common in the north and ssp. grayana more common in the south.
The western subspecies have yielded considerable taxonomic debate. Darrow (1966) considered the western types of F. virginiana to be a separate species F. ovalis, but Staudt (1962) did not give species rank to this material because of the lack of barriers to hybridization and the intermediacy of its characters. The separation of ssp. platypetala and ssp. glauca has also been questioned. Welsh et al. (1987) suggested that the two subspecies completely intergrade and probably should be referred to as a single taxon var. glauca. Others have followed this designation (Scoggan, 1978) or have not attempted to recognize infraspecific taxa (Dorn, 1984). What was formerly recognized as a distinct species, F. multicipita, was recently reduced to F. virginiana ssp. glauca f. multicipita when it was discovered that its unique morphology was caused by a phytoplasma (Catling, 1995).
Whereas F. virginiana ssp. virginiana and ssp. glauca appear to be largely distinct across their range, strawberries in the Black Hills and eastern front ranges of the Rocky Mountains may be introgressive swarms between ssp. glauca and ssp. virginiana (Luby et al., 1992; Hokanson et al., 1993; Sakin et al., 1997). In a mutivariate analysis of F. virginiana and F. chiloensis populations across the northern USA, Harrison et al. (1997b) found that Black Hill populations are intermediate between collections of eastern ssp. virginiana and western ssp. glauca for morphological traits, and when analysed with RAPD markers, the Black Hill populations were part of a large cluster group incorporating both ssp. virginiana and ssp. glauca (Fig. 1.9). The Black Hill populations probably represent relicts of the late Pleiocene when the Great Plains were mainly a boreal forest that provided a continuous habitat for hybridization between eastern and western forms of F. virginiana.
Fig. 1.9. Multivariate analysis of F. virginiana and F. chiloensis populations from across the USA. The factor scores are for (a) the first three principal components of the means of morphological variables; and (b) RAPD frequencies. Groups are labelled according to the Fragaria taxonomy of Staudt (1962). (From Harrison et al., 1997b.)
Natural hybrids of F. virginiana × F. chiloensis exist where native habitats of the two species overlap in British Columbia, Washington, Oregon and Northern California. Staudt (1999) has designated these hybrids as F. × ananassa ssp. cunefolia (Nutt ex Howell). The F. chiloensis populations noted on woodland-meadow sites by Jensen and Hancock (1981) probably represent stabilized hybrid derivatives of this cross. Hybrid populations of F. chiloensis × F. virginiana are common from Vancouver Island along the coast to Fort Bragg, California, and Staudt (1989) has indicated that ‘the further one goes from the coastal area the more the F. chiloensis characters decrease. Plants with somewhat thinner leaves but some other characters of F. chiloensis are combined in ssp. platypetala of F. virginiana … considered to be the final link of introgression of F. chiloensis into F. virginiana ssp. glauca.’ Luby et al. (1992) present evidence of interaction between these two species in the mountains of northern Idaho and western Montana, where individuals of F. virginiana have thick, roundish leaves and thick runners reminiscent of F. chiloensis even though they are more than 400 km from the Pacific Ocean.
In the recent study by Harrison et al. (1997b), variation patterns in the morphological traits suggested that F. virginiana ssp. platypetala is distinct from F. chiloensis ssp. lucida in the Pacific Northwest (Fig. 1.9). However, in the RAPD analysis, the two groups were combined even though they remained distinct from all the eastern F. virginiana. It is possible that the RAPD markers are selectively neutral traits that reflect ancient patterns of gene flow between F. chiloensis and F. virginiana in early post-Pleistocene times, whereas the morphological traits were moulded through selection as the genus Fragaria faced new environmental challenges.
Fragaria × ananassa L.
This is now the most important strawberry cultivated worldwide, however, its domestication was not based on natural hybrids between F. chiloensis and F. virginiana, but instead on accidental hybrids that appeared in European gardens in the mid-1700s (see Chapter 2, this volume). From a horticultural point of view, many of the traits distinguishing the two species are complementary (Table 1.3), and it is not surprising that hybrid-derived populations came to dominate commercial plantings of strawberry. Even after dozens of rounds of selection, many of the morphological traits found in F. × ananassa are still intermediate to its parent species, but considerable segregation has occurred. Using morphological traits, Darrow (1966) found that eastern cultivars expressed 27–57% of the characters from F. chiloensis.
Decaploids (10n = 10x = 70)
Fragaria iturupensis Staudt
This species is found solely on Iturup Island, north-east of Japan (Staudt, 1989; Hummer et al., 2011). Staudt’s original chromosome counts of F. iturupensis indicated that it was octoploid, but a later collection after the original plant was lost found it to be decaploid. (Hummer et al., 2009). It has obovate, subglaucous leaves that are bluish, much like F. iinumae. The petiole is covered with patulate hairs. The flowers are hermaphroditic, 16–20 mm wide with five petals. There are two to four flowers to an inflorescence. The fruit is similar to F. vesca, but larger. Stolons are branched with no secondary runners from axils of primary bracts. Berries are subspherical, bright red and shiny with reflexed calyxes and superficial achenes.
Fragaria cascadensis Hummer
Other than its decaploid chromosome number, F. cascadensis has a morphology similar to the octoploid F. virginiana ssp. platypetala (Staudt, 1999; Hummer, 2012). It can only be accurately distinguished by its hairy adaxial leaf surfaces and comma-shaped achenes. F. virginiana ssp. platypetala of the Oregon Cascades has no hairs on the upper leaf surface, and achenes are dome shaped. It is only known in a narrow distribution range in the western Oregon Cascades at elevations
