Dr. Cameron Peace checks block of seedlings resulting from the intercrossing of parents that have been used in WSU’s breeding program.
Tree fruit geneticist Dr. Cameron Peace is planting 2,500 trees in Wenatchee, Washington, that he expects will play a role in the development of superior apple varieties.
The 4.5-acre apple long-term germ plasm block he is planting at Washington State University’s Sunrise Research Orchard between Wenatchee and Quincy is part of an effort to widen the gene pool available to WSU’s apple breeder, Dr. Kate Evans, and develop strategies for efficient application of DNA information into crop improvement to address the industry’s needs and challenges.
Peace, a geneticist with WSU in Pullman, has travelled the world looking for apple germ plasm that could be used in breeding to introduce desirable and exciting traits into new varieties.
This spring, he planted 186 seedlings that originated from a large and diverse collection of apples at the U.S. Department of Agriculture’s Plant Genetic Resources Unit in Geneva, New York. Between 1989 and 1996, USDA-sponsored scientists visited Kazakhstan in central Asia, where the apple originated, to collect graft wood and seeds from wild apples of the species Malus sieversii, which is believed to be the main ancestor of today’s commercial apple, Malus x domestica.
Peace said if breeders are going to work with wild apples to look for new traits and valuable new genes, it makes sense to start with a close wild relative of the domestic apple. Though primarily seeking material with resistance to diseases, the scientists found about 50 wild trees with fruit that had other desirable traits also, such as good size and taste, and were almost of a high enough quality to be commercialized. Back in Geneva, they crossed several M. sieversii trees with Gala, with the goal of studying the genetic control of diseases and also to take the first step of crossing the wild material into domestic material to make new cultivars, Peace said.
Interesting traits
The seedlings he planted this spring came from one cross of M. sieversii and Gala, donated by Dr. Gennaro Fazio (USDA Geneva apple rootstock breeder) and Dr. Phil Forsline (curator of the USDA Geneva apple collection). Peace said he hopes to identify genes controlling interesting traits that have been reported from observations of the original trees of this cross still growing in New York.
The same trees are being grown in New York, Illinois, and Minnesota. Scientists in the different areas can combine their data for greater efficiency of DNA knowledge discovery and identification of valuable traits, a collaboration made possible by lack of competition as each of the regions’ breeders will select for new cultivars best suited to their particular conditions.
Scientists in the eastern United States are primarily focused on resistance to diseases, such as scab, he noted, but in Washington, where dry conditions lead to lower disease pressure, the primary focus is fruit quality. “We look at the same germ plasm in a different way, and this diverse material can actually be a source of novel fruit quality traits,” he said.
He will assess how the trees perform in Washington conditions and will do genetic studies with the goal of developing more efficient strategies for combining desirable traits in new cultivars.
Peace said there are genetic variations in wild material that are not found in newer cultivars and might have been lost in the process of selecting for certain desirable traits. “This cross between wild and elite apples will give us a window into desirable genes that have been lost within a traditionally narrow breeding gene pool,” he said.
But he’s also identifying undesirable traits so that breeders can be sure not to select for astringency and small fruit size, for example.
He suspects that in M. x domestica chunks of chromosomes might be responsible for multiple desirable traits, rather than scattered pieces of DNA. For example, one of the ethylene genes (with selection for certain gene variants providing better storability) might be linked in the chromosomes with other good traits such as sweetness. “If we only think about the ethylene genes, we may end up removing a lot of the good traits,” he said. “We want to find chromosome segments that are associated with elite performance, which we can then select for all at once with a DNA-informed breeding approach.”
He will also try to identify the DNA for undesirable traits, such as small fruit and astringency, so that breeders can select against them. When using wild material, there must be a two-pronged approach of selecting for good traits and against bad ones, he said.
Texture
In 2008, Peace planted a block of seedlings representing the WSU breeding program, resulting from the intercrossing of ten different parents that have been used in the program, with the most important being Honeycrisp, Cripps Pink, and Aurora Golden Gala. Unlike seedlings in the main breeding program plantings, which are eliminated after few years of evaluation, these trees will be kept and studied for many years. They will primarily be used to directly link new DNA information with application in the local breeding program. Genetic studies with this material will aim to discover the DNA behind important traits and to validate in local material any promising new DNA-trait associations that other scientists report. Promising seedlings may even be used as parents in further crosses.
Next year, he will plant 800 seedlings that are crosses of three commercial varieties noted for good texture—Honeycrisp, Cripps Pink, and Crimson Crisp (Co-op 39). Peace hopes to finely dissect the DNA associated with texture. Flavor is another high-priority set of traits that will be studied in these seedlings.
Genetic diversity remains the key. He said he’s found some DNA evidence that Honeycrisp is descended from a unique source of germ plasm—something other than the domestic apple—a few generations back. “If this is true, it’s an example of the type of amazing new qualities we can get from widening the gene pool that’s available to the breeder,” he said.
Honeycrisp has been used frequently as a parent for crosses made in the WSU breeding program and is the parent of a number of advanced selections.
Peace will complete most of the planting of the long-term germ plasm block next year with a core subset from the Geneva collection of extremely diverse material.
Dr. Kate Evans, WSU pome fruit breeder, said that the material that has been derived from the USDA’s Kazakhstan collection is totally novel germ plasm with many different quality traits from the named cultivars that have been used in the breeding program so far.
Many of the breeding programs around the world have been using the same pool of germ plasm, but they, too, are becoming interested in using a wider range of material with more diversity. She said the breeding program in New Zealand is now using a lot of wild apple material from central Asia.
Evans said her goal is to develop a portfolio of different varieties with a whole range of different quality attributes. It’s possible those attributes might include unusual attributes such as red flesh, though that is not a priority in the program.
The material from Geneva has been prescreened for fireblight resistance and appears to have some useful resistance to fireblight that the WSU program has not had access to before, she said. “I will be interested in having a bigger focus on fireblight resistance.”
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