Dave Byrne has sent along the latest newsletter from the RosBreed project which “is working towards marker enabled breeding on roses (black spot resistance) and other Rosaceae crops”.
Note that the fruit people in the project are now far enough along so that they have formed an extension service whereby breeders can send them leaf tissue from their seedlings to have analyzed for the presence of genetic markers to disease resistance thus greatly reducing the time needed to select seedlings for further breeding. This is where we are hopefully headed with roses.
Buried in the newsletter is a link to a free software package, Pedimap, that allows breeders to take advantage of the genetic marker information they thus obtain:
Is this not still firmly rooted in the Mendelian attitude that leads to vertical resistance at the expense of horizontal resistance?
I admire the ability to construct thoroughly loaded questions like this one. I’m way to busy to dive into it head first but I’ll stick some toes in.
I’m not sure what Mendel’s attitude was about disease resistance. I do know he was trying to work out the basic principles of inheritance which he managed to do pretty well. It took thirty years for the world to catch up with him then another two generations for biologists to suss out the particulars behind the basics before they could move on to advanced topics like jumping genes and quantitative inheritance, the latter being what I’m guessing that you to mean as being the specific alternative to ‘Mendelian’ - if that’s the case then nope, not necessarily.
Marker assisted breeding simply gives you a handle on a trait. Sometimes the markers are actual genetic elements of the traits, sometimes they are just nearby the actual genetic elements. The traits can be ‘Mendelian’ in the sense of being a simple binary like pea wrinkles but they can also be complex, or quantitative, like ‘Diplocarpon resistance’. The scope of that quantitative trait might be narrow (Diplocarpon only’, say) or broad (‘horizontal resistance’) or somewhere in between.
I’m already up to my ankles and I didn’t even want to get wet. Maybe our resident professorial types can jump in here - Larry, David?
I think it would be more accurate to mention the neo-Mendelist attitude. This is a real “thing”. I no longer have the books, but I did read in the USDA Yearbook (1936 or '37) where a neo-Mendelist author distinguished old-fashioned, vigor-related resistance from “true” or genetic resistance.
It had been observed that in some cases, in some breeding lines, resistance to a particular infection segregated as a unit character. This led too many students to ignore the old and established methods of plant breeding in favor of the new-fangled, neo-Mendelist methods. Over time, some folks learned that this gene-based resistance was more easily overcome by evolving pathogens. Others refused to get the message.
For example, Dr. Lammerts learned that he could make a great deal more money selling seeds of rust-resistant snapdragons than he did from teaching in a college. Alas, rust soon showed up in the rust-resistant snapdragons.
Later, Lammerts was confident that he had defeated blackspot in roses. But when he moved his nursery from Livermore, CA to Watsonville, his most resistant selections got spotty.
Today, despite the propaganda regarding Round-Up Ready crops, wild plants are evolving their own resistance to Round-Up – no Round-Up Ready gene required.
Breeding for disease resistance is like shooting at a moving target. This is especially true when we expect a cultivar to be UNIVERSALLY resistant … everywhere, every time, and under all conditions.
The notion that resistance to blackspot, mildew, etc. MUST be associated with specific gene-markers can restrict the investigations. It presupposes that a desert plant and one native to wetlands must share the same mechanism of resistance to a given infection. I doubt it.
Emsweller, et al.: Snapdragons, p. 967
“Recently in several localities in California, some of the supposedly resistant plants have again succumbed to rust. Such a situation is not unusual, being common in grain varieties bred for resistance to certain strains of the cereal rusts. It is possible that the condition in California is caused by a new strain or physiological form of the rust organism. Since the rust parasite on the snapdragon is itself a small plant, it is not unusual that it should produce a new strain able to attack otherwise resistant plants. If this should prove to be the situation, the production of rust-resistant strains of snapdragons will be more difficult in those sections where more than one physiological race of rust occurs.” http://bulbnrose.x10.mx/Heredity/Emsweller/Emsweller_Snapdragon.html
Improvement of Sweet Corn
C. F. Poole
p. 385
“The great popularity of Golden Cross Bantam is chiefly due to its yield, canning qualities, uniformity, and resistance to bacterial wilt (figs. 3 and 4). This disease is most serious in latitudes close to 40° north and is of importance mainly in sweet corns, sometimes destroying almost the entire crop. The earlier maturing varieties are most susceptible. A recent study of Stewart’s disease (26) recognizes two distinct genetic types of resistance: (a) Vigor-correlated, inferred from the fact that vigorous hybrids between some low-resistant inbred strains are more resistant than either parent; and (b) true résistance, shown by the fact that hybrids from low-resistant inbreds are less resistant than hybrids from high-resistant inbreds. Golden Cross Bantam may owe its high degree of resistance to both these causes, since it has hybrid vigor and one of its parents, Purdue 39, is also highly resistant.”