Impatience and tree fruit breeding are an unlikely match, blending together as well as water and diesel. But the combination might lead
to more efficient fruit breeding techniques and shorten the time required to develop new cultivars.

Dr. Jim Olmstead, the new manager of the cherry breeding program at Washington State University, is in a hurry to discover new sweet cherry varieties for the Pacific Northwest. As he looks for varieties that are resistant to diseases and rain cracking, self-fertile, produce large fruit, and taste exceptional, he is also working to shorten the time involved in cherry breeding. Olmstead, a Yakima Valley, Washington, native, studied under Dr. Amy Iezzoni at Michigan State University for his doctorate in breeding and genetics.

“I don’t want to wait the 20 to 30 years that my colleagues tell me it takes to develop new varieties,” he said.

WSU revived its cherry breeding program last year as a result of funding by the Washington Tree Fruit Research Commission and Oregon Sweet Cherry Commission. More than 7,000 seeds were generated in 2005 from crosses made from nearly 40 different parents. The seeds are being grown into seedlings this year in a greenhouse.

Olmstead is considering various techniques to reduce the normal amount of time involved with breeding. These techniques include budding seedlings on precocious rootstock or sleeping-eye bench grafts to reduce the time needed to reach flowering and fruiting stages, using shoot tip micrografting instead of chip budding, and using cold storage to trick the trees into growing the equivalent of two seasons in one calendar year.

DNA technology

Another way to make breeding more efficient, he said, is to use marker-assisted selection, technology he hopes to use to choose which seedlings are further evaluated. Advanced selections would be chosen based on genotype characteristics—the sequence of genes—instead of the phenotype, or physical characteristics that can be seen.

For example, if blue eyes were the desired trait, Olmstead explains that he could either visually look for blue eyes or assess whether DNA had the blue-eyed allele of the eye color gene. The brown eye color and blue eye color are different alleles of the same gene.

“We could know early on, say a few months after germinating the seed, if the seedling would have the traits we wanted and could select seedlings with traits like large fruit size,” he added.

MSU’s Iezzoni and other research colleagues received a $400,000 grant last year to develop genomic resources to implement marker-assisted selections. Researchers are working to identify the DNA markers involved with fruit size and other qualitative traits, like taste. They are also working on DNA marker technology for self-compatibility, self-fertilization, and powdery mildew for both sweet and sour cherries. A marker for self-compatibility in sweet cherries has already been identified, according to Olmstead.

Information gleaned from the grant project will be used in WSU’s breeding program, he noted.

Genetic quality traits involving taste are more difficult to isolate because a number of factors are involved. For example, the taste gene includes sugar and organic acid profiles.

Size

Olmstead is particularly interested in identifying the genes involved with determining cherry fruit size. His recently completed postgraduate thesis was titled “Linkage Map Construction and Analysis of Fruit Size in Sweet and Sour Cherries.”

“The number of cells in the flesh of the fruit determine its size,” he said, adding that fruit size is a quantitative trait, with many different genes influencing fruit size. He has chosen to concentrate on the gene that controls cell number.

Olmstead has found that the number of cells within a cherry is different between varieties, but that all fruit from the same variety have the same number of cells. The cherry variety Selah has an average of 79 cells, compared to Bing and Regina that average 47 and 48 respectively.

“The cell number does differ within varieties, but we can use that knowledge in our breeding program.”

In comparing large and small cherries of the three varieties that ranged in size from 11.3 to 13.7 grams for the large cherries and from 7.5 to 8.3 grams for the smaller fruit, he found the same number of cells in each cherry. The difference was that the size of the cells in large fruit was larger.

“Within the variety, they all have the same number of cells, but the size range varies due to environment,” Olmstead said, adding that growers can increase or decrease the size of the cells with different horticultural practices, but not the total number.

“The trick is to associate that trait, the cell number, with a marker on a chromosome of a cherry. That’s the genetic linkage or road map of the cherry.”

A DNA marker for size has already been identified for sour cherries, he noted. Data is currently being analyzed to identify the size marker for sweet cherries.

By using a DNA marker for size, those seedlings with undesirable alleles would be quickly discarded while seedlings with the marker associated with large fruit size would be kept, making the evaluation component of the program more efficient.

“It’s not bioengineering,” he added. “It’s just using DNA markers to help make better selections.”