Researchers have discovered that the cultivated garden strawberry’s complex evolutionary history started long ago on opposite sides of the world.
Until now, researchers have known little about the evolutionary origins of the cultivated strawberry. While most species, including humans, are diploid with two copies of the genome—one copy from each parent—the strawberry is an octoploid, with eight complete copies of the genome that multiple, distinct parental species contributed.
In a new study researchers explain how the strawberry became an octoploid, and the genetics that determine important fruit quality traits.
A globe-spanning story
“For the first time, analysis of the genome enabled us to identify all four extant relatives of the diploid species that sequentially hybridized to create the octoploid strawberry,” says Patrick Edger, assistant professor of horticulture at Michigan State University and coauthor of the paper, which appears in Nature Genetics. “It’s a rich history that spans the globe, ultimately culminating in the fruit so many enjoy today.”
The four diploid species are native to Europe, Asia, and North America, but the wild octoploids are almost exclusively distributed across the Americas. The paper’s results suggest a series of intermediate polyploids, tetraploid, and hexaploid that formed in Asia, prior to the octoploid event that occurred in North America, involving the hexaploid and a diploid species endemic to Canada and the United States. This makes the strawberry relatively unique as one of only three high-value fruit crops native to the continent.
Breeders began propagating these octoploids around 300 years ago. Since then, researchers have used them around the world to further enhance variety development. However, Edger hypothesized that—as with several other polyploids—an unbalanced expression of traits each diploid parental species contributes, called subgenome dominance, would likely also be present in the octoploid strawberry. He was right.
“We uncovered that one of the parental species in the octoploid is largely controlling fruit quality and disease resistance traits,” Edger says. “Knowing this, as well having identified the genes controlling various target traits, will be helpful in guiding and accelerating future breeding efforts in this important fruit crop.”
No longer ‘flying blind’
The genomic discoveries will advance the trait selection process, bringing about a more precise method of breeding for this important worldwide crop. The genome will also enable studies previously unthinkable in strawberries, and will help tackle difficult breeding and genetics questions, researchers say.
“Without the genome we were flying blind,” says coauthor Steven Knapp, a plant scientist from the University of California, Davis.
“I remember the first time I saw a visualization of the assembled genome, which went from a complex jumble of DNA molecules of 170 billion nucleotides to an organized and ordered string of 830 million base pairs. That was a special moment that changed everything for us in strawberry.”
Knapp says that, historically, scientists studying complex biological phenomena in strawberries have tended to focus on diploid relatives because of the complexity of the octoploid, even though genetic analyses in the octoploid are actually straightforward once one has a good road map.
“We have been on a crusade to shift the focus in the basic research community to the commercially important octoploid,” Knapp says. “The wild octoploid ancestors, together with cultivated strawberry, provide a wellspring of natural genetic diversity to support biological and agricultural research.”
Traditional breeding has been highly successful in strawberry, yielding outstanding modern cultivars that have been the catalyst for expanding production worldwide. As with other crops, remaining challenges will require breeders to continually redesign cultivars and introduce genes from wild species and other exotic sources. The genome is an essential vehicle for applying predictive, genome-informed approaches in strawberry breeding and cultivar development.
For the US, improved varieties could provide a boon to an already-thriving business. The US is the global leader in strawberry production, a yield comprising roughly one-third of the world’s total. In 2016, the country produced more than 1.5 million tons.
An international team of researchers contributed to the sequencing and analysis of the cultivated strawberry genome, which exposed new information about its origin and traits.
Michigan State’s AgBioResearch, UC Davis, the United States Department of Agriculture, the California Strawberry Commission, and the National Science Foundation funded the work.
Source: Michigan State University