Re: mitotic recombination
- To: hosta-open@mallorn.com
- Subject: Re: mitotic recombination
- From: h*@open.org
- Date: Sun, 4 Mar 2001 18:56:01 -0800 (PST)
Ben:
I've been going through the web sites I downloaded and I only found a
few references to mitotic recombination, and most of these only
speculate that mitotic recombination is the cause of the results they
see. Most of the cases are either contrived or beyond what we would
consider normal. In most of the cases HIGH doses of irradiation had
to be used. I'm presenting some of the material below for those who
might be interested, along with a few short comments.
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A search for interchromosomal mitotic recombination in maize
--Peterson, T
There are very few reports of spontaneous mitotic recombination in
plants (e.g. Carlson, 1974; Ashley, 1978). We have previously
identified deletions of the maize P gene which are thought to occur
via premeiotic intrachromosomal recombination between 5.2 kbp direct
repeats which flank the P-rr gene (Athma and Peterson, 1991).
Deletions between direct repeats at the Knotted locus have also been
reported (Lowe, Mathern and Hake, 1992).
To detect interchromosomal mitotic recombination, we crossed together
two P gene alleles which should produce twinned sectors after
reciprocal mitotic recombination. The P-wr allele specifies colorless
pericarp and red cob glumes, while the P-oo-13:255A-10 allele
specifies orange pericarp and orange cob glumes. The P-oo-13:255A-10
allele is essentially a weak-acting P-rr allele due to a 6 bp (2
codon) insertion in exon 1 of P-rr left as a footprint following
excision of the transposable element Ac. Plants of genotype P-wr/P-oo
were grown in an isolation field, detasseled, and allowed to pollinate
with a P-ww pollen donor. The progeny ears were inspected for sectors
of altered kernel pericarp pigmentation. In particular, twinned
sectors of red and white pericarp should be visible against the orange
pericarp specified by the P-oo allele. Among approximately 400 progeny
ears, two distinct and unambiguous red/white twinned sectors were
found. These twinned sectors could have arisen by mitotic crossing
over of chromosome 1 homologs in the four-strand stage between the P
locus and the centromere, followed by appropriate segregation of
chromatids to the two daughter cells. These daughter cells would
produce adjacent cell clones, with one clone carrying two doses of
P-oo (gives red pericarp) twinned with a clone carrying two doses of
P-wr (gives white pericarp). Unfortunately, both twinned sectors are
very small (approximately 1 mm in width) and unlikely to be
transmitted to the egg due to their location on the abgerminal side of
the kernel. However, molecular testing may be possible if sufficient
DNA can be obtained from twinned sectors for PCR analysis.
_____________________________________________________________________
Please note that the author says "These twinned sectors COULD have
arisen by mitotic crossing over", not that mitotic crossing over DID
cause the results. Also note that this is NOT L1-L2 layer chimera
formation as would be required for hosta variegation.
______________________________________________________________________
Concurrent Products of Premeiotic Recombination
Y. HIRONO and G. P. REDEI
Arabidopsis is the first higher plant in which premiotic exchange of
linked markers has been reported (HIRONO and REDEI, 1963, 1965).
Drosophila is the only other higher organism where non-meiotic
crossing over has been demonstrated (cf. WESTERGARD, 1964). In human
leukocytes there is only cytological evidence for spontaneous
chromatid exchange (GERMAN, 1964) and for mitomycin C induced
chiasmata (SHAW and COHEN, 1965)
Genetic analysis of somatic cells is virtually impossible since
critical progeny tests are limited to the "germ line", in higher
plants if the differentiation of the "soma" is reversible an adequate
analysis of the mechanism may become feasible. Interpretation of the
causes of somatic sectoring requires extreme caution. The term mitotic
crossing over should be used only if the recombination yielding exact
complementary products of exchange is achieved by parameiosis (HURST
and FOGEL, 1964) i.e., meiotic type synapsis and mitotic centromere
disjunction. Unfortunately these criteria have not yet been
ascertained in any organism including the lower plants, extensively
studied. Premeiotic or nonmeiotic exchange appears to be a better term
for the cases where interstrand reassortment of linked markers prior
to meiosis can be demonstrated, but the exact mechanism responsible
for the exchange is not identified. Since the mechanism of meiotic
crossing over is not understood (cf. WESTERGAASRD, 1964; UHL, 1965),
equating two unknowns does not contribute to any knowledge.
Somatic recombination may be useful as a general term to denote
sectoring. The term somatic crossing over has only historical value
since strictly somatic cells generally can not be subjected to a test
which may distinguish between the several mechnisms causing somatic
recombination.
A previous study in Arabidopsis (HIRONO and REDEI, 1963, 1965)
demonstrated premeiotic recombination in a single case by the analysis
of the half product of the exchange. Recently indications were found
of the appearance of double products of the exchange. Also additional
cases of non meiotic recombination of linked markers will be reported
below.
Again, recombination was induced by x-irradiation of triple
hetrozygotes (cf. HIRONO and REDEI, 1965). Seed was harvested of each
plant which displayed yellow (ch) sectors. Each seed sample was
individually analysed by progeny test to determine genotypic
frequencies.
______________________________________________________________________
Please note again that the author says "recombination was induced by
x-irradiation of triple hetrozygotes". This is not something that
happens very often to hostas growing in the garden.
_____________________________________________________________________
High non-reciprocal recombination in a barley cross
Hartmut Schreiber
Institute for Breeding Research, 0-4300 Quedlinburg, Neuer Weg 22
(Sachsen-Anhalt), Germany
In crosses of a sex-rowed line (vv) with a two-rowed variety (VV),
aberrant ratios were found in the kernel row number and the fertility
of lateral floerts, which are controlled by an incompletely dominant
gene. To test the reason for this instability, further crosses
involved the dominant gene Lk which suppresses the development of awns
on the lemma and is linked completely to the v locus in chromosome 2
(Nilan, 1964).
Ths six-rowed, awned Line 152 (vv, lklk) originated from the Dornburg
mutant collection (Hentrich, 1964) and the two-rowed, anless Ethiopian
barleys E.P. 79 and HOR 2937 (VV, LkLk) obtained from teh Gatersleben
World Collection were crossed. The F1 was swon in January in pots in a
glasshous without iluumination under cool condition, and the F2 in
March in a foliage house. Single ripe plants were harvested, and the
types of spikes and awns were classified.
Results
The F1 of the cross of two-rowed, awnless barleys (VV, LKLK) with
six-rowed, awned ones (vvlklk) commonly had intermediate, short-awned
spikes (Vv, Lklk). However, when the Ethiopian barleys were crossed
with the Line 152, three exceptional F1 plants (6.1%) carrying medium
length to long awns were also found. One of them was a chimeric plant
having two intermediate, short-awned spikes (No. 1/1 and 1/2) and a
two-rowed, long-awned one (No. 1/3). Table 1 shows the segregation of
single spike progenies of this chimera, the other two exceptional
plants (No. 2/1 and 3/1), and ratios of progenies from normal Fl
plants (No. 4 to 7). In the progenies of the exceptional spikes, the
parent type 1 (VV, LkLk) is absent, but the recombinant type (VV,
Iklk) appears frequently.
Discussion
Because the functional genes v and Ik are very closely linked, three
phenotypic classes of progeny are expected in the F2. The ratio is 1/4
two-rowed, awnless; 1/2 intermediate, shortawned; and 1/4 six-rowed,
long-awned plants. The surprising occurrence of one recombinant type
only indicates non-reciprocal recombination at the v-lk region. Such
events, termed gene conversion, were first discovered and well
documented hitherto in the fungi (see Catcheside, 1977). The
appearance of the recombinant, long-awned phenotype in the F1 and in
the intermediate class in the F2 suggests that the conversion of the
genes Lk into Ik and v into V, respectively, occurs in somatic cell
divisions of heterozygote plants. The chimeric nature of several
plants, which probably also causes the aberrant ratios, supports this
assumption. The occurrence of these events needs a high frequency of
mitotic chromosome pairing, which is likely promoted by crosses with
Line 152, and seems to be also influenced by growing conditions.
In conclusion, the Lk-V genetic system provides an opportunity to
investigate frequencies of non-reciprocal events, their dependence on
genetic and ecological factors, and their implication in plant
breeding.
____________________________________________________________________
Please note that this case is something very much out of the ordinary.
Also note that the author is not making an absolute claim that this is
unquestionably mitotic recombination.
______________________________________________________________________
Homologous Recombination in Arabidopsis Induced by HO Endonuclease.
A project at : University of Rochester.
Research by: One P. Investigator.
RAY. THE INTEGRITY, AS well as the diversity of chromosomes, is
maintained by recombination and yet little is known about how this
mechanism is judiciously performed, particularly in plants. A system
of induced mitotic recombination in Arabidopsis thaliana, in which a
single, defined, DNA double strand break is delivered to the
chromosome at a particular locus has been developed. The target site
of a site-specific DNA endonuclease, HO endonuclease of yeast, is
cloned into a unique chromosomal location and the break induced by the
expression of a cloned HO gene in the plant. Specific objectives are
1) to test the hypothesis that a chromosomal double strand break in
meiosis can induce homologous recombination in Arabidopsis and 2) to
determine whether ectopic recombination can be stimulated in mitosis
and meiosis by a DNA double strand break in the plant chromosome. A
better understanding of the basic mechanisms involved in recombination
of plant chromosomes will result from this study using the model
plant, Arabidopsis thalania. Moreover, any movement toward the goal of
achieving site-specific recombination in plants will have significant
impact in both basic and applied research.
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A little more complicated then anything that has been done with
hostas!
Ben, the evidence is clear - there isn't mich evidence for mitotic
recombination in higher plants, and NONE that I could find that
studies mitotic recombination in hosta. This isn't to say that there
might not be a variegated hosta out there somewhere that is the result
of mitotic recombination, but the event is so rare that we really can
dismiss it as a plausible cause for variegation in hostas until such
time as you can supply the necessary scientific studies to show that
it really is a common event in hostas.
Joe Halinar
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