Post by DarJones on Aug 16, 2010 22:20:45 GMT -5
Joseph,
I read through the share book re horizontal resistance. I can see the advantages of vertical resistance and I can see the advantages of horizontal resistance. What bothers me about this book is the substitution of supposition for hard facts and the omission of several significant items.
One of the supposition items that really irks me is the general thesis that horizontal resistance does not generally break down. I can think of at least 3 different ways for horizontal resistance to break down.
1. A pest/disease acquires a new way to attack the host organism - example Tracheal mites on honeybees which once were external parasites but a new form allowed them to move into the trachea to feed.
2. A new pest/disease moves in and there is no resistance in the host - example ink disease and chestnut blight on american chestnut.
3. One or more of the genes conveying tolerance is overcome by the parasitic organism - example Stuart pecan which was at one time highly resistant to scab but over time, so many millions of trees were planted that eventually scab overcame the tolerance.
Another issue I have is with a somewhat pie in the sky attitude of the advantages of horizontal resistance. In that regard, it reminds me of a used car salesman with the 'trust me, you'll love it!' spiel. don't just say that it works, point out concrete examples that prove it works.
There also seems to be deliberate glossing over of the way wild populations exploit horizontal resistance. It is a complex host/parasite relationship. Using one of his examples, if you have 10 genes for resistance that are widely spread in a plant population with each plant having a few of the genes but no plant having all of them, then the parasite develops keys to match one or more of the resistance genes until it can again effectively parasitize the population. Because the pest never develops keys to overcome all of the genes, it effectively becomes a permanent nuisance but never an overwhelming problem.
From the explanation he gives of stacking horizontal resistance genes, he would effectively mount overwhelming selective pressure on the parasite so that it develops keys to infect even the most maximally stacked horizontal resistance genes. The key here is time, as in with enough time, the parasite will overcome the pressure.
I did an analysis of stacking 3 highly effective genes into the tomato genome a few months ago. Stipulate that the three genes each convey a high level of tolerance to Late Blight. Stipulate that no late Blight variant at present can overcome the resistance of any of the three. Now grow out a population of a few billion tomato plants with those three genes. The late blight organism could break one of the genes in a few years, the second gene in about 30 years, and the third gene in about 100 years. It all gets down to the fact that with a large enough population and enough disease pressure, eventually it will break and the plant will be overwhelmed.
The most valid conclusion is that contrary to the theme of the book, the best long term strategy is to stack into a population a group of effective horizontal resistance genes and select for concatenation of those genes while also developing a similar line with stacked vertical resistance genes. Then combine the two.
You can throw most of the above in the garbage if we develop means to construct entire genomes from scratch using genetic engineering. It would be possible to construct a set of chromosomes that have all possible genes for tolerance which would eliminate all the plant breeding and crossing and selecting.
DarJones
I read through the share book re horizontal resistance. I can see the advantages of vertical resistance and I can see the advantages of horizontal resistance. What bothers me about this book is the substitution of supposition for hard facts and the omission of several significant items.
One of the supposition items that really irks me is the general thesis that horizontal resistance does not generally break down. I can think of at least 3 different ways for horizontal resistance to break down.
1. A pest/disease acquires a new way to attack the host organism - example Tracheal mites on honeybees which once were external parasites but a new form allowed them to move into the trachea to feed.
2. A new pest/disease moves in and there is no resistance in the host - example ink disease and chestnut blight on american chestnut.
3. One or more of the genes conveying tolerance is overcome by the parasitic organism - example Stuart pecan which was at one time highly resistant to scab but over time, so many millions of trees were planted that eventually scab overcame the tolerance.
Another issue I have is with a somewhat pie in the sky attitude of the advantages of horizontal resistance. In that regard, it reminds me of a used car salesman with the 'trust me, you'll love it!' spiel. don't just say that it works, point out concrete examples that prove it works.
There also seems to be deliberate glossing over of the way wild populations exploit horizontal resistance. It is a complex host/parasite relationship. Using one of his examples, if you have 10 genes for resistance that are widely spread in a plant population with each plant having a few of the genes but no plant having all of them, then the parasite develops keys to match one or more of the resistance genes until it can again effectively parasitize the population. Because the pest never develops keys to overcome all of the genes, it effectively becomes a permanent nuisance but never an overwhelming problem.
From the explanation he gives of stacking horizontal resistance genes, he would effectively mount overwhelming selective pressure on the parasite so that it develops keys to infect even the most maximally stacked horizontal resistance genes. The key here is time, as in with enough time, the parasite will overcome the pressure.
I did an analysis of stacking 3 highly effective genes into the tomato genome a few months ago. Stipulate that the three genes each convey a high level of tolerance to Late Blight. Stipulate that no late Blight variant at present can overcome the resistance of any of the three. Now grow out a population of a few billion tomato plants with those three genes. The late blight organism could break one of the genes in a few years, the second gene in about 30 years, and the third gene in about 100 years. It all gets down to the fact that with a large enough population and enough disease pressure, eventually it will break and the plant will be overwhelmed.
The most valid conclusion is that contrary to the theme of the book, the best long term strategy is to stack into a population a group of effective horizontal resistance genes and select for concatenation of those genes while also developing a similar line with stacked vertical resistance genes. Then combine the two.
You can throw most of the above in the garbage if we develop means to construct entire genomes from scratch using genetic engineering. It would be possible to construct a set of chromosomes that have all possible genes for tolerance which would eliminate all the plant breeding and crossing and selecting.
DarJones
Anyway, I'm waiting to make a fresh batch of sandwich bread and then I'm making a peanut butter and jelly sandwich. I can hardly wait!
;D
