Red raspberry is an outcrossing species that in the wild is highly heterozygous and heterogeneous between species and genotypes. Raspberry plant ancestors were
self-incompatible, but breeding efforts have resulted in modern cultivars being self-fertile (Keep 1968). During the breeding process, genetic variability is created in seedling populations.
Seedlings (stage 1) can be rasied from controlled crosses or open pollinated seed from wild species or germplasm accessions. Selection occurs among and within families based on phenotypic values for selection of superior types for cultivar testing (includes additive and non-additive genetic effects) or selected on breeding value (non-additive genetic effects) for use as parents for another generation (recurrent selection).
19 Selected seedling plants are clonally propagated and planted in replicated or non-replicated trials of various forms for testing for suitability as commercial cultivars. At Northwest Plant stage 2 trials typically consist of non-replicated 6-plant plots derived from deviding the original seedling plant. Stage 3 replicated trials (typically 3 reps x 6 plants) and stage 4 grower trials (typically several hundred or more plants) are established from plants
propagated by tissue culture. Decision for commercial release is typically made after grower trials. Typically raspberry plants for commercial fruit production are vegetatively propagated via suckers (derived from adventitious buds on roots of established plants), root cuttings or tissue culture methods (Hall et al. 2009).
Most raspberry breeding programmes use standard recurrent selection strategies for an out-crossing, heterozygous plant. Commercial cultivars are typically produced by repeatedly crossing ‗best‘ parents with each other followed by selection within these populations.
New traits such as disease resistance from a number of Rubus species can be incorporated into a programme via introgression and backcrossing techniques. However, inbreeding can produce mixed results in raspberry (Fejer and Spangelo 1974) with increasing homozygosity resulting in a rapid development of inbreeding depression and is therefore generally avoided by breeders. Despite this, the ancestry of current red raspberry commercial varieties are dominated by only five or six parents. For example, from European sources
(R. idaeus), ‗Lloyd George‘ and ‗Preussen‘ and from North American (R. strigosus) sources,
‗Newman‘, ‗Latham‘, ‗Cuthbert‘(Jennings 1988; Jennings et al. 1991). This has resulted in considerable relatedness in many cultivars (Dale et al. 1993). Many breeders are now sourcing new genetic material from other species to increase the genetic base, and cultivars have been released containing genes from R. occidentalis, R. arcticus, R. odoratus and
R. cockburnianus and it is expected that more cultivars will be released with genes from other Rubus species (Daubeny and Anderson 1993; Knight 1993; Daubeny 1996).
Raspberry has a number of simply inherited qualitative traits of significance to the breeder such as cane spininess, cane hairiness and Raspberry Bushy Dwarf Virus (RBDV) resistance.
However, a number of key traits are controlled by many genes and are quantitatively
inherited. Key traits that are quantitatively inherited include; yield, fruit colour, ease of fruit release from the receptacle, fruit firmness, root rot resistance, plant architecture and vigour. It has been shown some of these are largely additively inherited (Barritt 1982; Dale 1989; Keep
20
1989) suggesting that non-additive effects such as dominance and epistatic interactions have lesser effect.
2.3.1 Yield and yield components
Yield is the ultimate quantitatively controlled trait with probably most genes in a plant having an influence on it and it is therefore very complex in its nature. Over the period of crop domestication, breeders have increased yield significantly (Way et al. 1983), yet scope remains for further increases in crops such as raspberry, which have relatively short breeding histories (Jennings 1988). The difficulty breeders face in accurately assessing yield in
seedling and selection plots is time-consuming nature of the measurements required to
accurately describe yield. This has resulted in the use of yield estimates (Daubeny et al. 1986) in some raspberry breeding programmes. However, when making selections from seedlings, many small-fruit breeders still rely on simple ‗eyeball‘ observations, which may or may not turn out to be accurate. Furthermore, breeders may emphasise traits that are easier to measure or more ‗in fashion‘ at the expense of yield even though it remains a key commercial
objective. There is, however, great potential to improve raspberry plant yield by increasing or optimising any of the key components of yield identified in 2.2.3.
While these are relatively easily measurable components each can be further dissected for example; cane number per plant can be influenced not only by the number a plant produces but also by the ability of the plant to re-grow new canes after sucker removal (Waister et al.
1980). Fruit lateral number per plant is determined by node number, which in turn can be influenced by cane height and diameter (Crandall et al. 1974; Jennings and Dale 1982). Fruit number per lateral is influenced by lateral length and nodes per lateral (Dale 1976). Fruit size is influenced by ovule number, drupelet size and number (Dale 1989). These traits are under genetic control and thus can be influenced by breeding. Dale (1989) summarised first year cane and second year fruiting yield components for raspberry and made suggestions as to which could be targeted for breeding.
Measuring total yield is time-consuming and expensive and not feasible for large seedling populations. Sampling or estimating total yield could be used to reduce costs and increase heritability of yield in raspberry but there has been limited work in this area (Daubeny et al.
1986).
21 2.3.2 Yield component inheritance
While yield components interact with environmental influences to produce total yield certain components are likely to be more important than others from a breeding point of view.
Dale and Daubeny (1985) showed that high yield in raspberries was closely related to high lateral numbers in Abbotsford, British Columbia and by cane thickness in Invergowrie, Scotland. Several studies have shown that high yield in raspberry is highly correlated with large fruit size (Dale 1976; Cormack and Woodward 1977; Dale and Daubeny 1985) and this is probably the easiest component for breeders to select for. Certainly, increases in average fruit size of cultivar releases from raspberry breeding programmes around the world would suggest this has been achieved; however, other yield components are likely to be just as important and may hold the key to major advances in yield in the future. Dale (1976)
conducted a study of yield components in raspberry and produced a correlation matrix for 13 yield components from one family. This study concluded that, to increase yield, breeders should focus on increasing fruit numbers and this could be done in two ways; by developing types with increased plant vigour and large numbers of fruit per lateral as well developing plants with average vigour and high fruit numbers per cane.
For numbers of laterals and fruit weight, the quantitative inheritance has been shown to be additive in raspberry (Fejer 1977; Dale 1989). Daubeny (1996) suggests that other yield components would also be additively inherited in raspberry but there is limited published work in this area. More recently, Stephens et al. (2009) reported heritability estimates and genetic correlations of yield and yield components on red raspberry in New Zealand and found heritability was low (h2=0.24) for yield and some yield components, e.g. percentage bud break (h2=0.23) and cane length (h2=0.27) and high for others, e.g. berry weight (h2=0.82) and cane number (h2=0.64).
2.3.3 Inheritance of fruit quality traits: firmness, sugars, acids, anthocyanins, and ellagitannins in raspberry
While several sources of firmness in raspberry have been identified (Knight et al. 1989), including R. occidentalis, little has been published on the inheritance of fruit firmness.
Similarly, there has been little published on the genetic inheritance of sugars and acids in raspberry fruit and nothing published on the inheritance of ellagitannins. Dossett et al. (2008) reported narrow sense heritability for sugar and acid content in black raspberry fruit (h2=0.38 and h2=0.68, respectively). Anthocyanins have been reported to have high heritability in
22
raspberry (Connor et al. 2005; Dossett et al. 2008; Stephens et al. 2009) and therefore potential exists to breed fruit with increased anthocyanin content and thus antioxidant activity. However, anthocyanins have been shown to be negatively correlated with yield and other fruit quality traits in red raspberry in New Zealand (Stephens et al. 2009).