Since 1960, the majority of the genetic makeup of strawberry cultivars in North America came from only seven nuclear and ten cytoplasmic sources (Sjulin and Dale, 1987; Dale and Sjulin, 1990). This germplasm base also predominated in the international breeding programs (Hancock et al., 1993). Narrow genetic base may lead
to lethal inbreeding effects and lack of diversity to adapt to new environment (Hancock and Luby, 1993). The cultivated strawberry came from an accidental cross of F.
chiloensis and F. virginiana, but little use of the native germplasm has been made by breeders until recently (Hancock et al., 1993). Disease resistance, stress adaptability, and characteristics in wild strawberries can be used to expand the germplasm of the cultivated strawberry and improve its pleasant characteristics (Hancock and Luby, 1993). The key to use native germplasm in plant breeding is to catalog their horticultural useful traits (Hancock et al., 2003). Wild clones of Fragaria species were collected and evaluated for germplasm conservation and for utility in strawberry breeding.
Species with the same ploidy level can often be successfully crossed and all octoploid Fragaria species were completely interfertile (Hancock and Luby, 1993).
Fragaria chiloensis and F. virginiana, the octoploid species, which were the progenitors of cultivated strawberry, are the suitable materials for improving its genetic base. Although the lower ploidy strawberries were more difficult to be crossed with the octoploid F. ×ananassa (Hancock and Luby, 1993), they were valuable for holding potential to improve modern strawberry cultivars by introducing traits such as unique flavors and disease resistance while increasing genetic diversity (Harbut and Sullivan, 2004).
4.1. The octoploid species
The first step of utilizing wild strawberries should be collection of germplasm.
Possible useful traits could be speculated by the original habitat of the collection site.
Staudt (1999) and Hancock et al. (2001a) assumed that there were horticulturally useful genes in the native Fragaria since they distributed in a broad geographical range covering various biotic and abiotic stress environment. Native F. virginiana was
collected for representatives of North America from Pacific Northwest in 1985 and Northern Rocky Mountains in 1989 (Luby et al., 1992). The diverse habitat of which F.
virginiana was collected, ranged from dry pine forests to wet meadows. It suggested that certain accessions could possess resistance to drought or water-saturated soil. Some F. virginiana accessions were found near timberline where growing seasons were 6-8 weeks, with frost or snow occurring any time of the year indicating possible cold hardiness or blossom frost tolerant resource. Collections from sites with alkaline soils may be sources of higher pH (Luby et al., 1992). Similarly, F. chiloensis clones were collected with representatives of its wide geographical range and important horticultural traits like very large fruit, resistance to powdery mildew, red stele caused by Phytophthora fragariae, leaf spot, aphids, and two-spotted spider mites were observed (Hancock et al., 2001a). A bulk collection representing all octoploid strawberry species in North and South America was evaluated by Hancock et al. (2003). In this study, F.
chiloensis was generally superior in crown number, fruit weight, soluble solids and seed set while F. virginiana was superior for runner production, peduncle length, fruit number, fruit color, and winter hardiness.
After selecting elite accessions of the wild strawberries, the performance test should be conducted at various sites to affirm if the characteristics were truly genotypic or resulted from interaction between genotype and environment. The work of Hancock et al. (2001b) demonstrated that fruit characteristics could be assessed in one single site since the fruit weight, skin color, flesh color and firmness coincided throughout the five sites (Maryland, Oregon, Minnesota, Michigan, and Pennsylvania). However, multiple sites were necessary to predict physiological adaptations and disease resistance since the percentage bed fill, foliar disease incidence, 50% bloom date and number of flowering cycles showed interactions between genotype and location (Hancock et al., 2001b).
A subsequent step was to cross the elite accessions of wild strawberries with the cultivated strawberry or another elite wild strawberry accession for evaluation of the hybrid progeny and the transferability of traits from the parents. It was reported that fruits of hybrid from the cross of wild F. virginiana with F. ×ananassa were generally too soft and often irregular in appearance, but the fruits displayed high level of fertility, high flavor and highly productive behavior (Hancock et al., 2001a). Hancock et al.
(1993) and Luby et al. (2008) considered that reconstruction of F. ×ananassa using superior clones of F. chiloensis and F. virginiana might be more efficient than simply backcrossing to F. ×ananassa because the hybrid of wild strawberries possessed much higher proportion of unique genes that will be available for recombination in later generations.
4.2. The lower ploidy species
Fragaria species occupied various environments and they should carry some horticultural useful traits (Hancock et al., 2008). Fragaria vesca (2x) was often found in the same habitats as F. virginiana but frequently occupied drier and coarser sites where F. virginiana was absent. It suggests that F. vesca might be a source of extreme drought tolerance (Luby et al., 1992). F. moschata (6x) was found under heavy shade (Hancock and Luby, 1993). Harbut and Sullivan (2004) indicated that a species adapted to shade might maintain higher CO2 assimilation rate, and it could be beneficial for production in greenhouse, low light areas and high plant populated areas.
Disease resistance can be found in some lower ploidy Fragaria species. Resistance to Phytophthora cactorum (crown rot or leather rot on fruit) was found in several F.
vesca clones and its hexaploid and decaploid derivatives indicated that the wood strawberry might be a source of crown rot resistance and this ability was inheritable (Gooding et al., 1981). Xue et al. (2005) screened eleven non-octoploid Fragaria
species for resistance to Xanthomonas fragaria Kennedy and King, the bacterial angular leaf spot which may reduce yield for up to 75%. Their results indicated that some accessions of F. pentaphylla (2x) and F. moschata (6x) either showed no symptoms (highly resistant), hypersensitive reactions (resistant), or restricted water-soaked lesions (moderately resistant) and the two species harbored diversified resistant source. Bors and Sullivan (1997) observed immunity to aphids and leaf diseases in F. nilgerrensis, and winter hardiness and excellent leaf disease resistance in F. moschata.
Some useful fruit characteristics can be found in the lower ploidy strawberries.
Fruit of F. viridis (2x) had a spicy, cinnamon-like flavor (Bors and Sullivan, 1997); the flavour of mature fruit of F. nilgerrensis (2x) was described as similar to melons or peaches (Oda et al., 1990 in Noguchi et al., 2002), apricots and/or bananas (Staudt et al., 1975); F. moshchata tasted like ‘Concord’ grape when grown in the greenhouse (Bors and Sullivan, 1997). These strawberries could add new elements to typical strawberries and have the potential in aroma breeding. Fragaria pentaphylla had very bright red and firm fruits (Bors and Sullivan, 1997) which would be ideal for strawberry shipping (Harbut and Sullivan, 2004). Fragaria nilgerrensis subsp. hayatae (F. hayatae) was found to have anthocyanins in all parts of the plant (Hancock, 1999), and this unique characteristic may lead to high antioxidant capacity (Harbut and Sullivan, 2004).
4.3. Crossbility
1.) Crosses between diploids
Four subspecies of F. vesca were used as female parent to cross with four diploid Fragaria species (F. nilgerrensis, F. nubicola, F. pentaphylla and F. viridis) in the study of Bors and Sullivan (2005a). Although the rate of fruit set varied from 39% (F.
vesca × F. nilgerrensis) to 89-100% (other combinations), hybrids were obtained from all combination (Bors and Sullivan, 2005a), indicating that crosses between these
diploid species were generally successful.
2.) Crosses between diploids and hexaploids
Interspecific hybridization was administrated by Evans (1974) using F. vesca, F.
viridis, F. nubicola, F. nilgerrensis, and F. (vesca × viridis) as female plants to cross with F. moschata (6x) and only two seedlings were obtained from the 43 pollinated flowers and the two seedlings died before true leaves formed. This result indicated crossing barriers between these species. However, the diallel crosses of F. moschata with F. nubicola and F. viridis in the study of Bors and Sullivan (2005b) were more successful. They got 1.4 healthy plants/ pollination in the combination of F. moschata × F. viridis, 3.3 healthy plants/ pollination in F. nubicola × F. moschata and 0.1 healthy plants/ pollination in F. viridis × F. moschata. The success rate was raised probably because the germination technique was improved using in vitro culture (Bors and Sullivan, 2005b).
3.) Crosses between diploids and octoploids
Interspecific hybridization was conducted in numerous studies and clear crossing barriers were observed (Evans, 1974; Li et al., 2000; Marta et al., 2004). In F. × ananassa ‘Honeoye’ × F. vesca ‘Changsen’, a relative low number of hybrid seedlings (84 hybrid seedlings from 303 seeds) was obtained. The germination rate of F. vesca pollen on F. ×ananassa stigmas was low, some germinated pollens did not penetrate the stigma. Elongation of pollen tubes in the style was irregular and the growth of embryos and endosperms was aberrant (Li et al., 2000). In the study of Marta et al.
(2004), pollen tube growth was observed to arrest in the first-third of the style and it produced only two aborted seeds in the cross of F. ×ananassa and F. vesca. In the reciprocal cross, 35 seeds were obtained, but the germination rate was only 14% (5 seeds) and seedlings died shortly after germination (Marta et al., 2004). The work of Li
et al. (2000) and Marta et al. (2004) revealed pre-zygotic and post-zygotic barriers between the interspecific hybridization of octoploids and diploids.
The above studies suggest that crosses in the same ploidy level are much easier than interploidy hybridizations. Although species at lower ploidy levels were more difficult to cross with F. ×ananassa, they had not been ignored by plant breeders (Hancock and Luby, 1993). Incorporation of traits from lower ploidy Fragaria species into the cultivated strawberry had been accomplished by artificially doubling chromosome numbers and making numerous crosses (Hancock and Luby, 1993).
Evans (1977) came up with the system called synthetic octoploid (SO system) in which germplasms of 2x, 4x, and 6x Fragaria species were incorporated into octoploid hybrids. In this system, Fragaria species of the lower ploidy were crossed to obtain tetraploid hybrids and the hybrids were further treated with colchicine resulting in octoploid hybrids that contained various germplasms (Evans, 1977; Harbut and Sullivan, 2004). It bypassed ploidy level differences and facilitated introgression of 2x, 4x and 6x species into the cultivated strawberries (Evans, 1977; Bors and Sullivan, 2005).
The use of this method has led to two SO clones, Guelph SO1 (Evans, 1982a) and Guelph SO2 (Evans, 1982b). Guelph SO1 originated from colchicine treated tetraploid hybrids between F. moschata (6x) and F. nubicola (2x). It was a staminate clone possessing late flowering, upright flower stalks, high number of flower stalks and its flavor, aroma and flesh color of fruit were distinctive to F. moschata (Evans, 1982a).
The origin of Guelph SO2 came from crossing the amphidiploid (4x) hybrid of F. vesca (2x) and F. viridis (2x) with F. moupinensis (4x) and the chromosome of the interspecific hybrid was doubled again to form the synthetic octoploid strawberry. SO2 was a staminate clone and had a reasonable resistance to powedery mildew, leaf scorch and leaf blight (Evans, 1982b). SO1 and SO2 were evaluated by means of outcrossing
(recurrent selection) for their potential to contribute horticultural useful traits in strawberry improvement. The yield and berry weight of some hybrid progenies were improved to be as good as or greater than the average of the check cultivars within 3-5 generations (Sangiacomo and Sullivan, 1994).
Fragaria ×ananassa ‘Toyonoka’ (female parent) and F. nilgerrensis ‘Yunnan’
(male parent) were crossed, their hybrid chromosome number was doubled and then backcrossed to F. ×ananassa ‘Pajaro’ in the study of Noguchi et al. (2002) in order to breed a new aromatic strawberry. This hybrid strawberry performed aroma of peach, light pink skin and soft flesh and was registered as Kurume IH No.1 in Ministry of Agriculture, Forestry and Fisheries of Japan at 2005. Some decaploid strawberries had been produced and released from crosses of F. ×ananassa and F. vesca, namely
‘Spadeka’, ‘Annelie’ and ‘Sara’ (Bauer, 1979 and Trajkovski, 1997).
The works of Bauer (1979), Evans (1982a, 1982b), Sangiacomo and Sullivan (1994), Trajkovski (1997) and Noguchi et al. (2002) verified possibility to incorporate germplasms of lower ploidy Fragaria into the cultivated strawberries via using the species of higher ploidy level as female parent when possible, or try to increase the chromosome complement of the species at lower level, or to use means such as embryo culture mentioned by Evans (1974).