Seeds of Hybrids

[Junglekeeper]

I noticed instances of seed suppliers offering seeds of hybrid plants. An example is x Citrofortunella microcarpa which is often sold as Citrus mitis. Is it not true that seeds of hybrids are either sterile or will likely grow to something other than the parent? I assume the answer will apply to hybrids between plants within the same genus as well as those between different genuses(sp?).

[mr.shep]

If the seed was collected from a grafted parent then the
propensity for the offspring to be true to the grafted
parent is unlikely for Citrus. If the seed was collected
from a cutting grown parent then the frequency of the
offspring more closely resembling the parent goes way
up but still may not be true to the parent. In order to
achieve a closer phenotypic similarity to the parent it
will require a few generations of back crossing, probably
no less than three to five filial generations.

We have a cutting grown Meyer Lemon that produces
viable seed. About 50% of the seed germinated will
yield seedlings that resemble a Trifoliate Orange which
tells me the parent of our Lemon was a grafted individual.
That alone should tell us that seed gathered from most
Citrus is not going to be true unless a few generations
back have all been cutting grown or propagated by air
layering. As soon as we graft a plant we force the
parent plant to become bastardized (sorry for the term
but it is an accepted and used term in the field of Plant
Breeding).

Sterility in so-called hybrid offspring many times is the
norm but is not always true in plants. If the genotypes
are close enough and the number of gene pair are roughly
the same then we can expect some of the hybrids to be
fertile. Most will be sterile however. Even male steriles
can still be a parent for cross pollination studies and
eventually the male sterile parent can yield offspring
that will be fertile in a few generations later. We've
seen that true with Corn among other crops.

Jim

[Ron B]

Jim, what's the mechanism by which rootstocks used for citrus (and relations) are influencing the genetic makeup of the seedling offspring of the scions?

[mr.shep]

Hi Ron:

what's the mechanism by which rootstocks used for
citrus (and relations) are influencing the genetic makeup
of the seedling offspring of the scions?

Not sure this is what you want but I'll let it rip anyway.

Seed collected and germinated from grafted parents are
not pure due to the fact that the genetics of the plant is
dominant for the rootstock. The original source is
dominant to the recessive grafted parent. When we
germinate seeds, all things being equal as there are
some exceptions, the seedling will more closely
resemble the root stock parent. We may see for the
short term characteristics we think resemble the grafted
parent but in time those characteristics will fade out
on us. In Citrus we have been programmed by some
people to believe that seed gathered from various
Citrus would be true. They can be close at times
but they are not true at all from my experience. Even
seedlings grown from seed collected from grafted
Limes will not be true such as Kaffir, Bearss and Key
Lime. We have a better chance of having a closer to
true seedling if the seeds were gathered from a cutting
grown parent and even then there is an impurity factor
or genetic contamination factor we may have to deal
with based on the history of the plant in question.
Just like my Meyer Lemon producing some seedlings
with characteristic Trifoliate shaped leaves still has
Lemons on it but they are not as sweet as the parent
Meyer Lemon, nor will the fruit color up like a Meyer
Lemon will. The offspring that has a definite Lemon
characteristic leaf do produce Lemons that do color up
once they get some cold chill.

In Citrus with grafted plants we have several genetic
impurities to overcome. The grafted parent has both
root stock genes as well as its parent genes in its system.
Then we have to allow for other either induced impurities
if it also came from seed or dirty pollen from other nearby
Citrus. Almost all Citrus used for breeding are cutting
grown as the plant is closer to being pure than a grafted
plant will be. After a few back crosses and lastly an
out cross we can then have a true hybrid that has had its
genetic make up cleaned up so to speak and then outcrossed
to a known source. We have a better idea as to the actual
genetic make up of the hybrid then. Several old Citrus
were found in the wild and were not raised under controlled
situations. Even with genetic studies we are still not sure
what Mandarin was used as a parent for the Meyer Lemon,
neither are we certain the Mandarin was the pollen parent
or the seed parent. All we know is the Meyer Lemon was
found and it shows characteristics of a Mandarin with the
skin and the coloring once it gets some cold chill, the higher
percentage of sugar and it has the Lemon smell, appearance
and flesh texture and acidity of a Lemon. Offspring from
the original plant would not be pure to start with but when
we graft onto Calamondin or Trifoliate Orange root stock
we end up mixing up the genome even more by virtue of
the fact that the root stocks gene expression will be dominant
to the grafted parent in the endosperm of the seeds. The root
stock will hold dominant over the graft and we see the
dominance in the seedlings being more like the root stock
parent than the grafted parent. When we see reversion in
Japanese Maples we are seeing the plant become more like
the root stock parent was. Some physiologists attribute this
to the polarity of the plant but a geneticist will reply on the
phenotype (what we see) and the genotype (the actual genetic
make up) instead. The root stock in some other plants does not
always hold up as being dominant but in most angiosperms it
is more true. There are exceptions, even in Fruit Trees, as
evidenced by the Nut Trees. Even then when an Almond is
grafted onto Peach root stock the seeds would yield a higher
percentage of Peach like seedlings than Almonds. Especially
true for Nemaguard root stock more so than Nemared. Things
have changed when Titan (Almond) root stock came about.
Even Pecans are grafted onto Pecan root stock and even many
Walnuts are grafted onto either Persian or English Walnuts so
seedlings from them will yield Pecans and Walnuts but are
they pure to the grafted parent? No, they are not. The seedlings
are essentially mulattos instead.

Now, if we were to graft a Meyer Lemon onto Lemon root stock
is when the fun begins. Then, like the Pecans and the Walnuts we
will get seedling Lemons that will at first seem like to us a Meyer
but I bet they will end up being closer in time to a Eureka or a
Lisbon Lemon for color, acidity and tartness. We may just even
produce a seedling closer in tartness to a Ponderosa than a Meyer
and I've seen that happen before from seeds germinated from a
grafted Meyer Lemon onto of all things a grafted Meyer Lemon
root stock. When the Meyer Lemon was grafted onto a Meyer
Lemon cutting grown root stock is when the seedlings more
closely resembled our original Meyer Lemon!

Jim

[Ron B]

What I'm wondering is how the genes of the rootstock are getting into the genes of the scion and changing the genetic makeup of its offspring, if that is what you are saying happens.

[Junglekeeper]

Interesting discussion from a rather innocent question. So...as for seeds of hybrids (and less so for others), to quote the Bits n' Bites commercial, "A different handful each time. You never know what you're gonna to get.".

[Ralph Walton]

This is getting very interesting!
I've had the perhaps over-simplified view that the genetics of the scion were fixed and the rootstock was simply a nutrient gathering and support system. I can see the rootstock producing and delivering hormones that would influence the scion (early/late bloom and fruit set, vigor, even survival) but when the scion cells divide and begin to produce seed surely it can only be the scion's genetic material that is the source "code" for the process.
Actually, I'm talking myself thru this situation as I type. In mitosis (regular cell division = growth) the daughter cells are identical to the mother cells. That's why we propagate by cutting, layering etc. to breed "true" offspring. In meiosis (producing a
gamete which has only a single spiral of DNA), that spiral is a "random" combination of the genetic spirals of the scion's original parents. It must be this "randomness" that is influenced by the rootstock.
When two of these gametes (think sperm & egg) come together to make a seed, even if they come from the same plant (self fertilization) there is great potential for variability. If both have been pushed in the same direction by the rootstock hormones or some other rootstock characteristic that we are as yet unaware of, then we would see what Jim has been seeing (and passing on - thanks again Jim).
Hmmm.. sounds like thesis material, don't you think? Come on some of you youngsters, fly at it!
Ralph

[Ralph Walton]

Or maybe the genetic makeup isn't being "pushed", but perhaps the seeds that have some familiarity with the rootstock have a better chance of surviving, so those are the ones we see and attempt to grow. A sort of pre-selection?
Ralph

[mr.shep]

Why is it that we trim off the suckers on various plants?
Even with Japanese Maples the vigor of the plant is not in
the grafted scion, the vigor of the plant is in the root stock.
Why is it that if we let the under stock of an Acer negundo
'Flamingo' or 'Sensation' send out an array of suckers that
it can lead to the grafted scion dying out in some cases?
Why is it that the grafted scion may be killed due to a
disease or due to an insect and the root system can still
be alive? What is the hardiest portion of the plant, the
grafted scion or the root stock?

I think we are trying to equate sexual reproduction with
asexual methodology. Once the egg has been fertilized
by either wind blown pollen, by pollen carried by bees
or in some cases wasps, by emasculation by hand or
through natural means, then we can think in terms of
sexual reproduction. What I am saying is that not all
times, actually in most cases, the seed parent is not
pure if the seed parent has been a grafted individual.
In the case of Fruit & Nut trees, Citrus and various
flowering trees, we have a seed parent that indeed
does have a mixed gene genetic make up. Granted,
in more cases than not the egg will indeed have more
gene expression originating from the scion than from
the root system but there is no doubt there is a mix of
both genes in the plants genome.

Let's put things in this perspective, let's say I had my head
transplanted recently, are any of you going to say that my
original DNA will not be changed as a result of placing
someone else's head on my body?

There will be a degree of change caused by the union of
two unlike gametic parents. Thus, when we see a Pecan
seedling grown from a Mahan grafted onto a Choctaw root
stock the seedling may show characteristics of the Mahan
but it is not a Mahan in its genotype as the Choctaw genes
have altered the gene expression of the Mahan scion. If we
back cross the seedling with a Mahan we still do not have
a clean individual to come about but in roughly 3-5 back
crosses we can get close to the original genotype of the
original Mahan. Through cutting growing, the siblings
from the original Mahan are offspring we do not have to
worry so much about genetic impurities brought about
from the merging of two parents into one.

In Japanese Maples it is the root stock that serves
as the healer of the union of the scion, not the
reverse. It is the root stock that will produce
the growth hormones needed for the scion
to remain viable and alive until the two
cambium layers merge together. By virtue
of what we know in Plant Physiology it is the
root stock that is dominant to the scion as
without a sturdy and fully working root stock
the scion cannot survive. Even when the top
of the tree develops and becomes weakened
the root system can take over and we have
seen that happen with various trees in which
there may have been a large amount of sucker
buildup and the scion will suffer or the scion
itself dies out because it is recessive in a
two parent marriage. I am talking dominance
in the plant but not so much dominance in the
genome of the female flower but there are
changes in the genetic make up of the egg,
as a result of the mixing of two parents, that
the flower produces and that should be not
be a mystery to us.

Jim

[Junglekeeper]

Originally Posted by Ralph Walton

I've had the perhaps over-simplified view that the genetics of the scion were fixed and the rootstock was simply a nutrient gathering and support system. I can see the rootstock producing and delivering hormones that would influence the scion ...

Rightly or wrongly, this was my understanding as well.

I can't offer any enlightening information on the genetics of seeds since I lack the technical background. However I can report that the limited number of x Citrofortunella microcarpa seeds that I managed to germinate were albinos that eventually died since they lacked chlorophyll.

[mr.shep]

What I'm wondering is how the genes of the rootstock are getting
into the genes of the scion and changing the genetic makeup of its
offspring, if that is what you are saying happens.

I've based most of my thoughts on how we as plant breeders would
be looking at the plant in a puritanical sense, not necessarily from
a scientific point of view. Our thoughts would be that the plant has
become a caricature of its old genetic self once we have forced
the root stock to live in combination with a foreign scion. With
the physical altering to both hosts we will in effect have a change
in the base genetics of the original plant as well as in the original
scion. Essentially we have forced the plant to develop its own
organelles otherwise the scion will not live as a result of the two
cambiums not melding together. From the organelles we get
replication and from replication we get newly made "hybrid"
germ cells in the plant. So, in effect the female flowers will yield
what some people may feel are hybrid eggs (they are genetically
altered somewhat but are not a true hybrid) as a final result of the
interactive marriage of the two plant systems.

Jim

[Ralph Walton]

And thank goodness for plant breeders.
Science operates in an atmosphere of theory, proof, focus (often necessarily narrow), and more recently competition. We can all come up with a favorite story or horror story of scientific "fact" disproven and debunked by ongoing experience. The one unchangeable "fact" in this world is that we don't know all the facts.
When our eyes and our experiences are at odds with the "facts", the most likely solution is that our facts are not necessarily wrong, but very possibly incomplete. We have to be prepared to add "except when..." to our understanding.
I have heard what Jim is saying before, and I accept that he and the others have seen what they have seen, so hence my challenge (which I repeat): this is thesis material. I'm too damn busy with the cows and the fencing, plus I don't need the glory.
Ralph

[Ron B]

I've never seen a cow fence before. How do they hold the foil?

[photopro]

Junglekeeper, some hybrid plants do produce viable seeds while others do not. Aroids such as the rare Philodendron 'Marijke' never produce vaible seeds while many hybrid Anthurium plants continue to produce viable seeds until the third or fourth generational cross.

LariAnn Garner produces many exceptional hybrid aroid plants and is a very good expert in this subject so I'm going to ask her to comment. http://aroidiaresearch.org/aboutus.htm

[bjo]

Hi,

I know in some citrus grafts and in many other grafted plants, you end up with a mixture of cells from both the scion and the rootstock known as a "graft-chimera".
See:http://en.wikipedia.org/wiki/Graft-chimaera.

Therefore the reproductive cells in the flowers may be derived from the rootstock rather than from the scion as you would imagine... hence the unpredicted offspring?

Brian

[vitog]

I wish a biologist would step into this discussion and set the record straight. I'm no biologist, but everything I've ever learned leads me to believe that it's impossible for the rootstock to have any genetic influence on the scion after a normal graft union. The "graft-chimera" described in the Wikipedia article is not the normal outcome of a graft. You can see that from the picture. In a normal graft the flowers and the fruit are genetically 100% identical to the plant used for the scion. How could it be otherwise? Grafting is used to produce fruit that is identical to a particular parent and wouldn't be useful if the rootstock had any influence at all on the genetics of the fruit. Since the seeds are part of the fruit, they also must be identical to those of the scion. Of course that doesn't mean that the seeds will produce a plant identical to the scion; that's an entirely different topic.

[David in L A]

What about nucellar embryony?
http://en.wikipedia.org/wiki/Nucellar_embryony