Seeds of Hybrids

Discussion in 'Plants: Science and Cultivation' started by Junglekeeper, Jan 7, 2005.

  1. Junglekeeper

    Junglekeeper Esteemed Contributor 10 Years

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    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?).
     
  2. mr.shep

    mr.shep Well-Known Member 10 Years

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    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
     
  3. Ron B

    Ron B Paragon of Plants 10 Years

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    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?
     
  4. mr.shep

    mr.shep Well-Known Member 10 Years

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    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
     
  5. Ron B

    Ron B Paragon of Plants 10 Years

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    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.
     
  6. Junglekeeper

    Junglekeeper Esteemed Contributor 10 Years

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    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.".
     
  7. Ralph Walton

    Ralph Walton Active Member 10 Years

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    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
     
  8. Ralph Walton

    Ralph Walton Active Member 10 Years

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    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
     
  9. mr.shep

    mr.shep Well-Known Member 10 Years

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    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
     
  10. Junglekeeper

    Junglekeeper Esteemed Contributor 10 Years

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    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.
     
  11. mr.shep

    mr.shep Well-Known Member 10 Years

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    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
     
  12. Ralph Walton

    Ralph Walton Active Member 10 Years

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    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
     
  13. Ron B

    Ron B Paragon of Plants 10 Years

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

    photopro Well-Known Member

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    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
     
  15. bjo

    bjo Active Member 10 Years

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    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
     
  16. vitog

    vitog Contributor 10 Years

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    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.
     
  17. David in L A

    David in L A Active Member 10 Years

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