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Need Help from Geneticists/Biologists  RSS feed

 
Charlie Michaels
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I need help understanding a concept, something that probably confuses many other people too.

There are thousands and varieties of tomatoes. I can be assured that if buy a packet of tomato seeds and they will definitely "come true". Its astonishing the amount of variety that can be grown; tomatoes that fruit all in one patch, tomatoes that fruit for months; green, purple, orange, striped, huge, grape sized, super hardy, not hardy, ex, and these  VERY SPECIFIC characteristics will all come true from seed.

Then look at something like an apple. Again VERY SPECIFIC characteristics are bred that are unmistakable to the variety, I mean, can you mistake a macintosh for anything else? Yet if you grow out the seeds of a macintosh, good luck getting an apple that looks anything like a macintosh apple! You need cuttings to ensure you'll get another macintosh.

I don't understand it. Why will very specific traits (funky colors, patterns ect) come true in tomato seeds, but never in apple seeds?

Is it just the amount of years we've been breeding them, perhaps with a hundred years of breeding we can get apple seeds that will come true?
 
Paul Cereghino
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Location: South Puget Sound, Salish Sea, Cascadia, North America
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Not my specialty... but I think two mechanisms may be interacting.  First is the degree to which the characteristics of the individual reflect a narrowing of the genetic range -- in other words has the species been bred generation over generation to be more like it is now?  Thus if you collect seed from a field with lots of different varieties of beet you will get a new variety of beet, but it will not likely revert to the scrubby seashore biennial Beta vulgaris.  Apple varieties by contrast, like most woody plants, are weakly bred (probably due to generation length?), such that the plants we cherish are more or less mutants harboring tremendous genetic diversity, and perpetuated by cloning.

The other issue is how pollen is transfered.  Plants that rely on bees are very promisucuous in that pollen comes from far and wide, increasing opportunity for diverse genetic expression.  In most tomatos (where stigma don't extend outside the enveloping flower petals) the pollen is released and remains within the individual flower, dramatically reducing the opportunity for cross polination, making tomato come unusually true to individual.  There is likely other factors and more complexity, but this is a start.
 
tel jetson
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I think it's all about the length of generations.  it's not too difficult to get two generations of tomato in one year.  but how long does it take to get fruit from an apple tree from seed?  I would bet that a stable, open-pollinated apple cultivar could be created in roughly the same way that stable vegetable varieties are created, but it would take a very long time and an awful lot of space.  it wouldn't surprise me to find out that folks are attempting this, but it could easily take more than one lifetime.
 
                            
Posts: 126
Location: Ava, Mo, USA, Earth
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Tel Jetson has it right, but I just want to add one thing.  Since apple trees CAN be started from clones (cutting/grafts) there is no reason to breed one to come true from seed.  You can start tomatoes from cuttings, but seeds are a lot easier for home use.  Some big places (both growers and plant suppliers) might be better ahead with cuttings for tomatoes, but that's not an option for many annuals.
 
tel jetson
steward
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homesteadpaul wrote:
Since apple trees CAN be started from clones (cutting/grafts) there is no reason to breed one to come true from seed.


I could see some advantages to having apples that seed true.  if they were isolated from other apples, they could be left alone to propagate themselves.  the disadvantages of grafting could be avoided: weak unions, disease susceptibility, shortened life-span, &c.  and seedlings would likely have stronger roots than grafted trees or rooted cuttings.  that isn't to say that those advantages make it worth the immense time investment required.  I'll leave that for everyone to decide themselves.

there was another thread all about seedling apples last year.
 
Joshua Msika
Posts: 66
Location: Nova Scotia
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I don't know much about apples or tomatoes in particular but it seems to me that Paul found the main issue:

The other issue is how pollen is transfered.  Plants that rely on bees are very promisucuous in that pollen comes from far and wide, increasing opportunity for diverse genetic expression.  In most tomatos (where stigma don't extend outside the enveloping flower petals) the pollen is released and remains within the individual flower, dramatically reducing the opportunity for cross polination, making tomato come unusually true to individual.


That is the deciding factor. If you can precisely control which plants pollinate each other, you can precisely determine the traits of the next generation. For tomato plants, it seems to be pretty easy since Paul points out that the pollen remains within the flower. For apple trees, to produce offspring with similar traits, using cuttings to make clones is much easier and faster.
 
Charlie Michaels
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I was thinking that if you make woody plants that come true to seed they would eventually naturalize and it would work out real well for a Permaculture in the 100+ year sense. Still there's a great use for apple seeds that come true to seed for us now. Seeds are easier to transport. You can store seeds for a long time, can't really store cuttings for a long time I think.
 
ronie dee
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Location: NW MO
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Genetics was not near as advanced when i was in college as it is today, but the simple answer is still the same.

The reason that you can't plant a Fuji apple seed that you get from a Fuji apple(and get a Fuji apple tree) is because that Fuji apple had two parents. One parent was a Fuji female part and the other parent was a male Gala apple tree . In this example the seed would be a genetic cross between the Gala and the Fuji...with each parent donating 26 pairs of chromosomes to the seed. (This is sexual reproduction.) (When each parent donates 26 pairs of chromosomes you wind up with 42 and these determine the final traits of the offspring.)

When you plant a Fuji apple tree, you need a pollinator apple tree (the Gala is an acceptable pollinator but there may be others). 

Some apple trees are self pollinating - with self pollinating apple trees you could get a seed that was true. You would still have to isolate the female part and make sure that nothing else pollinates the female except the male pollen. (This is sexual reproduction.) (42 chromosomes all came from the genetic same type parent plants.)


OK I left it up in the air as to where the original Fuji apple tree came from...so the answer is that it is a cross between two other trees that results in a Fuji... It may be that it was a genetic mutation that occurs rarely or it could be some thing that could be forced between two parent trees anytime the breeder wants...I don't know... If it is a rare genetic mutation, then the breeder would always make more by cuttings.

Like others have said you can root a cutting from a particular tree and get a tree that is like the parent tree..(All 42 chromosomes come from the same tree.) (I believe this is vegetative reproduction.)
 
                    
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The simple answer involves the idea of "stable breeds." If you cross a poodle with a poodle, the typical result is a poodle. Some poodles are still different from each other (size, color, temperament) but we recognize their "poodleness" or "poodleosity."

If you really love Airedoodles (half airdale terrier, half poodle) and you tried to breed them by crossing one Airedoodle with another, the litters might have some dogs that were very poodle-like, some that were very terrier like, and others that were various blends. By selection and inbreeding, it should be possible to eventually come up with airdoodles that breed true - but it will take time time and effort. And it is not clear that the new 'pure bred' airdoodles would be as vigorous as the F1 hybrid airdoodles that are easier to breed.

Hundreds or thousands of genes control fruit flavor, size, color, and productivity ... things we use to define cultivars of fruit trees. Some reasonably good fruit do come true from seed, but it is rather complicated. We are looking for an Einstein or Mozart or Galileo, a rare combination of genes (which also happened to be in the right environment at the right time). So we clone those rare individuals when possible, at least in the world of plants. Without human selection, most fancy plants would regress ... apples would be small and sour.
 
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