A strawberry Multi-parent Advanced Generation Intercrosses (MAGIC) population, derived from crosses using six strawberry cultivars was successfully developed. The population was composed of 338 individuals; genome conformation was evaluated by expressed sequence tag-derived simple short repeat (EST-SSR) markers. Cluster analysis and principal component analysis (PCA) based on EST-SSR marker polymorphisms revealed that the MAGIC population was a mosaic of the six founder cultivars and covered the genomic regions of the six founders evenly. Fruit quality related traits, including days to flowering (DTF), fruit weight (FW), fruit firmness (FF), fruit color (FC), soluble solid content (SC), and titratable acidity (TA), of the MAGIC population were evaluated over two years. All traits showed normal transgressive segregation beyond the founder cultivars and most traits, except for DTF, distributed normally. FC exhibited the highest correlation coefficient overall and was distributed normally regardless of differences in DTF, FW, FF, SC, and TA. These facts were supported by PCA using fruit quality related values as explanatory variables, suggesting that major genetic factors, which are not influenced by fluctuations in other fruit traits, could control the distribution of FC. This MAGIC population is a promising resource for genome-wide association studies and genomic selection for efficient strawberry breeding.

PK23-2, a line of six-rowed barley (Hordeum vulgare L.) originating from Pakistan, has resistance to Japanese strains I and III of the barley yellow mosaic virus (BaYMV). To identify the source of resistance in this line, reciprocal crosses were made between the susceptible cultivar Daisen-gold and PK23-2. Genetic analyses in the F(1) generation, F(2) generation, and a doubled haploid population (DH45) derived from the F(1) revealed that PK23-2 harbors one dominant and one recessive resistance genes. A linkage map was constructed using 61 lines of DH45 and 127 DNA markers; this map covered 1268.8 cM in 10 linkage groups. One QTL having a LOD score of 4.07 and explaining 26.8% of the phenotypic variance explained (PVE) for resistance to BaYMV was detected at DNA marker ABG070 on chromosome 3H. Another QTL having a LOD score of 3.53 and PVE of 27.2% was located at marker Bmag0490 on chromosome 4H. The resistance gene on chromosome 3H, here named Rym17, showed dominant inheritance, whereas the gene on chromosome 4H, here named rym18, showed recessive inheritance in F(1) populations derived from crosses between several resistant lines of DH45 and Daisen-gold. The BaYMV recessive resistance genes rym1, rym3, and rym5, found in Japanese barley germplasm, were not allelic to rym18. These results revealed that PK23-2 harbors two previously unidentified resistance genes, Rym17 on 3H and rym18 on 4H; Rym17 is the first dominant BaYMV resistance gene to be identified in primary gene pool. These new genes, particularly dominant Rym17, represent a potentially valuable genetic resource against BaYMV disease.

Fruit shape of cultivated strawberry ( Fragaria × ananassa Duch.) is an important breeding target. To detect genomic regions associated with this trait, its quantitative evaluation is needed. Previously we created a multi-parent advanced-generation inter-cross (MAGIC) strawberry population derived from six founder parents. In this study, we used this population to quantify fruit shape. Elliptic Fourier descriptors (EFDs) were generated from 2 969 two-dimensional binarized fruit images, and principal component (PC) scores were calculated on the basis of the EFD coefficients. PC1–PC3 explained 96% of variation in shape and thus adequately quantified it. In genome-wide association study, the PC scores were used as phenotypes. Genome wide association study using mixed linear models revealed 2 quantitative trait loci (QTLs) for fruit shape. Our results provide a novel and effective method to analyze strawberry fruit morphology; the detected QTLs and presented method can support marker-assisted selection in practical breeding programs to improve fruit shape.

Male sterility is defined as the loss of pollen fertility, and it represents a plant reproductive isolation symptom, along with self-incompatibility. It plays an important role in the efficient production of F 1-hybrid seeds, which results in affordable seed prices for farmers. Male sterile cultivated strawberry Fragaria × ananassa Duch. plants were found in an F 1 population and reciprocal backcrossed populations derived from a cross between 'Fukuoka S6' and 'Kaorino'. Male sterile plants were clearly distinguished from male fertile plants in those populations based on the anther color. The pollen of the male sterile plants was a lighter yellow color and not maturely shaped compared with pollen of male fertile plants. Genotyping was performed using EST-SSR markers in the three populations. Quantitative trait locus analyses for pollen fertility were conducted independently using three kinds of populations, and this revealed that male sterility was controlled by three independent chromosomal regions in these populations, which corresponded to chromosome 4 in the wild strawberry (Fragaria vesca) genome. One region was derived from 'Fukuoka S6' and the other two regions from 'Kaorino'. The segregation patterns of fertile and sterile plants in each population clearly supported the three gene theory of male sterility in cultivated strawberries. The accumulation of recessive alleles at the three regions led to male sterility, and the existence of a dominant allele in at least one region resulted in fertile pollen. Male sterile plants were also found in two self-pollenated populations derived from 'Fukuoka S6' and 'Kaorino', and the effects of the three regions were validated. The adaptability levels of the three genes with different genetic backgrounds were also evaluated using core collection cultivars and selected lines derived using recurrent selection. We also detected flanking DNA markers for the three regions associated with male sterility. The use of these markers, which are in the vicinity of quantitative trait loci and responsible for malesterility, could increase the efficiency of producing seed-propagated strawberry F 1-hybrids.

This study was conducted to select a DNA marker linked to the new recessive resistance gene rym7t, which is derived from 'Tokushima mochihadaka' and confers resistance to Barley yellow mosaic virus (BaYMV) strains I, II and III. A genetic linkage map neighboring rym7t was constructed using doubled haploid lines derived from a cross between the resistant parent 'DH21-73,' which includes rym7t, and susceptible parent 'Haruna Nijo'. Two SSR markers, Bmac0031 and Bamc0167 were selected, adjacent to each side of the rym7t locus. The genotype of the two SSR markers corresponded well to the phenotype of BaYMV resistance. The field test of Barley yellow mosaic disease (virus strain I) resistance of F 2 individuals derived from a cross between the susceptible parent 'New Golden' and resistant parent 'DH42-84,' which includes rym7t, showed that the correspondence rates between phenotypes of BaYMV resistance and genotypes with Bmac0031 and Bamc0167 were 98.6% and 98.8%, respectively. In the case of correspondence between the two markers' genotypes, they corresponded exactly with the phenotype of BaYMV resistance of F 2 individuals. The two SSR markers were co-dominant, so both could clearly identify susceptible plants with heterozygous rym7t. The two SSR markers could be useful for marker-assisted selection of resistant lines including rym7t.