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, 1951; Islam, 1960; Bringhurst and Gill, 1970; Dickinson et al., 2007). Staudt (1984) observed restitution in microsporogenesis of an F1 hybrid of F. virginiana × F. chiloensis. In a study of native populations of F. chiloensis and F. vesca, Bringhurst and Senanayake (1966) found frequencies of giant pollen grains to be approximately 1% of the total. Over 10% of the natural hybrids generated between these two species were the result of unreduced gametes.
Although there may not be sufficient differentiation among the diploids to warrant the designation of separate genomes (Staudt, 1959), cytogenetic studies have indicated that there are distinct sets of chromosomes associating in the hexaploid and octoploid species (Federova, 1946; Senanayake and Bringhurst, 1967). Lerceteau-Köhler (2003), studying segregation ratios of AFLP markers in a full-sib family, found that 92% (727 out of 789) had simplex ratios and 8% (62 out of 789) fitted a multiplex ratio. This suggests that inheritance in the octoploid strawberries is mixed, being mostly disomic but not completely.
Cytogenetic studies indicated that at least two pairs of genomes are represented in the octoploid species. When they are crossed with the diploids F. vesca or F. viridis, bivalent or multivalent numbers approaching 14 are commonly observed in pentaploid hybrids, suggesting that there is pairing between one set of diploid and octoploid chromosomes, and another set of octoploid chromosomes (Ichijima, 1926, 1930; Federova, 1946; Bringhurst and Khan, 1963; Senanayake and Bringhurst, 1967). An additional set of chromosomes is left as largely univalents, either due to non-homology with the other sets or competition with a homologous set of chromosomes from the diploid. Similar results have commonly been obtained whether F. chiloensis, F. virginiana or F. × ananassa was used as the octoploid parent, although a few studies have reported much higher numbers of bivalents (21–28) in diploid × F. × ananassa crosses (Yarnell, 1931; Ellis, 1962).
Based on the cytogenetic studies, three genome formulas were suggested for the octoploids: AAAABBCC (Federova, 1946), AAA′A′BBBB (Senanayake and Bringhurst, 1967) and AAA′A′BBB′B′ (Fig. 1.10; Bringhurst, 1990). It seems likely that species similar to F. vesca and F. viridis are in the background of all the octoploid strawberries, as chromosomes from both pair regularly with those of F. chiloensis, F. virginiana and F. × ananassa. Federova suggested that the A genome came from an ancestor of F. orientalis other than F. vesca, the B from F. nipponica and the C from F. vesca.
