Fig. 2.8. Population divergence [Weir and Hill F-statistic (F) arrow labels] of wild progenitor species (South American F. chiloensis and F. virginiana), temperate F. × ananassa (TM-F×a), California coastal (CC-F×a) pre-1970, and CC-F×a post-2000. Internal values reflect average marker diversity (π) within the populations. (From Hardigan et al., 2018.)

AMERICANIZATION OF EUROPEAN BREEDING PROGRAMMES

      As previously described, strawberry breeding began in Europe in the early 1800s, with a collection of New World representatives of F. chiloensis and F. virginiana. In the mid-1800s, the early hybrid cultivars from Europe found their way across the Atlantic and were used by the first North American breeders to initiate their programmes. Over the next 100 years, the European and North American breeding programmes tended to operate in isolation with only limited germplasm exchange. However, this separation dramatically changed in the 1960s, when European breeders began to widely employ improved North American cultivars from California in their breeding programmes.

      The appearance of US germplasm in European breeding programmes has been documented in a couple of recent studies using diagnostic DNA markers. Gil-Ariza et al. (2009) used simple sequence repeat (SSR) markers derived from expressed sequence tags to determine similarity relationships of 92 mostly US and European cultivars from the mid-1800s to present. They identified three clusters: (i) a clade of 26 old accessions (most of them released before 1969) of European or American origin with little California genetics incorporated; (ii) a clade of 44 later-released accessions from both continents represented by ‘Camarosa’ and carrying a goodly proportion of genetics from the Strawberry Breeding & Research programme at the University of California, Davis (Cal-Davis); and (iii) a clade of 22 cultivars from different breeding periods represented by ‘Selva’ and also carrying a strong contribution of genes from Cal-Davis.

      Horvath et al. (2011) employed 23 SSR markers to examine diversity patterns and population substructuring in 57 European cultivars dating from the late 1800s to modern times and 29 others developed outside Europe (mostly North American) from the early 1900s to the present. They observed ‘a progressive abandonment in European breeding efforts of old European cultivars towards mainly North American ones’ and a distinct separation between old European cultivars developed before 1960 and those bred more recently. They found a clustering of cultivars into four groups: (i) mostly old European cultivars; (ii) mixtures of American and modern European cultivars grown across the continent; (iii) mixtures of mostly American cultivars from California and modern cultivars grown in southern Europe; and (iv) an ‘intermediate’ group of cultivars introgressed with F. chiloensis and F. moschata. They warned that the older European material needs to be protected and maintained so that potentially valuable genes from the infancy of strawberry breeding in Europe are not lost.

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