In article , mel turner mturner@sni
pthis.acpub.duke.edu writes
In article ,
wrote...

A true graft hybrid is a plant that has genetic material from both
parents in all cells and which has arisen from cells right at the
graft union. True graft hybrids are very rare, because you have to get
actual mixing of cell contents and then development of a shoot from
these mixed cells.

Ummm, I don't think that's accurate. As I recall it, "graft hybrids"
are just chimeras, with tissues composed of a mix of cells of both the
stock and scion of the original graft union. The cells don't actually
fuse, but both cell types take part in the same growing tips that
produce the shoots, leaves, flowers, etc. of the "hybrid".

Such periclinal chimeras in angiosperms can be quite stable [forming
shoots with outer tissues from one "parent" over inner tissues of the
other], and graft hybrids will sometimes arise as adventitious buds
formed from the graft union tissues. It can be possible to destroy all
the lateral buds on a grafted plant, cut it off through the graft union,
and get "graft hybrid" shoots regenerating from callus formed from the
graft union region.

Hybrids formed by somatic cell fusions such as you describe are also
possible [especially in laboratory cell cultures], but they're
a separate thing from "graft hybrids".

I was also of the opinion that graft hybrids and chimeras were the same,
but I was recently browsing a book which had them as different. It
appears that the original hypothesis for various plants
(Hawthorn/Medlar, Laburnum/Broom) now recognised as chimeras was that
they were true hybrids arising from fusion of cells in the graft, and
therefore described as "graft hybrids". It would appear that there are
two usages for the term "graft hybrid". (And checking B. Daydon
Jackson's "A Glossary of Botanic Terms" I find a 3rd - "effect produced
by one or other of the united individuals on its grafted fellow" - which
would seem to make a fruit tree on a dwarfing rootstock a "graft
hybrid".)
--
Stewart Robert Hinsley