A new genetic study of blackberries by scientists from the University of Arkansas Department of Horticulture in the USA could make it easier for breeders to develop new varieties of the cane fruit without thorns. The researchers, led by Margaret Worthington, associate professor of fruit breeding and genetics for the Arkansas Agricultural Experiment Station, have identified the genetic region responsible for blackberries thorns, which can harm pickers and damage fruit. She explained that until now blackberry breeders have not had the necessary genetic information to identify why some plants lacked thorns.
All fresh market blackberry varieties are tetraploids, meaning that they have four copies of every chromosome. The higher number of copies makes genetic analysis more difficult, with Worthington working on the ‘prickly’ trait for almost ten years. She added that, to her knowledge, the results reveal the “first diagnostic marker for any trait that’s been developed and published in blackberry.”
Ellen Thompson, co-author of the study and Global Rubus Breeding and Development Director for Hortifrut Genetics, highlighted the significance of the work. “These are the world’s first publicly available markers developed for fresh-market and processing blackberries,” she said. “Markers save time and money, allowing breeders to make decisions faster, before seedlings are planted in the field. Though Hortifrut Genetics blackberry breeding programme is already 100% thornless, something many other companies are trying to achieve, using these markers to screen seedlings in a high-throughput manner allows us to incorporate diverse and rustic traits of thorny germplasm, study segregation ratios more quickly, and identify the associated desirable prickle-free phenotypes at a very early stage”.
The research team used a genome-wide association study to determine the locus responsible for the thornless trait and gathered DNA from a total of 374 blackberry varieties, some with the prickly trait and some without. These samples were then analysed through genotyping. Another key finding of the study was the lack of genetic variation around chromosome Ra04 leading to a “linkage disequilibrium block,” or a region where genetic markers are more likely to be inherited together than by happenstance.
For the thornless gene included in this block, this means it is often passed on from parent plant to child plant with many other genes, including undesirable traits like high acidity, lack of cold tolerance, and canes that don’t grow upright unless supported. Worthington explained that in selecting so specifically for thornless plants, these negative traits have been brought in and variation around the locus has been lost. She said conducting crosses with thorny plants could help bring the variation back.
Co-author Hudson Ashrafi, associate professor of horticultural science at North Carolina State University, added that the study’s finding of a thornless locus enables breeders to avoid “waiting years to phenotype mature canes, which reduces breeding cycle length and improves selection efficiency for thornless blackberries”.
