Parental potential, or breeding value, was evaluated for strawberry (Fragaria × ananassa Duch.) genotypes selected for high, intermediate, or low phenotypic expression of soluble solids content (SSC) and titratable acid content (TA). Progeny means and genetic variance parameters were estimated using seedlings from 40 crosses among these selections, conducted in two factorial sets. Selection response for breeding value was detected for SSC in the upward direction and for TA in both upward and downward directions. Populations created by divergent selection of parents followed by intermating expressed additive genetic variances 2.3 and 9.3 times larger than those estimated for appropriate controls for SSC and TA, respectively. For TA, the response to selection for breeding value was consistent with results for genotypic selection reported earlier. The significant selection response in breeding value obtained for SSC differed from the results of previous clonal tests that had detected no significant genotypic selection response. These results, together with previous assessments, suggest that cumulative genetic gain can be obtained for both SSC and TA. However, gain for SSC will be contingent on selection under conditions that approximate commercial treatment; this will not be necessary for TA, as expression for this trait is stable across test location and cultural treatments.
Douglas V. Shaw
The extended production season of strawberries raised in mediterranean environments depends on plant development that occurs during the winter months. Seedling genotypes from 20 bi-parental crosses and their nine parent genotypes were fully vernalized and grown at 11, 14, and 17C, to test for adaptation to growth at minimal temperatures. Genetic variance parameters were estimated and tests for genetic x temperature interactions were conducted for five vegetative growth traits. Highly significant (P < 0.01) genetic effects were detected for all traits, and broad-sense heritability estimates ranged from 0.09 to 0.41. None of the genetic x temperature interactions were significant for seedling genotypes, and interactions were significant only for leaf dry weights for parental genotypes. These results indicate a genetic basis for variable vegetative growth rates, but provide no evidence for specific adaptation to growth at low temperatures.
Douglas V. Shaw
The heritabilities of, and genetic correlations among, variables that describe internal and external color in fresh strawberry (Fragaria × anarrassa) fruit were estimated using factorial analyses of seedlings from 20 controlled crosses. Hunter L and a values, and a subjective score generated by comparison with color plates were obtained for seedling genotypes and their parents at two locations. Genetic effects were responsible for 33% to 61% of the phenotypic variance for color traits, after correction for location effects. Means for objective color variables differed significantly between locations, but means for subjective color scores did not. Genetic × location interaction variances were usually nonsignificant, and were < 12% of the phenotypic variance for all variables. Phenotypic and genetic correlations between objective and subjective color scores were significant and large (absolute values of r = 0.42-0.69; rg = 0.84-1.00). Multiple regression of subjective scores on L and a explained 69% and 59% of the phenotypic variation for external and internal color, respectively. Genetic correlations between measures of internal and external color were small and mostly nonsignificant, suggesting that separate sets of genes condition these traits.
Douglas V. Shaw
Selfed progenies were generated using 10 day-neutral genotypes from the University of California (UC) strawberry breeding program as parents and their offspring were classified for late-summer flowering response. The grandparents of each selfed progeny included one of four day-neutral genotypes and one of eight short-day genotypes. Under the null hypothesis of genetic control by a single locus with the allele for day-neutrality dominant to the allele for short-day flowering response, all of these day-neutral parent genotypes must be heterozygous and their selfed offspring were expected to fit a 3:1 ratio of day-neutral: short-day phenotypes. The percentage of day-neutral offspring observed over all progenies was 70.9%, and was significantly smaller than the expected value of 75% (χ2 1 = 5.08, P < 0.02). The percentage of day-neutral offspring for individual progenies ranged from 41.4% to 84.8%, and highly significant heterogeneity was detected among progenies (χ2 9 = 40.3, P < 0.01). Selfed progeny means for the cumulative late-summer flowering score calculated using the day-neutral fraction of offspring varied from 1.31 to 2.35 and progeny means for the number of inflorescences per plant ranged from 3.5 to 9.9; these differences among progenies were highly significant (P < 0.01). These observations can be used to conclusively reject the hypothesis that day-neutrality in this domestic strawberry population is controlled by a single locus.
Douglas V. Shaw
Ingram Olkin and Douglas V. Shaw
Douglas V. Shaw and Thomas R. Gordon
Strawberry (Fragaria ×ananassa Duch.) genotypes retained for resistance to Verticillium wilt (Verticillium dahliae Kleb.) after two cycles of a two-stage (TS) selection procedure consisting of full-sib family selection followed by within-family selection of individuals, and genotypes retained for resistance using genotypic mass (GM) selection were crossed to a common set of moderately susceptible genotypes. The relative resistance of the seedlings from these progenies was compared using a resistance score and the percentage of stunted plants. Although the two sets of resistant parents had performed similarly in genotypic comparisons, those genotypes selected using the TS procedure yielded test cross offspring with significantly higher resistance scores (X̄ = 3.84 ± 0.09 vs. X̄ = 3.46 ± 0.09, t = 3.11**) and significantly lower rates of plant stunting (X̄ = 38.1% ± 3.1 vs. X̄ = 50.2% ± 2.9, t = 2.87**) than the parents chosen using GM selection. Further resolution using analysis of variance and general combining ability (GCA) estimates showed that these between-set differences resulted from higher resistance breeding values for parents selected using the TS procedure. The five genotypes with largest GCA for resistance score and four of the five genotypes with minimum GCA for percentage stunting were obtained by TS selection.
Kirk D. Larson and Douglas V. Shaw
Three preplant soil fumigation treatments were applied on 5 Apr. 1993 to a nursery site that had not been planted previously to strawberries (Fragaria ×ananassa Duch.): 1) a mixture of 67 methyl bromide: 33 chloropicrin (CP) (by weight, 392 kg·ha–1) (MBCP); 2) 140 kg CP/ha; and 3) nonfumigation (NF). On 26 Apr., cold-stored `Chandler' and `Selva' strawberry plants of registered stock were established in each treatment. Soil and root/crown disease symptoms were absent in all treatments during the course of the study. In October, runner plants were machine-harvested and graded to commercial standards. The cultivars produced a similar number of runners per mother plant. Fumigation with MBCP, CP, and NF resulted in 18.56, 15.75, and 7.89 runners per mother plant, respectively. For `Selva', runner root and crown dry weights were similar for the MBCP and CP treatments, but NF resulted in significant reductions compared to the other two treatments. For `Chandler', fumigation with CP resulted in reduced root dry weight, and NF resulted in reduced crown and root dry weights compared to fumigation with MBCP. The results demonstrate the marked decreases in strawberry runner production and runner size that can occur in the absence of preplant soil fumigation, even on new strawberry ground. Also, small, but significant, reductions in runner production and runner size may occur with CP applied at a rate of 140 kg·ha–1 compared to standard fumigation with MBCP. Chemical name used: trichloronitromethane (chloropicrin).
Erik J. Sacks and Douglas V. Shaw
Color change in fresh, ripe strawberry (Fragaria ×ananassa Duch.) fruit stored at 0C for up to 7 days was recorded using the Commission Internationale de l' Éclairage color space (L*, a*, and b*). External (skin) fruit color became darker and less chromatic but did not change hue. Internal (flesh) fruit color became darker and more chromatic. Regression coefficients calculated for individual genotypes were homogeneous for each of the color traits except internal hue. Depending on genotype, the red fruit flesh either became a bluer red or did not change hue. In all cases, rates of change were small. Color change for fresh strawberry fruit during several days of storage at 0C likely is not an appreciable source of error in plant breeding experiments.
James J. Luby and Douglas V. Shaw
Breeders of horticultural food crops are usually concerned with multiple traits related to yield and quality as well as other traits such as biotic and abiotic stresses. Yield in these crops is not solely tonnage of biomass produced in the field. Rather, it is the proportion of the crop that can be harvested and brought to market in a condition and at a price acceptable to the consumer. Quality may include flavor, color, shape, size, degree of damage, nutrient levels, and traits that permit greater perceived food safety or environmental sustainability. Some traits may exhibit phenotypic associations. Traits with unfavorable associations will be of concern to the breeder if the cause is unfavorably correlated genetic effects, especially those resulting from pleiotropy. Several multiple trait selection schemes have been developed, including independent culling levels, tandem selection, and index selection. These schemes can result in improvement even for traits with unfavorable associations. However, the breeder must have a strong rationale for each trait addressed in a breeding program because each additional trait necessitates larger breeding populations and more resources. Thus, the breeder's first challenge for each crop is to determine which traits are most important and which issues are most amenable to a breeding solution.