Strawberries. James F Hancock
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rel="nofollow" href="#ulink_c3904b1a-34a7-5bb9-a561-87f1b9fb1261">Edger et al., 2020; Liston et al., 2020).
Edger et al. (2019) did not propose a genome formula for the octoploid strawberry but based on earlier crossibility studies, it could be represented by Bringhurst’s (1990) proposal of AAA′A′BBCC. F. vesca (AA) and F. viridis (A′A′) can be crossed with limited fertility so they warrant a similar letter designation. F. iinumae (BB) and F. nipponica (CC) cannot be crossed with any other diploid species so warrant separate letter designations.
Hardigan et al. (2019) employed whole-genome shotgun genotyping of interspecific segregating populations to identify 1.9 million subgenome variants spanning 3394 cM in F. chiloensis ssp. lucida, and 1.6 million subgenome variants spanning 2017 cM in F. × ananassa. Through comparative genetic mapping of these variants, they were able to show that the genomes of the wild octoploids are effectively diploidized as predicted by Bringhurst (1990) and completely collinear. This genetic structure has allowed for ‘unimpeded gene flow’ during the domestication and interspecific hybridization of the strawberry.
In comparisons of the nuclear and organelle genomes of the octoploids, it appears that F. vesca ssp. bracteata was the chloroplast donor of the octoploid strawberries, while both F. vesca ssp. bracteata and F. iinumae were the sources of the mitochondrial genomes, which subsequently recombined (Mahoney et al., 2010; Njuguna et al., 2013; Govindarajulu et al. 2015). Little is known about the origin of F. iturupensis, except it shares the same plastid donor as F. chiloensis and F. virginiana. A plastid genome phylogeny generated by Dillenberger (2018) found that the octoploid F. chiloensis is monophyletic, while all other polyploid taxa are para- or polyphyletic. F. cascadensis has biparental plastid inheritance and has four different plastid donors.
The tetraploid F. orientalis and hexaploid F. moschata likely represent a polyploid series (Harrison et al., 1997a; Njuguna et al., 2013). Based on the molecular studies of Lin and Davis (2000), Rousseau-Gueutin et al. (2009) and DiMeglio et al. (2014), F. orientalis appeared to be an allopolyploid derived from F. vesca and F. mandshurica, and F. moschata contains the additional genome of F. viridis. Based on the work of Edger et al. (2019), F. orientalis is more likely derived from F. vesca and F. nipponica. F. moschata crosses readily with F. viridis (Schiemann, 1937), F. nubicola (Ellis, 1958), F. nipponica (Lilienfeld, 1933), F. orientalis (Federova, 1946), and with difficulty with F. vesca (Mangelsdorf and East, 1927).
In the older studies on crossibility, F. orientalis and F. viridis were shown to cross relatively easily and the resulting hybrids were partially fertile (Federova, 1946). Hybrids of F. orientalis and 2x F. vesca are much more difficult to make, although Staudt (1952) was able to produce a fertile hexaploid hybrid between them, and Bors and Sullivan (1998) found the cross of 4x F. vesca and F. orientalis to be relatively easy to make. Schiemann (1937) described plants that looked like F. orientalis that were derived from a pentaploid F2 population of F. vesca × F. moschata.
The diploid–tetraploid relationships previously proposed in the ‘China’ clade by Staudt (2008) based on geographical and morphological similarities (F. pentaphylla > F. tibetica, F. chinensis > F. gracilis/F. corymbosa, F. nubicola > F. moupinensis) has not been supported by recent molecular phylogenetic studies (Kamneva et al., 2017; Yang and Davis, 2017). It appears that F. corymbosa, F. gracilis and F. moupinensis all share F. pentaphylla and F. chinensis as parent, and F. tibetica is most likely derived from F. pentaphylla and F. nubicola.
Dominance of subgenomes in octoploid strawberries
The octoploid genome sequence provided by Edger et al. (2019) along with gene expression data show that the last species to enter into the octoploid species, F. vesca, is the dominant subgenome ‘with significantly greater gene content, gene expression abundance, and biased exchanges between homoeologous chromosomes, as compared with the other subgenomes’. Their data fits the ‘subgenome-dominance theory’ which predicts that genome-wide expression disparity can arise when the merged genomes differ in their transposable element (TE) complement and in their level of TE-mediated repression of gene expression (Bertioli, 2019). Edger and his group found that F. vesca has 20% fewer transposable elements, has retained 20% more genes and has generally higher gene expression. The metabolic pathways that give rise to strawberry flavour, colour and fragrance are largely controlled by this dominant subgenome.
Origin of the North American octoploid strawberries
The phylogenetic study of Edger et al. (2019) combined with the geographic distributions of extant species supports a North American origin for the octoploid strawberry with F. vesca ssp. bracheata being the last diploid to be added in the formation of the ancestral octoploid strawberry. It is likely that F. chiloensis and F. virginiana are extreme forms of the same biological species, which emerged in Beringia and subsequently evolved differential adaptations to coastal and mountain habitats of North America. The north-eastern Asia distribution of F. iturupensis suggests that it may have been part of the genomic pool that originated in north-east Asia before spreading across the Bering Strait to north-western North America. The rise of the octoploid clade is estimated to have occurred 0.37–2.05 million years ago (Njuguna et al., 2013).
While F. chiloensis and F. virginiana are completely interfertile, there are significant morphological distinctions between them. F. chiloensis has thick, dark-green, coriaceous leaves and large achenes, whereas F. virginiana has thin, bluish-green leaves and smaller achenes (Staudt,