"nicollas" a écrit :
L'idée de base est que les parties immatures de l'arbre peuvent être influencées plus "facilement" par les conditions extérieures (comme le PG), et donc qu'un PG aux fruits inférieurs pourrait influencer les fruits de l'arbre issu du pépin. Je suppose que si cette influence est réelle, et que cette influence est retirée (en faisant pousser l'arbre sur ses propres racines par exemple), il y a plus de chance que le résultat soit plus conforme aux lois mendéliennes ?

Horizontal genome transfer as an asexual path to the formation of new species

Here we have examined the possibility that allopolyploidization can also occur by asexual mechanisms. We show that upon grafting—a mechanism of plant–plant interaction that is widespread in nature—entire nuclear genomes can be transferred between plant cells. We provide direct evidence for this process resulting in speciation by creating a new allopolyploid plant species from a herbaceous species and a woody species in the nightshade family. The new species is fertile and produces fertile progeny. Our data highlight natural grafting as a potential asexual mechanism of speciation and also provide a method for the generation of novel allopolyploid crop species.

"nicollas" a écrit :

http://libgen.org/scimag/get.php?doi=10 ... ature13291

http://i853.photobucket.com/albums/ab98/permactiviste/graft_hybrid_chimera_zpseb5eaed1.png
Diagram explaining the method of producing graft chimera—Winkler’s method. (A)
Scion of nightshade is grafted onto the stock of tomato. (B) After union, the junction
is cut transversely. (C) A shoot of sectorial chimera is produced from the junction.
~~~
http://i853.photobucket.com/albums/ab98/permactiviste/graft_hybrid_mentor_zpsbde81b4d.png
The most widely adopted mentor-grafting method in annual plants. (1) A seedling at
cotyledonary phase is grafted onto an older stock. (2) Leaves of the scion except for two
to three at the top are removed during the entire growth.

During recent decades, however, several independent groups of non-
Soviet scientists repeatedly showed that graft-induced variant characteristics
were stable and heritable. To take several examples: in 1955, Dr. Shinoto, the
president of Japanese society of genetics, claimed to have obtained graft hybrids
of eggplants: (1) The variety of Kantoao (blue fruits) was grafted onto the
variety of Sinkuro (black fruits). Twenty were successfully grafted of which 16
bore fruits in the branches of the scions; in 9 plants the fruits were blue, while in
the other 7 plants they were black. (2) Sinkuro was grafted onto Kantoao; out of
four grafted plants two showed that the fruits on the stock were all blue, while in
the other two plants bodies, leaves and fruits of the stocks all changed into
black. (3) The first generation raised from seeds obtained by selfing in the
blackened fruit borne on the scion of Kantoao showed segregation into the
two types of plants: 25 blues and 10 blacks. It is worth mentioning that
similar positive results were obtained by Zu and Zhao (1957) in China, Stroun
et al. (1963) in Switzerland, and Rajki and Pal (1966) in Hungary, in their
experiments involving eggplant graft hybridization.
Glavinic (1955) claimed to have heritably transmitted three single-
gene Mendelian characters in tomato (cut leaf rather than potato leaf, yellow
fruit rather than red, and short fruit rather than long) from the variety
“Kartofelisn,” which was used as the stock, to the variety “GoldenTrophy,”
which was used as scion. Scions were grafted as young seedlings at the cotyle-
donary stage. Significantly, it was the first-generation seedlings, produced from
selfed fruits of the scion, and not the scion itself, that produced various combi-
nations of stock and scion characters. When one plant of each combination was
selfed, there was a tendency to breed true for the three single-gene Mendelian
characters, although considerable segregation also occurred. Glavinic checked
the homozygosity of these varieties for four generations before she started the
grafting experiments. The buds on the scion were isolated by pergameneous
bags, which were kept in place until they were removed by the fruit that had
been formed in it. The parental Kartofelisni and Golden Trophy were grown
as controls. Dean (1962) recognized that Glavinic’s work satisfied the mini-
mal requirements for experiments presuming to prove the possibility of graft
hybridization, although he was suspicious of Glushchenko’s claims.
During a span of time between the 1950s and 1970s, Frankel demon-
strated graft transfer of cytoplasmic male sterility in Petunia. Male-fertile main-
tainer scions were grafted onto male sterile stocks. Scion flower male-fertility
remained autonomous, but male-sterile progeny were produced when scion
flowers were selfed or crossed to other maintainer clones (Frankel, 1956, 1962,
1971). Edwardson and Corbett (1961) confirmed Frankel’s experimental claims.
Attempted graft transfer of cytoplasmic male sterility has also been successful in
sugar beets (Curtis, 1967) and alfalfa (Thompson and Axtell, 1978).
Kasahara and his coworkers conducted extensively graft hybridiza-
tion experiments in pepper (Capsicum annuum L.). In 1967, Ohta, a Japanese
“Mendelist,” was allowed closely to examine Kasahara’s original data. He
became aware that some anomalies could be attributed to mistakes but inferred
that other incongruities seemed potentially real and beyond the knowledge of
Mendelian genetics. He and his coworker carried out grafting experiments by
using Kasahara’s materials and methods and produced variants similar to those of
Kasahara’s. In their experiments, reciprocal grafts were made between two
cultivars of red pepper: Tochigisantaka (with erect, fasciculate, and red fruits)
and Kiiro (with pendent, nonfasciculate, and yellow fruits). An overall rate of
variant occurrence of 0.84% was obtained in the first, second, and third genera-
tion self-crossed progeny of the scion (Ohta and Choung, 1975a). In another set
of experiments Ohta and Choung (1975b) found that in stocks artificially
infected with the single-stranded RNA virus for “broadbean wilt” the rate of
gene transfer increased dramatically from 2% for noninfected stocks to 16.5%
for infected stocks. Apparently the hereditary changes associated with grafting
were enhanced by virus infection. It should be noted that, unlike Kasahara,
Ohta paid special attention to the characteristics of fruiting direction, fruiting
position, and ripe pericarp color, since he knew that these were Mendelian traits
with gene symbols already established; and they were clear enough to be
understood by Mendelists, who had either refused to accept or been skeptical
of Michurinists. From the beginning of his experiments, a special concern was to
eliminate possible contamination at every step of the experiments. He used pure
materials, which had been maintained for five generations by selfing, and sown
the seeds in sterile soil and boxes. The genotype of each variant was determined
by selfcrossing and testcrossing to both parents to check for the possibility of
accidental contamination. Ohta and Chuong (1975a) concluded that contami-
nation could not be responsible for the majority of the variants and that the rate
of variant occurrence was too high for spontaneous mutation. Thus, as one
citation of his paper noted—the work is thoroughly documented and there is
no cause to doubt the genuineness of the results because these observations were
made by professional Mendelian geneticists and have been repeated with similar
results in independent laboratories ( Pandey, 1985).
Yagishita and coworkers have made clear the existence of graft hybrids
in pepper, eggplant, and other plants (Hirata and Yagishita, 1986; Hirata et al.,
2003; Taller et al. , 1998, 1999; Yagishita and Hirata, 1986, 1987; Yagishita et al. ,
1990). In pepper, variant fruits were obtained from a “Yatsubusa” scion, which
was grafted onto a “Spanish Paprika” stock, and had been stably inherited for at
least 27 generations by seed propagation (Yagishita et al., 1990). Both the stock
and the scion cultivars differed in many characteristics, and each repetition of
grafting enhanced the range of variations in the variants (Taller et al., 1999).
After the induction of genetic changes by repeated grafting, they selected
variant lines based on variant fruit shape, and finally established stable graft
hybrid lines, explained in part by invoking the term “G.” By using G 5 S 45
generations (i.e., the 45th generation of sexual hybridization by inbreeding,
after five successive grafting), they analyzed the graft-induced genetic changes
and the inheritance of several characteristics (Taller et al., 1998). Now they are
pursuing the possibility of foreign gene transport from stock to scion through the
vascular system, integration into the genome, and sexual transmission to the
scion progeny in graft system at molecular level (Hirata et al., 2003).
Dole and Wilkins (1991) made auto and reciprocal grafts among
different poinsettia cultivars. When the scions of Eckespoint C-1 Red (CR), a
restricted-branching cultivar, were grafted onto the stocks of Annette Hegg
Brilliant Diamond (BD), a free-branching cultivar, vegetative characteristics of
branching pattern and leaf morphology of CR plants were altered when com-
pared to the control graft combination CR/CR (scion/stock). Eckespoint C-1
Red scions grafted onto BD stocks produced a plant very similar to BD plants
when axillary shoot length and node number were compared. However, axillary
shoot diameter and leaf morphology were intermediate between CR and BD
plants. Changes were retained after two generations of serial vegetative propa-
gation and are considered permanent.
In July 1965, Fan grafted a bud taken from an adult purple-leaved plum
tree onto a 1-year-old apricot seedling. In March 1966, the main stem of the apricot
seedling was wiped out, and then the bud of the purple-leave plum began to sprout
and grow vigorously. In March 1980, surprisingly, an offshoot with purplish red
leaves was produced from the root of the apricot and then was transplanted. The
variant plant began to blossom in 1985. Interestingly, it closely resembled the scion
of the purple-leaved plum but was totally different from its original plant—the
stock of the apricot in characteristics (Fan, 1999). This provided evidence for the
phenomenon underlying Michurin’s mentor-grafting method.

B. Graft Transformation
In the early 20th century, genetic dogma proclaimed that although the
stock may influence the phenotype of the scion (e.g., size control), the
scion and stock maintain their genetic identity (Bailey 1928). This
paradigm, which might be called conservation of genetic identity, was
then challenged by reports of graft transformation. The theory of graft
transformation has an infamous history in the former Soviet Union. The
Russian pomologist and fruit breeder Ivan Vladimirovich Michurin
(1855–1935), with a popular reputation similar to Luther Burbank in
the United States, made the neo-Lamarkian assertion that genetic
effects, could be induced by the environment, including grafting,
although this assertion was not based on specific experiments. The
issue became profoundly political when Trofim Lysenko, a Soviet crop
physiologist best known for his explanation of the vernalization of
winter wheat (cold induction of wheat flowering through seed
treatment), developed a theory of environmentally induced genetic
effects. He held the position that formal genetics based on the gene,
which he termed Mendelian-Morganism, was reactionary and bour-
geois and contrasted it with Darwinism-Michurinism, the reconcilia-
tion of inheritance of acquired characters with Marxist dialectical
materialism. Supported by Stalin, the Lysenko faction succeeded in
making genetics a political issue (Glass 1948). In 1940, N. I. Vavilov, the
plant breeder and botanist best known for his work on the domestica-
tion and centers of origin of crop plants, was relieved of his position as
head of the Institute of Genetics and imprisoned in 1943. There he later
died of starvation, to become a martyr for science. In 1964, the physicist
Andrei Sakharov spoke out against Lysenko in the General Assembly of
the Academy of Sciences and charged him with being ‘‘responsible for
the shameful backwardness of Soviet biology and of genetics in
particular’’ (Gorelik 2005). Vavilov’s reputation has been restored in
Russia, and he is now considered one of the most revered personalities
in science.
The claims of graft transformation were generally unreproducible by
appropriate methods of experimentation (Stubbe 1954; Bohme, 1957;
Topoleski and Janick 1963). However, over the past several decades,
many papers have been published in peer reviewed journals, such as
Science, Proceedings of the National Academy of Science (USA),
Genetics, Journal of Heredity, Euphytica, and Theoretical and Applied
Genetics, reporting observations that appear to support the concept of
graft hybridization, particularly a series of papers published by
Japanese researchers using a range of species including red pepper,
eggplant, tomato, tobacco, and soybean. The most intensive analysis
focused on changes observed in grafted Capiscum annum cultivars
with varying fruit morphology (Hirata et. al. 1986; Yagishta 1961;
Kashara et al. 1971). This work employed ‘‘mentor grafting’’ in which a
very young seedling that is continually defoliated is grafted to a mature
rootstock, so that the scion is a sink for stock-derived nutrients. A
range of stocklike phenotypic characteristics were observed in the
scion fruit, and in some case, these characteristics were transmitted to
seedling progeny (Taller 1998). Another Japanese scientist repeated
this work (Ohta 1991) with largely similar results. Among the stocklike
phenotypes reported to be inherited in the scion progeny were
alterations of fruiting direction, fruiting habit, and pericarp color.
Ohta states that genetic analysis indicates that these three traits are due
to independently inherited Mendelian genes that are highly stable in
the cultivars used in the grafting studies. The frequency of transmis-
sion of stocklike traits in the progeny of the scion was reported to be
highly variable across different experiments, but with an average rate
of 0.84%.
A second line of experimentation with graft-induced variation
involves the generation of cytoplasmic male sterility through grafting
of petunia (Frankel 1956; Edwardson and Corbett 1961), sugarbeet
(Curtis 1967), and alfalfa (Thompson and Axtell 1978). In these
studies, cytoplasmic male sterile plants were used as rootstock and
lines expressing nuclear maintainer/restorer genes were the scions.
The scions were observed to flower normally; however, a significant
percentage of progeny plants were scored as male sterile.

All of these studies run against the mainstream understanding of
genetics and are viewed with considerable skepticism. None of the
authors cited has proffered an explanation for these observations that
can be supported by modern molecular biology. Ohta (1991) specified
that the mechanism responsible for graft-induced variation was ‘‘graft
transformation,’’ in which chromatin was translocated from the stock
to the scion. A variant of this model was offered by Liu (2006) in a
review in which he states: ‘‘I propose that the stock mRNA molecules
are being transferred to the scion—then reverse transcribed into cDNA
that can be integrated into the genome of the scions germ cells,
embryonic cells, as well as the somatic cells of juvenile plants—may be
the main mechanism of graft hybridization.’’
The evidence provided for the transfer of chromatin or DNA
sequences is highly questionable. Ohta (1991) built his case on a
microhistological analysis that he interpreted as showing chromatin
masses moving through the cell walls of dying cells. Taller et al. (1998)
presented random amplified polymorphic DNA (RAPD) analyses
indicating that a stock-specific DNA marker could be detected in
graft-induced variants; however, it must be noted that standard
controls for RAPD experiments were not included in the data
presented. It is difficult to assess the significance of observations
associated with graft transformation given the lack of rigorous
experimental characterization and, with the exception of Ohta, the
absence of independent experimental replication. If these observations
are in fact artifactual, the most likely explanation may be pollen
contamination, although efforts to avoid this were noted in some of the
papers. Modern plant molecular biology has certainly not provided
support for the models involving translocation of DNA from one cell to
another.
However, before leaving this topic, it is worth considering how the
recent insights into RNA-mediated gene silencing might apply to
grafting and could provide a plausible mechanism for some of the
phenomena associated with graft-induced variation. It is now known
that the accumulation of double-stranded RNA (dsRNA) activates a
homology-dependent mechanism that cleaves the dsRNA into 21 to 25
base-pair fragments, known as small interfering RNAs (siRNAs). These
are utilized to direct the sequence-specific degradation of mRNA or to
suppress transcription via DNA methylation (Baulcombe 2005). A
particularly fascinating aspect of this process is that the silencing
signal can be propagated through the phloem so that gene silencing
occurs elsewhere in the plant (Tournier et al. 2006). It should be noted
that such movement would be stimulated in mentor grafting. This was
demonstrated by experiments in which transgenic tobacco plants
expressing green fluorescent protein (GFP) were grafted onto rootstock
containing another GFP construct that was silenced. In this case, the
rootstock GFP transgene was silenced because it produces dsRNA
homologous to the GFP coding sequence. In these grafted plants, the
silencing signal is propagated into the scion where GFP expression is
consequently abolished via degradation of the mRNA.
The demonstration of the transmission of silencing across the graft
junction raises the question whether it could be involved in graft-
induced variation. Several reports describing graft hybrids claim that
graft-induced variation can be inherited. Heritable genetic changes can
also result from the RNA-mediated gene silencing mechanism. One of
the actions known to result from production of siRNAs is sequence-
specific methylation of DNA. Heritable transcriptional silencing of
GFP genes in transgenic tobacco has been observed when siRNAs
target methylation to the promoter (Jones et. al 2001). Approximately
30% of seedlings that display GFP silencing will maintain this state
throughout their life cycle; plants that remain fully silenced give rise
to silenced progeny, the majority of which will eventually revert to
being nonsilenced.
Although all of this experimentation involves the silencing of
transgenes rather than endogenous genes, most aspects of the silencing
mechanism apply to naturally occurring genes and transgenes alike.
Based on what is now known, RNA-mediated silencing could provide
an explanation for variation reported in graft hybrids, provided the
phenotypic changes result from gene inactivation. If this mechanism
does underlie such variation, several conditions would be predicted to
exist in the rootstock and scion. It would be predicted that the gene in
the rootstock responsible for triggering the graft-induced variation
would be silenced and have a configuration such that it produces
dsRNA. If graft-induced variation is heritable, it would also be
predicted that the homologous gene, which becomes silenced in the
scion, would not be transcribed in the progeny and would display
DNA methylation.
The point we want to make is that although the topic of graft-
induced variation has been surrounded by much controversy and
generated much skepticism, aspects of the phenomena associated with
it have interesting parallels with RNA-mediated gene silencing. We
certainly are not prepared to state that any of the graft-induced changes
reported in these papers result from RNA-mediated gene silencing.
However, if similar observations could be produced in a model plant
like Arabidopsis, the resources would be available to rapidly
determine which genes were being affected. Given the wealth of tools
available for studying RNA-mediated gene silencing, this system could
be employed to carry out a rigorous reexamination of graft-induced
variation (Brosnan et al. 2007).

"belinsecte" a écrit :
J'aimerais savoir si …