Tomato (Solanum lycopersicum) breeders have observed that plants with uniform green-shouldered fruit are less prone to yellow shoulder (YS) than are plants with (dark) green-shouldered tomatoes and thus have selected for tomato cultivars with uniform green-shouldered fruit. However, a recent publication reported that a cultivar with green-shouldered fruit had significantly higher soluble solids content (SSC) than an isogenic cultivar with uniform green-shouldered fruit and postulated that selection of uniform green shoulder has negatively affected tomato flavor and processing quality. Lines with dark green (u +), uniform green (u), uniform gray-green (ug), apple green (u Ag), medium green, and pale green (u Pg) immature fruit colors were crossed in all combinations to produce F1 plants that were self-pollinated to produce F2 seed. Parents, F1, and F2s were planted in the field in completely randomized block designs over two seasons. Plants were visually phenotyped for immature fruit color, and fruit from each plant were selected to measure shoulder and base color with a colorimeter. Ripening fruit were harvested to measure the incidence of YS, and SSC was measured on ripe fruit from each plant with a refractometer. In the spring season, fruit from F2 plants with green-shouldered fruit had significantly higher YS incidence than all phenotypes with uniform fruit colors. In the fall, phenotypes with medium-green shoulders were also tested, and these had greater YS than all other phenotypes except green shoulder. YS incidence for green shoulder was not significantly greater than that in the other phenotypes. Fla. 7956, the apple green parent, had 0% YS and appeared to be resistant. Higher SSC was observed in the spring season than in the fall season. However, in both seasons, when comparisons were made between phenotypes that segregated in the F2s, the SSC of green-shouldered phenotypes was not significantly higher than that of other phenotypes. Plants with apple green fruit tended to be higher in SSC in the fall, but this may relate to the dark green foliage of apple green plants and not just the fruit color. A hypothesis that stress may relate to reported SSC increases because of u + is discussed.
Matthew R. Mattia and John W. Scott
Yuan Yu, Chunxian Chen, Ming Huang, Qibin Yu, Dongliang Du, Matthew R. Mattia, and Frederick G. Gmitter Jr.
Citrus (Citrus sp.) germplasm collections are a valuable resource for citrus genetic breeding studies, and further utilization of the resource requires knowledge of their genotypic and phylogenetic relationships. Diverse citrus accessions, including citron (Citrus medica), mandarin (Citrus reticulata), pummelo (Citrus maxima), papeda (Papeda sp.), trifoliate orange (Poncirus trifoliata), kumquat (Fortunella sp.), and related species, have been housed at the Florida Citrus Arboretum, Winter Haven, FL, but the accessions in the collection have not been genotyped. In this study, a collection of 80 citrus accessions were genotyped using 1536 sweet orange–derived single nucleotide polymorphism (SNP) markers, to determine their SNP fingerprints and to assess genetic diversity, population structure, and phylogenetic relationships, and thereby to test the efficiency of using the single genotype-derived SNP chip with relatively low cost for these analyses. Phylogenetic relationships among the 80 accessions were determined by multivariate analysis. A model-based clustering program detected five basic groups and revealed that C. maxima introgressions varied among mandarin cultivars and segregated in mandarin F1 progeny. In addition, reciprocal differences in C. maxima contributions were observed among citranges (Citrus sinensis × P. trifoliata vs. P. trifoliata × C. sinensis) and may be caused by the influence of cytoplasmic DNA and its effect on selection of cultivars. Inferred admixture structures of many secondary citrus species and important cultivars were confirmed or revealed, including ‘Bergamot’ sour orange (Citrus aurantium), ‘Kinkoji’ (C. reticulata × Citrus paradisi), ‘Hyuganatsu’ orange (Citrus tamurana), and palestine sweet lime (Citrus aurantifolia). The relatively inexpensive SNP array used in this study generated informative genotyping data and led to good consensus and correlations with previously published observations based on whole genome sequencing (WGS) data. The genotyping data and the phylogenetic results may facilitate further exploitation of interesting genotypes in the collection and additional understanding of phylogenetic relationships in citrus.