Raspberry varieties can be classified into four groups on the basis of the extent to which outcrossing is thought to have occurred in their evolution. The groups are considered to differ in genetic structure, particularly in respect of the amount of inbreeding which they will tolerate without showing inbreeding depression. The immediate task of the breeder is largely to incorporate major genes for specific improvements while retaining and stabilising the good qualities already present. The achievement of this stability through inbreeding is most easily obtained in varieties which, by the nature of their origin, are able to give good phenotypic performance following inbreeding. The alternatives of basing new varieties on F1 hybrids or on material selected for inbreeding tolerance are discussed: it is concluded that F1 hybrids should be more easy to produce and may have the advantage of possessing greater developmental stability. Some evidence for this is presented.
Three possible explanations are discussed for the frequent occurrence of heterozygosity at the gene loci H: h (hairy: non-hairy canes) and T: t (pigmented canes and red fruits: non-pigmented canes and yellow fruits) but it is unlikely that heterozygosity at these loci is important for cultivated varieties. It is suggested that virus resistance may play an important role in the breeding system of the species: if this is so, high levels of resistance may not always be desirable in wild populations, and their occurrence in a cultivated variety may therefore be influenced by the nature of the variety’s origin. Protection against fungal diseases of the canes, however, is unequivocally desirable in both wild and cultivated raspberries. Since escape from two diseases of this kind is favoured by the presence of cane hairs (gene H), the choice of forms with sub-glabrous canes (gene h) for many modern varieties has perhaps been unfortunate.