Strawberries. James F Hancock
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 from 1000 to 3800 m. Many have studied strawberries from this region without realizing a wild polyploid Fragaria species was there with more than 56 chromosomes (Darrow, 1966; Harrison et al., 1997b; Hokanson et al., 1993, 2006). Hancock et al. (2001) even unknowingly included the decaploid cytotype CFRA 110 (PI 551527) in their core selection of native octoploid germplasm.
A much clearer understanding of the phylogeny of Fragaria has emerged in the last decade as a number of molecular phylogenies have been published (Liston et al., 2014; Sobczyk, 2018). There are two major clades with nine species each (Rousseau-Gueutin et al., 2009; Njuguna et al., 2013). One is named ‘vesca’ representing the closely related diploids F. bucharica, F. mandshurica and F. vesca; the tetraploid F. orientalis; the hexaploid F. moschata; the octoploids F. chiloensis and F. virginiana; and the decaploids F. iturupensis and F. cascadensis. The other is named ‘China’ representing four diploid species (F. chinensis, F. daltoniana, F. nubicola, F. pentaphylla) and four tetraploid species (F. corymbosa, F. gracilis, F. moupinensis, F. tibetica) endemic to China and adjacent Himalayan countries, and one diploid species endemic to Japan (F. nipponica). Unfortunately, the placement of F. iinumae, F. hayatai, F. nilgerrensis and F. viridis was left poorly resolved, as was the parental ancestry of most of the polyploids.
Although there appear to be some barriers to interfertility among the diploid strawberries, they can all be crossed to some extent, and meiosis in the hybrids is regular, even in cases where the interspecific hybrids are sterile (Federova, 1946; Staudt, 1959; Fadeeva, 1966). This suggests that they may share the same genome, with only cryptic structural differences. Iwatsubo and Naruhashi (1989) found that the chromosomes of F. nipponica and F. vesca are very similar in morphology, although F. iinumae had some distinguishing features. It seems likely that F. vesca is ancestral to all the diploids, as its geographical range overlaps or touches almost all the other diploid species and it has been successfully crossed with most of them, including F. nilgerrensis, which is sexually isolated from all the other species tested (Fadeeva, 1966).
Based on levels of interfertility, there are at least three overlapping groups of species (Bors and Sullivan, 1998): (i) F. vesca, F. nubicola, F. pentaphylla and F. viridis; (ii) F. vesca, F. daltoniana, F. pentaphylla and F. nilgerrensis; and (iii) F. pentaphylla, F. gracilis and F. nipponica. No fertile seeds were recovered when F. iinumae was crossed with Group 1 and Group 2 species, but Bors and Sullivan (1998) did not attempt to cross it with Group 3 species.
Polyploidy in Fragaria probably arose through the unification of 2n gametes, as several investigators have noted that unreduced gametes are relatively common in Fragaria (Scott,