Submitted to: Theoretical and Applied Genetics
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/1/2013
Publication Date: 7/16/2013
Citation: Castro-Lopez, P., Stafne, E.T., Clark, J.R., Lewers, K.S. 2013. Genetic map of the primocane-fruiting and thornless traits of tetraploid blackberry. Theoretical and Applied Genetics. DOI:10.1007/s00122-013-2152-3. Interpretive Summary: Breeding of blackberries, a fruit which has many valuable nutritional properties, is slow in part because seedlings derived from breeders’ crosses must be grown to maturity for evaluation of many traits, including whether the plant will produce two crops or just one crop each year. The breeding process would be greatly accelerated, and would be much more efficient, if a breeder could test a small seedling and know with confidence what traits that seedling will have if grown to maturity. A DNA based method, called “marker assisted selection” is available to accomplish this, but requires DNA “markers” that can be used to identify the seedlings the breeder should “select”. This research reports the discovery of markers that can be used to select seedlings that produce two crops a year and have no thorns. This is the first time that any DNA marker has been reported for use in marker assisted selection of blackberry seedlings. Blackberry breeders and geneticists worldwide will use these and new markers as they are discovered to develop improved blackberry cultivars.
Technical Abstract: Blackberry primocane fruiting, fruiting on first-year canes, has the potential to expand blackberry production both seasonally and geographically. The incorporation of the primocane-fruiting trait into cultivars with desirable horticultural attributes is challenging due to its recessive nature and tetrasomic inheritance in cultivated blackberry. Molecular-marker-assisted selection has high potential to facilitate incorporation, because breeders already use morphological-marker-assisted selection of seedlings without marginal cotyledonary hairs to identify progeny that will be thornless as mature plants. The development of a genetic linkage map with these two traits is the first step to utilizing molecular markers in breeding for thornless primocane-fruiting blackberry cultivars. A full-sib family segregating for thornlessness and primocane fruiting, from a cross between ‘APF-12’ and ‘Arapaho,’ was used to construct the first genetic map of tetraploid blackberry. Segregation patterns of several dominant markers and the two phenotypic traits fit those expected uniquely with tetrasomic inheritance (e.g. 5:1, 11:1 and 35:1). Some loci showed a significant double reduction frequency, but genotypes that could have originated only from double reduction were not found. The map consists of seven linkage groups (LG) in each parent, consistent with the basic number of chromosomes (2n = 4x = 28). Naming of LG1-LG6 followed that of the recently revised system for raspberry using SSR markers in common between blackberry and raspberry, and LG7 was tentatively defined by default. The loci controlling primocane fruiting and thornlessness were not linked to each other; thornless/thorny, the S Locus, was mapped on LG4, and the primocane-/floricane-fruiting locus, named in this work the F Locus, on LG7.