are probably encoded
in the chloroplast".  They go on to state, "For pigment biosynthesis, 16
genes in chlorophyll biosynthesis and 31 genes in carotenoid biosynthesis
were found".  Since, in Drosophila Melanogaster, more than one gene can
influence the same trait, e.g. eye color, it seems likely that redundant
genes and the placement of genes in the genome sequence are potentially
part of this mitotic recombination to which Ben Z. refers, but that the
colorations of sports are more likely due to mutations in the chloroplasts
since so many more protein sequences of the chloroplasts are determined by
chloroplast DNA, to which Jim H. refers.

I am still searching for answers (no kidding).  When the pollen is
transferred to the egg, is the DNA that will eventually reside in
chloroplasts and mitochondria part of the chromosomes of the nucleus, or
are they separate?  Seems like something I ought to know about this
process.

It is easy to see how just one base-pair substitution from the nuclear DNA
could lead to a replacement of a single amino acid.  The post-transcription
gene expression from the nucleus, through to the chloroplast could mean
that the teeniest mutation in the nuclear DNA translates to significant
mutations in Chloroplast DNA.

So, maybe you're both right?  Back to the "big books", as Carolyn would
say.  Don't ya just love that Carolyn Brashear?  Unlike me, she never says
anything mean about anybody, not even the people she KNOWS are mean!

--
Andrew Lietzow
#1 Plantsman at http://hostahaven.com
1250 41st St
Des Moines, IA 50311-2516


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