Validation of Molecular Markers Associated with Perpetual Flowering (PF) and Soluble Solids Content (SSC) in Strawberry (Fragaria xananassa Duch. ex Rozier) Public Deposited

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  • Strawberry is economically the most important berry crop grown worldwide and breeders are continuously striving to develop improved cultivars. So far, marker assisted breeding (MAB) in strawberry has been limited to the private sector. However, loci controlling some traits of economic importance such as perpetual flowering (PF) and soluble solids content (SSC) associated with fruit sweetness were recently mapped in the cultivated strawberry, a prerequisite to enabling MAB. The first objective of this research was to investigate usefulness of markers linked to the FaPFRU locus controlling PF in 'Capitola' and strawberry germplasm containing the same F. virginiana subsp. glauca source collected by Bringhurst in Utah’s Wasatch Mountains through 'Tribute' and 'Seascape', and from other sources such as 'Pan American' through 'Fort-Laramie', and an unknown source through 'Sarian'. The second objective was to assess the ability of an SSC-associated simple sequence repeat (SSR) marker for predicting high soluble solids content in different environments in strawberry. Five SSR markers (EMFvi136, Bx89, Bx215, Bx56 and Bx63) that are linked to PF on linkage group IVb were used to genotype 893 strawberry individuals. Based on Mendel's Law of Inheritance, parentage was confirmed for most seedlings. Those that had unlikely genotypes or alleles were identified in each of the mapping and breeding populations from Michigan State University (MSU) and those of the USDA-ARS in Corvallis (ORUS). Off type frequencies in breeding populations were 17.1%, on average. Furthermore, multiple genotypes of parents were identified and included: 'Fort Laramie' and FRA 1701 in the MSU breeding programs and 'Tillamook' and 'Puget Reliance' in the ORUS program. Genotype of the FRA 1701 used as parent was different between both programs. Six alleles were associated with PF in the entire dataset: EMFvi136_167, Bx89_251, Bx89_262, Bx215_129 and Bx56_209. However, none of these alleles were present in the FRA 1701-derived progeny and could not be used to predict PF. Since Bx215 mapped closest to the FaPFRU locus, we used repeated rounds of ANOVA to identify combinations of alleles at this SSR that best explained the PF trait in the breeding populations. The presence of 129 and absence of 160 increased predictivity of PF from 63% to ~80% while the absence of 129, 123, 127 and 134 predicted OF in 100% of the germplasm where the original source of PF was that obtained from the F. virginiana subsp. glauca from the Wasatch Mountains through 'Capitola', 'Seascape' and 'Tribute'. In 'Sarian'-derived germplasm, other alleles, 123 and 134 or 127, were predictive of PF. Establishing allele composition at each subgenome using microsatellite allele dose and configuration establishment MADCE for Bx215, Bx56 and Bx63 identified the associated alleles as shared among three or four of the subgenomes, thus unable to predict PF based on allele presence in all PF individuals. Absence of these alleles was more predictive of once flowering and could be used to eliminate those individuals and therefore increase the proportion of PF progeny to evaluate. Determining haplotypes in this region and evaluating haplotype association with PF could be much more accurate in predicting PF than allele presence or absence and should be investigated. Soluble solid content-associated EMFv006 was used to genotype 602 individuals. Alleles and genotypes associated with low (209, 215), medium (207, 213, 217) and high SSC (219, 221) were obtained. Plants with the 213:219 EMFvi006 genotype always had higher SSC than those with the 213:209 or 213:215 genotypes in all states except for California. When converted to functional alleles and analyzed by ANOVA in each of the states, a consistent increase in SSC mean was observed between genotypes with the lower SSC types and the higher SSC genotypes in each of the states and years. Heritability (H²) was low (0.02-0.09) in all states but NH_12 indicating that this trait is highly influenced by environmental conditions. The proportion of phenotypic variance (PVE) was high in comparison with those of H² in all states except for CA_12 and ranged from 0.65 in MI_11 to 0.86 in NH_12, indicating that most of the genotypic variance observed can be explained by this locus that can strongly predict differences in SSC content. Much of the variability seen in the SSC trait in this study could be explained by differences in environments and fruit ripening at the time of harvest and their effects on SSC as SSC is very sensitive to environmental factors such as temperature and harvest date. It may be necessary to develop a more standardized protocol of measuring SSC across states and years to obtain more reliable SSC data for each individual. Most importantly, this study illustrates the usefulness of introgressing novel alleles from wild accessions into the cultivated gene pool. Both alleles consistently associated with high SSC, 219 and 221, originated from wild F. virginiana parents (FRA 1701-I1, and FRA 1701, respectively) from the "supercore" and were not present in any of the F. ×ananassa cultivars genotyped in this study except for an old European selection, 'Jucunda', of unknown pedigree. Based on the results obtained in this study, breeders may be able to select progeny with desired flowering type and increased soluble solid content after evaluating the alleles present in their parental material.
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