The root-knot nematode (M. hapla Chitwood) poses a threat to carrot (Daucus carota L.) production in the United States. Little information is available concerning the genetic control of nematode resistance in carrot. Crosses between two inbreds, a resistant genotype (R1) and susceptible genotype (S1) identified in previous screening tests of carrot were studied in the F2 and BC1 generations to determine the heritability of resistance to the root-knot nematode. Seedlings of F2 (R1/S1), BC1S1, and BC1R1 generations were evaluated for their responses to infestation of M. hapla Chitwood based on gall number per root, gall rating per root, and root rating per root in a greenhouse experiment conducted during 1994. Narrow-sense heritabilities were calculated according to the method of Warner (1952). Narrow-sense heritability was 0.16 for resistance based on gall number, 0.88 for resistance based on gall rating, and 0.78 for resistance based on root rating. This information may be of importance to geneticists and carrot breeders for the development of nematode-resistant carrot cultivars.
Bong-kyoo Kim, Don R. La Bonte, Christopher A. Clark, and Mario I. Buteler
Narrow-sense heritabilities for reaction to chlorotic leaf distortion (CLD), incited by Fusarium lateritium Nees: Fr., were estimated in sweetpotato [Ipomoea batatas (L.) Lam] by variance component analysis and parent–offspring regression. Visually rated severity reactions to CLD varied greatly among the 20 parents used to generate half-sib progeny from open-pollinated nurseries in 1990 and 1991. Progeny from each nursery were evaluated along with parents in a completely randomized design in two consecutive years. Narrow-sense heritability (h 2) estimates based on variance components were moderate on an entry mean basis at 0.61 in 1990, 0.38 in 1991, and 0.33 for the two years combined. Slightly higher, but still moderate, estimates were obtained on an individual plant basis. Narrow-sense heritability estimates using parent–offspring regression were 0.35 in 1990, 0.33 in 1991, and 0.33 for the two years combined. Predicted next-generation response was highest using a half-sib family recurrent selection among three schemes compared at a 10% selection intensity. Our data indicate –0.63 improvement in the half-sib family CLD severity rating in one breeding cycle.
John R. Stommel and Kathleen G. Haynes
Inheritance of resistance to tomato anthracnose caused by Colletotrichum coccodes (Wallr.) S.J. Hughes was evaluated in parental, F1, F2, and backcross populations developed from crosses between adapted resistant (88B147) and susceptible (90L24) tomato (Lycopersicon esculentum Mill.) breeding lines. Resistance was evaluated via measurement of lesion diameters in fruit collected from field-grown plants and puncture inoculated in a shaded greenhouse. Backcross and F2 populations exhibited continuous distributions suggesting multigenic control of anthracnose resistance. Anthracnose resistance was partially dominant to susceptibility. Using generation means analysis, gene action in these populations was best explained by an additive-dominance model with additive × additive epistatic effects. A broad-sense heritability (H) of 0.42 and narrow-sense heritability (h2) of 0.004 was estimated for resistance to C. coccodes. One gene or linkage group was estimated to control segregation for anthracnose resistance in the cross of 90L24 × 88B147.
Douglas V. Shaw and Kirk D. Larson
The genetic opportunity for selection of early fruiting strawberry cultivars was evaluated using seedling populations from the Univ. of California (UC) breeding program in three years. Narrow-sense heritabilities for early season yield and for the proportion of an individual's total yield expressed early were moderate (h2 = 0.24-0.53) and broad-sense heritabilities were slightly larger (H2 = 0.31-0.70), suggesting the presence of some nonadditive genetic variance for these traits. These two traits were genetically correlated with each other (rg = 0.78-0.98), but only early yield was consistently genetically correlated with seasonal yield (rg = 0.52-0.82). Selection was performed for each trait using an index on full-sib family means and individual phenotypic values in two of the three years, and predicted response was compared with that obtained using vegetatively propagated runner plants from selected genotypes in the subsequent fruiting season. Statistically significant (P < 0.05) selection response was obtained in one of two years for each trait, and combined analysis demonstrated highly significant (P < 0.01) response for both traits. However, realized response over all traits and years was just 27.3% of that predicted based on the estimated heritabilities and applied selection intensities. These results suggest that selection for early yield should be based at least in part on runner plant evaluations rather than exclusively on seedling performance.
Peter M. Hanson, Jen-tzu Chen, and George Kuo
High-temperature fruit set (heat tolerance) is a critical trait of tomato (Lycopersicon esculentum Mill.) cultivars targeted for lowland wet season production in the tropics and subtropics. Heat-tolerant Asian Vegetable Research and Development Center (AVRDC) tomato line CL5915-93D4-1-0-3 (CL5915) is a valuable source of heat-tolerance genes for tomato genetic improvement. The gene action of heat tolerance in CL5915 was determined by evaluating the F1, F2, BCP1, and BCP2 of a cross between CL5915 and heat-sensitive line UC204A for fruit set traits in two wet-season trials at AVRDC. Parent-offspring regression of F2-derived F3 (F2:3) family means on the F2 plants from CL5915 × UC204A was used to estimate the heritability of F2 single plant selection for heat tolerance. Mean percentage of fruit set and fruit number per cluster of the F1 and BCP1 exceeded midparent values and were not significantly different from those of CL5915, indicating complete dominance for heat tolerance. Generation means analyses indicated that a model including simple additive and dominance effects adequately explained the inheritance of mean fruit number per cluster both years. For mean percentage of fruit set, a model including simple additive-dominance effects produced an adequately fitting model in the 1996 season but the best-fitting model included an epistatic component in the 1997 season. Heritabilities estimated for fruit set traits in 1996 and 1997, respectively, were: 0.31 and 0.21 for percentage of fruit set; 0.28 and 0.14 for mean fruit number per cluster; and 0.53 and 0.15 for flower number per cluster. The low heritabilities for percentage of fruit set and mean fruit number per cluster under high temperatures imply that single plant selection in the F2 for heat tolerance from crosses involving CL5915 is not effective and that selection should be based on replicated family testing in the F3 and later generations.
R. Bruce Carle and J. Brent Loy
Genetic experiments were initiated to assess the potential for combining large seed size from PI 285611, a large-fruited, hullless seeded accession, with small fruit size from a hullless seeded breeding line (NH29-13-5-4). An F2 population and parental line were field-grown during Summer 1993 to determine inheritance and heritability of large seed size and the relation between fruit and seed size. Seed size variables of weight, width, length, and thickness were regressed against fruit weight. There was a moderate, positive correlation between large fruit and seed length (R2 = 0.46). However, seed thickness, a major determinant of seed weight, was not correlated with fruit size. In an F2 population of ≈450 plants, there was a small number of plant selections with fruit under 1.5 kg and seed size approaching that of PI 285611.
Ann Marie Connor, James J. Luby, and Cindy B.S. Tong
Narrow-sense heritability and among-family and within-family variance components were estimated for antioxidant activity (AA), total phenolic content (TPH), and anthocyanin content (ACY) in blueberry (Vaccinium L. sp.) fruit. AA, TPH, and ACY were determined in the parents and in 10 offspring from each of 20 random crosses for each of 2 years at Becker, Minn. Offspring-midparent regression analysis provided combined-year heritability estimates of 0.43 ± 0.09 (P ≤ 0.0001) for AA, 0.46 ± 0.11 (P ≤ 0.0001) for TPH, and 0.56 ± 0.10 (P ≤ 0.0001) for ACY. Analyses of variance delineated variation among and within families for AA, TPH, and ACY (P ≤ 0.001). Year-to-year variation in the means for all offspring genotypes was not significant for AA or TPH, but there were changes in rank between years for families and for offspring within families for these traits. Year-to-year variation in the mean for all offspring genotypes was significant for ACY, but rank changes were observed only among offspring within families, not among families. In total, 18 of 200 offspring from 7 of the 20 crosses were transgressive segregants for AA, exceeding the higher parent of the cross by at least two sds. Estimates of variance components showed that variation among families accounted for 24% to 27% of total variance for the three traits. However, variation within families was greater than that among families, accounting for 38% to 56% of total variance for the three traits. These results suggest that increasing antioxidant activity in blueberry through breeding is feasible, and that the breeding strategies utilized should exploit the large within-family variation that exists.
Glenn M. Ito and James L. Brewbaker
Pericarp thickness in maize (Zea mays L.) was analyzed by generation mean analysis for backcross and F2 populations from eight hybrids, derived from two thin-pericarped sweet corn inbreds—AA8 and 677a (55 and 51 μm)—crossed with four field corn inbreds—B37, B68, H55, and Hi26 (range 82-132 μm). Average heterosis was −12.5% and segregating progeny distributions were skewed toward those of thin-pericarped parents. Narrow-sense heritability was high, averaging 55.2%, and the number of effective factors was low, ranging from 1.4 to 5.9 and averaging 3.3. Epistatic effects were as large as additive or dominance effects in many crosses, urging caution in applying models that exclude gene interactions to determine variance components and heritabilities. The mode of action in reducing pericarp thickness appeared to differ among the two thin parents, with AA8 affecting the differential thickening of germinal vs. abgerminal walls, and 677a reducing the number of pericarp cell layers. All genetic parameters suggested that genetic progress in backcross conversions to thin pericarp in sweet corn breeding would be rapid irrespective of the pericarp thickness of exotic parents.
Thomas A. Greene
Height and caliper (age 3 years), and flower count (age 3.5 years) were evaluated for 36 open-pollinated families of Magnolia grandiflora L. outplanted in two genetic tests in Bexar County, Texas. Significant family differences existed for height, caliper, number of flowers per tree, and percent of trees flowering in both tests. Family heritability estimates for all traits ranged from 0.72 to 0.92. Coefficients of genetic prediction (CGP) between growth and flowering characteristics were small but positive; CGP between height and percent of trees flowering was 0.28 and 0.24 in the two tests. Early growth rate and flowering appeared to be under strong genetic control; thus, improvement through selection would be efficient. However, both traits should be evaluated since the genetic relationship between them was weak.
R.L. Bell and Jules Janick
Narrow-sense heritability estimates were computed for five fruit quality characteristics and their weighted total index. Grit content and skin russeting were moderately heritable traits, while flesh texture, flavor, appearance, and the weighted total score were of relatively low heritability. Within sub-populations of crosses, defined by the species ancestry of the parents, the relative magnitudes of heritabilities for each trait varied, but were in general agreement with those for the entire population. The general combining ability variances were 4.5 to 12.0 times those for specific combining ability, although both were statistically significant for all traits and the weighted quality index. The species ancestry of a parent had no effect on its general combining ability rank. While selection of individual seedlings on the basis of their own phenotype will result in genetic improvement for grit and russet, selection based on a combination of full-sib family means and individual phenotypes is recommended for flavor, texture, appearance, and overall fruit quality.