Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: thedoc on 22/09/2013 21:07:17
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Where do seedless grapes come from?
Asked by Corinne, in the Netherlands
Visit the webpage for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/naked-scientists/show/2006.12.17/)
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We answered this question on the show...
The correct answer is that the plants that grow them are actually clones. So instead of growing them from seeds, they're grown from cuttings taken from [img float=right]/forum/copies/RTEmagicC_Shiraz_Grapes_01.jpg.jpg[/img]existing plants.
Obviously, the first seedless grapes were from a plant that arose through mutation - a genetic change - that meant that it didn't have seeds. And, presumably, some grower noticed this. He or she would have taken a little shoot or a stem off the plant, put it in the ground, and a new plant - genetically identical to the seedless parent - would have grown.
In fact, this is how a lot of plants are cultivated now, and also a lot of seedless varieties. But, this technique is causing problems in some cases. Bananas, because they're all clones, are getting struck down by fungi - like Panama disease.
And if a population is created by cloning, all of the plants are genetically identical, so they can very easily be wiped out because the plants have the same defences to a pathogen. And if the pathogen evolves to sumount this defence, every cloned plant is susceptible...
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Apparently the seedless grapes are descended from a natural mutation causing "aborted seeds". Commercial grown seedless grapes are then treated with the hormones that naturally would have occurred with seeds to produce larger fruits.
Watermelons, however, must be propagated by seed. The trick with the watermelons is to breed a tetraploid and a diploid watermelon. Then by crossing the tetraploid and diploid watermellons, the resulting cross is triploid and sterile.
Plant growth is by mitosis and allows replication of odd numbers of chromosomes, but sexual propagation is through meiosis (resulting in haploid cells, or otherwise having half the chromosomes), which has errors with odd chromosome numbers.
Seedless bananas are also similarly a triploid cross, and thus presumably additional varieties could be bred as desired.
See further notes here:
http://www.madehow.com/Volume-6/Seedless-Fruits-and-Vegetables.html
http://waynesword.palomar.edu/hybrids1.htm
Mules, of course, are also sterile due to donkeys and horses having different numbers of chromosomes. Perhaps similar crosses could be made with plants, or potentially chromosomes could be artificially split or merged in a lab to produce plants with sterile hybrids.
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Growing fruit from seed can be a long drawn-out affair, particularly in the case of trees. Take a peach stone and plant it in a pot, see how many years it will take before the new tree produces a commercialy viable crop. Alternatively, take a cutting from a peach tree and graft it on to a stock root and you will have a commercialy viable crop in perhaps half the time. The same applies to vines and fruiting bushes. Growing a raspberry bush from seed will take longer than growing from a cutting, or 'stick'.
Cuttings, layering and grafting are not only quicker means of replacing old plants or increasing your stock, they also provide the stability of produce which we have come to enjoy. When we buy a Cox's Orange Pippin apple, we know exactly what to expect. This does seem to suit both us and the retail chains which serve us in much the same way as suits both the retail chain and the producer.
But, as the doc eluded to, if a problem with a particular variety of a fruit arrises, say a new strain of honey fungus or blight, the old variety of plant may not have the ability to defend itself. If such a problem were to spread in the same way as has affected Elm, Poplar, Oak, Bees and amphibians, we could find ourselves battling to conserve a particular type of fruit. It would only take a chance mutation to leave us facing a fungal growth which could threaten raspberries, loganberries and other such soft fruits worldwide.
We need to put aside standardisation and get back to enjoying variation. It may cost us more, but it could save us from the ever present possibility of disasterous shortages.
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Cuttings will presumably slowly mutate over time, so potentially one could select the best graft/cutting propagated plants for future planting. However, seed grown plants will certainly have more crop variability, and thus greater potential for selecting beneficial traits. Presumably a research facility will use a combination of cutting propagation and seed propagation.
If gibberellin helps seedless grapes grow larger, I can imagine the next generation of GM plants designed to amplify specific growth hormones such as gibberellin.
I had thought that one of the plums that I ate this year may not have a pit. I wonder how long until we start seeing plums, peaches, and cherries without pits.
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can anybody tell me when the first seedless grapes would have hit the markets?