Resistance to colonization by the raspberry aphid (Amphorophora agathonica Hottes) has been an important objective in North American red raspberry (Rubus idaeus L.) breeding programs since the 1930s because of its effectiveness in controlling the spread of aphid-transmitted viruses in red raspberry. The most widely used source of resistance in North America has been the gene Ag1 from ‘Lloyd George’. The widespread use of Ag1 to control aphids led to the appearance of a resistance-breaking biotype in British Columbia, Canada, in 1990. Our objective was to identify biotypes of A. agathonica present in the commercial red raspberry production region of southwestern British Columbia and northwestern Washington and determine what sources of resistance may still be effective against this pest. We collected 12 aphid isolates and screened them against 15 raspberry cultivars and four selections. Although it has been widely believed that only two biotypes (regular and Ag1-breaking) of A. agathonica were present in the region, we identified six distinct biotypes and characterized them by their ability or inability to colonize a differential set of raspberry cultivars. This has confirmed the loss of previously recognized and unrecognized sources of resistance in some cultivars. The data also support the presence of a seventh biotype that has not yet been observed. In addition, we confirmed resistance from three sources of wild North American red raspberries that hold up to each of these biotypes. Our results will serve as a guide for future efforts to characterize the prevalence of different aphid biotypes in the region and the identification of new sources of resistance for breeding.
The large raspberry aphid (Amphorophora agathonica Hottes) is an important vector of viruses in Rubus L. across North America. Although breeding for aphid resistance has long been recognized as an important tool for protecting red raspberries (Rubus idaeus L.) from viral infection, this is the first report of resistance to A. agathonica in black raspberry (Rubus occidentalis L.). Seedlings from 132 wild populations of black raspberries, representing the species' native range, were screened for resistance to A. agathonica. Strong resistance was found in three of these populations, one from Ontario (ORUS 3778), one from Maine (ORUS 3817), and one from Michigan (ORUS 4109). Resistance to the large raspberry aphid in ORUS 3778 and ORUS 3817 is dominant and appears to be conferred by different genes. We propose that the genes for resistance in ORUS 3778 and ORUS 3817 be designated Ag4 and Ag5, respectively. Resistance to A. agathonica in ORUS 4109 also appears to be controlled by a dominant allele at a single locus, but cannot be differentiated from Ag4 at this time.
In recent years, there has been renewed interest in black raspberry (Rubus occidentalis L.) breeding. This has been spurred by an increase in black raspberry consumption resulting from studies that have shown them to be particularly high in anthocyanin content indicating high levels of antioxidants. Present cultivars are ill-adapted to the biotic and abiotic stresses of the Pacific northwestern United States, where the commercial black raspberry industry is centered, and fields must be replanted after three to five seasons. An incomplete partial diallel, consisting of 10 parents and 26 sibling families, was constructed for the study of variation and inheritance of phenological, vegetative, and fruit chemistry traits in black raspberry. Sibling families were established at the Oregon State University Lewis Brown Farm in Corvallis and were arranged as a randomized complete block design with four blocks of one to eight plants. Phenological development and vegetative measurements were recorded for each plant in 2005 and 2006. In addition, 25-berry samples of ripe fruit were collected from each plant. To study variation in fruit chemistry properties, including pH, titratable acidity, percent soluble solids, anthocyanin profiles, and total anthocyanins, additional samples of 25 ripe berries were collected from each plant and pooled by family within blocks. Although there were many striking similarities, strong trends in phenotype based on pedigree were observed for most traits indicating a strong genetic component. General combining ability (GCA) effects were significant and larger than specific combining ability effects for all traits, except for fruit size (mass). With the exception of fruit size, narrow-sense heritability estimates were generally moderate to high (0.30 to 0.91), indicating the potential for breeding progress within the population of plants studied. Despite these results, statistically significant and large GCA values were limited to just a few of the parents, indicating a lack of heritable genetic variation in much of the germplasm base and a need for greater diversity.
Luther Burbank, the quintessential nurseryman of the early 20th century, remarked that small fruit was the “Cinderella of the pomological family.” He stated that although tree fruits had been improved to the point of an almost uncountable number of cultivars, it was the time and responsibility of his generation and those to follow to develop the small fruit for human consumption. Burbank had a penchant for detecting potential qualities of unusual plants and his broad association with plant explorers at the U.S. Department of Agriculture and elsewhere allowed him to examine diverse wild berry species. He obtained seeds of many small fruit species from throughout the world. He made wide crosses within and between these genera and species. Burbank selected and named many cultivars to be introduced through his nursery and elsewhere. He named and released ≈40 blackberries, raspberries (Rubus L.), and strawberries (Fragaria L.); four grapes (Vitis L.); and a hybrid Solanum that he named ‘Sunberry’. He sometimes exaggerated their descriptions for promotion or public recognition. For example, Rubus ×loganobaccus ‘Phenomenal’ was, he stated, “far superior in size, quality, color, and productivity…” to ‘Loganberry’. Unfortunately, this cultivar was not a commercial success. Burbank made a few crosses and sold what he considered to be improved species, e.g., ‘Himalaya Giant’ blackberry (R. armeniacus). He created new common names for foreign species, e.g., balloon berry (R. illecebrosus) and Mayberry (R. palmatus), to better market them. However, his amazingly keen observations of thornlessness, pigment diversity, and recognition of repeat flowering and fruiting in blackberries, raspberries, and strawberries, were insightful of the needs of future industry. Burbank was a disciple of Darwin and his theory of natural selection. Burbank’s classic breeding approach, to make wide crosses, produce large numbers of hybrid seedlings, choose significant seedlings with his traits of choice, and backcross to the desired parent for several generations, was successful, although he did not know of ploidy or gene recombination. Unfortunately, the ‘Himalaya blackberry’, now ubiquitous in hedgerows and fields throughout the Pacific Northwest in the United States, is designated as a federal noxious weed. Although not presently in commercial production, three of his Rubus cultivars (‘Burbank Thornless’, ‘Snowbank’, and ‘Phenomenal’) are preserved in the U.S. Department of Agriculture, National Clonal Germplasm Repository, in Corvallis, OR.
The advent of next-generation, or massively parallel sequencing technologies has been a boon to the cost-effective development of molecular markers, particularly in nonmodel species. Here, we demonstrate the efficiency of microsatellite or simple sequence repeat (SSR) marker development from short-read sequences in black and red raspberry (Rubus occidentalis L. and R. idaeus L., respectively), compare transferability of markers across species, and test whether the rate of polymorphism in the recovered markers can be improved upon by how marker sequences are chosen. From 28,536,412 black raspberry reads and 27,430,159 reads in red raspberry, we identified more than 6000 SSR sequences in each species and selected 288 of these (144 from each species), for testing in black and red raspberry. A total of 166 SSR primer pairs were identified with informative polymorphism in one or both species. SSRs selected based on different percentages (90% to 97% as compared with ≥98%) of read cluster similarity did not differ in polymorphism rates from each other or from those originating from singletons. Efficiency of polymorphic SSR recovery was nearly twice as high in black raspberry from black raspberry-derived sequences as from red raspberry-derived sequences, while efficiency of polymorphic SSR recovery in red raspberry was unaffected by the source of the primer sequences. Development of SSR markers that are transferable between red and black raspberry for marker-assisted selection, evaluation of genome collinearity and to facilitate comparative studies in Rubus L. will be more efficient using SSR markers developed from black raspberry sequences.