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Re: mitotic recombination


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.

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

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 

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 

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 
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. 


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. 


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 

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. 

A little more complicated then anything that has been done with 

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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