Let’s start first by saying that “thorns” is somewhat of a misnomer in blackberries. The proper botanical term is spine, since it these are derived from tissues outside of the vascular cortex. However, it is convention that blackberries are “thorny”, so that is how I will continue.
Blackberries from the wild and sports from cultivated clones were the first selected that had no thorns. Some of these early selections included ‘Thornless Youngberry’, ‘Cory Thornless’, ‘Thornless Loganberry’, and ‘Thornless Evergreen’. When breeders started using these as parents to create improved varieties, they found that all the seedlings had thorns. How is that possible? Eventually, it was discovered that the thornless trait in these varieties was chimeral — meaning that it only had thornless genes within one layer of tissues, the epidermal layer. Thus the usefulness of these particular varieties was diminished.
Other sources of thornlessness exist though, two of which have been exploited in breeding to create some of today’s varieties. One source is from Rubus ulmifolius (a diploid species) and the variety ‘Merton Thornless’. The second is from the octoploid cultivar ‘Austin Thornless’. The source from ‘Merton Thornless’ is a recessive gene (s), whereas the gene from ‘Austin Thornless’ is a dominant gene (Sf).
Some of the more important varieties released that have the recessive gene are ‘Smoothstem’, ‘Thornfree’, ‘Chester Thornless’, ‘Triple Crown’ and all of the University of Arkansas thornless varieties (‘Arapaho’, ‘Navaho’, ‘Apache’, ‘Ouachita’, ‘Natchez’, and ‘Osage’). Interestingly enough, the s gene is epistatic with the trait for eglandular cotyledons. This is a fortuitous trait, since seedling derived from crosses with thornless blackberry parents can be screened easily when they are very small and thorny (those with glandular, or hairy, cotyledons) can be eliminated. One problem with breeding for thornless blackberries using this trait is inbreeding. Since ‘Merton Thornless’ is the source of the trait, then it must be in the parentage on both sides of the lineage of any thornless variety. Other drawbacks of using this gene are lack of winter hardiness, links to trailing growth habit, late harvest season, and high acid fruit. On the plus side, the the thornless trait is linked to tolerance of the double blossom (aka Rosette) fungal disease. This has allowed growers in Southern states to have more success in growing blackberries.
The dominant gene from ‘Austin Thornless’ has primarily been used in breeding cultivars for the Western U.S. and New Zealand. Some of the problems with using this gene in breeding include, short fruiting laterals, tight fruit clusters, small plant growth habit, brittle canes, trailing growth habit, susceptibility to downy mildew, and thorns at the base. Yes, you read that correctly — thorns at the base. This gene does not confer total thornlessness but rather after the canes are 30 cm (about 12 inches) tall. Below 12 inches to the ground is thorny. There is also no way to screen the seedlings for their thornlessness until after they are planted in the field and grow past 12 inches tall — a very time consuming and expensive process. Recent cultivars to be released using this gene source include ‘Douglass’, ‘Murrindindi’, ‘Adrienne’, ‘Helen’, ‘Black Diamond’, ‘Black Pearl’, and ‘Nightfall’.
Those of us in the eastern U.S. (east of the Rocky Mountains) will primarily be growing the thornless cultivars with the recessive gene from ‘Merton Thornless’. Although the cultivars display inbreeding, there does not appear to be an imminent concern for loss of heterogeneity (see Stafne and Clark, 2004; http://www.springerlink.com/content/xj657k725018w611/).
This topic of thornlessness in blackberries is covered in more depth in the publication Plant Breeding Reviews volume 29 (http://onlinelibrary.wiley.com/doi/10.1002/9780470168035.ch2/summary).