AgBioResearch

Strawberries and blueberries are among the most widely enjoyed fruits in the United States. Each year farmers produce 3 billion pounds of strawberries and 690 million pounds of blueberries.

While the nutritional value and sweet taste of these fruits are widely cherished, their origins remain a bit of a mystery. Humans and most other animals have diploid genomes, meaning they inherit one set of chromosomes from each of two parents that determine their genetic characteristics.

Strawberries, blueberries and many other plants are polyploids, however, which means they have three or more sets of chromosomes. Polyploidy can take two different forms – autopolyploidy, in which the chromosomes are copies of genetic material from a single progentior species, and allopolyploidy, in which each chromosome comes from a different ancestral species.

Both strawberries and blueberries are the latter, arising from two and four ancestral species, respectively. Discovering the identities of those species could open the next chapter in fruit breeding, and it’s a challenge that MSU AgBioResearch plant breeder Patrick Edger and his team aim to unravel.

“By disentangling each of the progenitor species that gave rise to crops like blueberries and strawberries, we can understand how their genomes are formed and use that to breed better cultivars for growers,” Edger, assistant professor in the MSU Department of Horticulture, said. “Many genes that encode favorable traits were unintentionally lost during domestication and modern crop improvement efforts. My team is identifying the valuable genes from the original sources – the wild progenitor species – and reintroducing them into our breeding programs.”

Traditional plant breeding is a lengthy process in which scientists crossbreed parent plants with promising, advantageous characteristics – such as higher yields, resistance to pests or diseases, and drought resilience. They wait for the plants to mature, then select the most promising of those with which to repeat the process, often for many generations.

Recent years have seen great strides in plant breeding, with new molecular technology emerging to allow researchers to identify which offspring possess the genes for desired traits when they are still seeds. This has the potential to eliminate much of the most time-consuming aspect of plant breeding, because researchers can grow only the most promising offspring, without having to cultivate an entire generation in search of the desired traits.

“For breeding perennial crops like blueberries it can take years to observe specific traits in the field,” Edger explained. “Molecular breeding can give us predictive power to know which seedlings will have the traits we want before they grow. It reduces costs and speeds up the process, and lets us deliver cultivars that address the needs of growers much more quickly.”

Without knowing which genes, and combinations of genes, lead to which traits, however, breeders are unable to identify those promising candidates. Polyploids, inherently more genetically complex than diploid organisms, present a greater challenge, but it’s one for which Edger’s team is more than prepared.

“The underlying genetics of polyploid organisms are certainly complex, but their study forms the core of my lab’s work,” Edger said. “My students and I are looking at how the fusion of tens of thousands of genes from ancestral species was coordinated is coordinated in polyploids.”

Unlocking the future of fruit breeding

Edger and his team have already made great strides in deciphering the genetics of important blueberry traits. Through a three-year project supported by MSU AgBioResearch and conducted in collaboration with MSU scientists including Kevin Childs, Tony Schilmiller and Robin Buell, Edger’s team successfully determined the genetic pathways that contribute to many of the fruit’s signature characteristics, including its color, sugar content, and level of phytonutrients and antioxidants.

Thanks to these successes, Edger’s lab is now tackling major challenges facing the blueberry industry in Michigan. Native to the Midwest, the blueberry stem gall wasp infests wild and cultivated blueberries alike.

Emerging during bloom, the wasp lays its eggs in the stem of blueberry plants, severely damaging the plant and causing the formation of the hard, kidney-shaped galls that give the pest its name. The activity of the pest has increased in recent years, with some estimates that about 40 percent of blueberry acreage in Michigan is affected, according to Edger.

Using wild species in combination with existing plant material in the breeding program and in collaboration with MSU AgBioResearch entomologist Rufus Isaacs, Edger’s team has already identified sources of genetic resistance to the blueberry stem gall wasp, enabling them to breed the trait into future cultivars and provide growers with a powerful new tool against a growing threat.

Pest resistance is not the only trait being targeted. With funding from the U.S. Department of Agriculture, Edger is beginning work on resistance to anthracnose fruit rot, the most common and widespread disease of blueberry plants in the United States.

As outward signs of the disease don’t ordinarily appear until close to harvest, growers depend almost entirely on preventive strategies to control it. Adding blueberry cultivars with heightened resistance to the disease would represent a crucial new tool for the industry.

To bring the same molecular breeding benefits to strawberry as to blueberry, Edger’s team has similarly developed a genomic platform to guide breeding and research efforts in strawberries. Taking a molecular approach is what Edger sees as the future of plant breeding.

 “With this technology, we can not only breed individual traits faster, we can also work on them in combination,” Edger said. “We want to be able to breed a cultivar with the best set of traits in order to give farmers and consumers a great crop. Before, we would have to wait two to three years to see if the traits we want will emerge. Now, we can assess them on day one right here in the lab.”