Euphytica 106: 7–13, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

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Heritability estimates and progeny testing of phenotypic selections forsoluble solids content in dehydrator onion

Arthur D. Wall & Joe N. CorganDepartment of Agronomy & Horticulture, MSC 3Q, P.O. Box 30003, New Mexico State University, Las Cruces,NM 88003–8003, U.S.A.

Received 3 March 1998; accepted 2 September 1998

Key words: Allium cepa, dry-mass, dry-matter, heritability, non-structural carbohydrates, soluble solids

Summary

Soluble solids comprise most of onion bulb dry mass, and dehydrator onion cultivars are developed from breedingpopulations that have high dry mass content. Realized and narrow-sense heritability estimates were obtained forthe soluble solids content (SSC) trait in two open-pollinated dehydrator onion breeding populations (BP) usingresponse to selection and half-sib family analysis. Parental populations, designated as BP9335-U and BP9243-U, were derived from two-way crosses of lines advanced as open-pollinated (OP) populations to the F7 or F6generation, respectively. BP9335-U had one previous selection cycle for increased SSC and BP9243-U had threeSSC selection cycles. In these experiments, parental populations were screened again for high SSC, and selectedbulbs were intermated to form half-sib progeny groups, designated as BP9335-S and BP9243-S. Mean SSC wasincreased by 6.6% and a realized heritability estimate of 0.64 was obtained for BP9335-S. Mean SSC was increasedby 6.3% and a realized heritability estimate of 0.36 was obtained for BP9243-S. Narrow-sense heritability estimatesof 0.58±0.05 and 0.30±0.03 were obtained for parental populations BP9335-U and BP9243-U, respectively.Narrow-sense heritability estimates of 0.40±0.03 and 0.63± 0.23 were obtained for progeny populations derivedfrom selected high-SSC bulbs of these lines (BP9335-S and BP9243-S), respectively, indicating that there issignificant additive genetic control of the SSC trait in these populations. Significant differences in half-sib familyperformance in the advanced groups BP9335-S and BP9243-S demonstrate that progeny testing was effective forevaluating phenotypic selections.

Introduction

White-skinned onion cultivars with high dry mass(‘solids’) content are processed into dehydrated onionflakes and powders (Fenwick & Hanley, 1990). De-hydrated onion products are primary ingredients forthe food processing and the spice industries (Amer.Spice Trace Assoc., 1993). Most dehydrated onionproduction in the United States is located in Cali-fornia (Amer. Dehydrated Onion & Garlic Assoc.,1993). Southern New Mexico has a suitable climatefor dehydrator onion production, and development ofregionally adapted, high-yielding cultivars could pro-vide incentive to expand dehydrator onion productionand processing in this region. Improved dehydrator

onion populations also can be good sources of parentsfor development of F1 hybrids.

Onion water soluble carbohydrate (WSC) com-prises 80 to 87% of onion bulb dry mass, and includesglucose, fructose, sucrose, and fructans (Darbyshire& Henry, 1979; Darbyshire & Steer, 1990; Sinclairet al., 1995). Onion fuctans are fructose oligosaccha-rides, with degree of polymerization (DP) from 3 to15 monosaccharide sub-units, depending on popula-tion. Other species ofAllium are known to accumulatefructans with DP>15 (Darbyshire & Henry, 1981).The most basic onion fructans are DP3 trisaccha-rides, synthesized by transfer of fructose to sucrose,to form two isomers, 1F-fructosylsucrose, and 6G-fructosylsucrose. Longer fructans are formed by addi-tion of fructose sub-units to the DP3 fructan (Endle-

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man & Jefford, 1968; Henry & Darbyshire, 1979;Darbyshire & Henry, 1981). However, Cairns (1993)states that previous reports on fuctan synthesis requirereappraisal.

Fresh market onions usually have 6 to 10% drymass and contain relatively high amounts of glucose,fructose and sucrose, and relatively low amounts offructan. Conversely, high-solids dehydrator onion cul-tivars contain≥20% dry mass, have relatively lowamounts of glucose, fructose and sucrose, and highamounts of fructan (Darbyshire & Henry, 1979; Sin-clair et al., 1995).

Onion dry mass content is usually measured in-directly by using a refractometer to assay for solu-ble solids content (SSC) in onion juice (Pike, 1986;Dowker, 1990). Sinclair et al., (1995) reported a 0.99correlation between refractometer SSC readings andactual dry mass content in a study of 49 diverse onioncultivars. Several other studies have also reported thatSSC is an effective selection criterium for dry masscontent (Mann & Hoyle, 1945; Nieuwhof et al., 1973).Bulb SSC directly indicates dry mass content, butdoes not indicate ratios and proportions (‘profiles’) ofconstituent soluble (non-structural) carbohydrates. Ithas been postulated that free sugars in onions causebrowning, or carmelization, reactions during dehy-dration. It is not known how recurrent selection forSSC will affect onion carbohydrate profiles. Non-destructive gravimetric methods (Kehr, 1952) andnear-infrared spectrophotometry (Birth & Dull, 1985)are also used to test for SSC and dry mass in onions.

SSC increases from the top to the bottom of bulb-scales, and also from outer to inner scales (Mann &Hoyle, 1945; Darbyshire & Henry, 1978), therefore, itis important to obtain a representative sample of onionbulbs for dry mass or SSC testing.

Accumulation of dry mass has been attributed togreater efficiency of carbohydrate synthesis, transloca-tion and storage in high SSC cultivars (Sinclair et al.,1995; Nieuwhof et al., 1973), or to relative hydrationof bulbs (Darbyshire & Henry, 1979). Both factors arelikely to contribute to bulb SSC, and bulb dry mass ac-cumulation is likely to vary according to both geneticand environmental regulation.

Heritability estimates (h2) describe the degree ofresemblance between relatives, are used to predictthe effectiveness of phenotypic selection, and helpto define appropriate breeding strategies. Also, half-sib family progeny evaluations can be used to indi-cate the breeding performance of parents (Hallauer &Miranda, 1988). Heritability estimates are useful to

predict response from selection, but are only relevantto the study population, and to environmental condi-tions that were applied (Simmonds, 1979). Narrow-sense heritability estimates the ratio of additive ge-netic variance to total phenotypic variance (VA/VP).Narrow-sense heritability estimates are made with theassumption that non-additive gene action is negligible,or minimal, and if non-additive gene action is present,heritability estimates will tend to be biased upward(Hallauer & Miranda, 1988). Realized heritability in-dicates genetic gain or progress during a generation ofselection, as an index derived by dividing the responseto selection (R) by the selection intensity differential(S), or h2 = R/S (Falconer, 1981).

McCollum (1968) reported heritability estimatesfor SSC of 0.79± 0.14 from parent-offspring regres-sion and 0.82 from intra-class correlation in a ‘WhiteSweet Spanish’ cultivar. Madalageri et al. (1986)studied a cycle of selection for SSC in 18 intermediate-solids dehydrator onion breeding populations with 13to 17% SSC, and reported significant additive ge-netic effects, narrow-sense heritabilities (0.69 to 0.92),and genotype× environment interactions. Kadams &Nwasike (1986) studied an OP Nigerian white onionpopulation and reported SSC heritability estimates of0.72 and 0.58 using half-sib correlations and parent-offspring regression, respectively, although standarderrors were not provided. Both Lin et al. (1995) andSimon (1995) evaluated SSC of storage onion inbredsin diallel crosses and found that additive gene actionaccounted for most genetic variability, with a smalldegree of partial dominance observed in some crosses.In addition, broad-sense heritability estimates of 0.08to 0.56 for SSC were indirectly estimated from gen-eration means analysis study. Havey & Randle (1996)performed test-crosses in long- and intermediate-dayonion populations, and reported significant generalcombining ability for SSC, and in some cases heterosisfor this trait was evident.

Our research objectives were to: 1) calculate re-alized heritability estimates for the SSC trait in twodehydrator onion breeding populations that had beenpreviously selected for SSC; 2) calculate narrow-senseheritability estimates for both the parental and progenygenerations of these two populations; and 3) determineif progeny testing of half-sib families is effective toevaluate phenotypic selections during recurrent selec-tion. These experiments differ from previous studiesbecause dehydrator onion breeding populations werestudied that had already been subjected to at least oneprevious selection cycle for increased SSC.

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Table 1. Model for half-sib family analysis used to calculate narrow-sense heritabilityestimates of onion soluble solids

Source of variation df MS Expected mean square

Replicates r-1

Among half-sib b-1 M1 σ2ω+kσ2ρ+rkσ2β

families

Error (reps× families) (r-1)(b-1) M2 σ2ω+kσ2ρ

Within half-sib rb(k-1) M3 σ2ω

families

Corrected total n-1

1) σ2β = M1-M2/rk = variance among half-sib families.

M1 = mean square of half-sib families.

M2 = mean square error.

r = number of replications.

k = number of individuals sampled in a half-sib family per replication.

2) σ2A = 4σ2β = additive genetic variance.

σ2β = variance among half-sib families.

3) h2 = σ2A

σ2A+σ2ρ+σ2ω

h2 = narrow sense heritability.

σ2A = additive genetic variance.

σ2ρ = M2-M3/k.

= variance due to interactions of half-sib families within replications.

σ2ω = M3 = variance within a half-sib family line.

Materials and methods

Onion breeding populations

The OP breeding populations, BP9335 and BP9243,were under development for dehydrator characteris-tics, and were disease and bolting resistance, and hadwhite bulb color. BP9335 was an F7 population orig-inating from the cross of ‘Ben Shemen’× ‘SouthportWhite Globe’ that had been selected once for SSC,and had an intermediate-day photoperiod response.BP9243 was an F6 population derived from crossinga bolting-resistant ‘Yellow Grano’ selection× ‘South-port White Globe’ that had been selected three timesfor SSC, and had a short-day photoperiod response.Earlier selection cycles were made sequentially andjust previous to the cycle performed in this experi-ment. BP9335 had a maturity range from July 24 toAugust 7, and BP9243 had a maturity range fromMay 23 to June 6 in southern New Mexico. A min-imum of 21 bulbs from each line were intermated toreduce the potential for inbreeding depression, andselection differential targets of 10 and 5% were cho-sen for BP9335 and BP9243, respectively. Plants from

bulbs that survived the screening process, summerstorage and over-wintering formed the final study pop-ulations. BP9335 was screened for SSC in 1994, and9% (29/324) of the highest SSC bulbs were intermatedin isolation cages with bees (Apis melliferaL.) during1995, to form the 9335 selected (progeny) population(9335-S). BP9243 was screened for SSC in 1993, and3.7% (21/571) of the highest SSC bulbs were inter-mated in 1994 to form the 9243 selected population(9243-S). A random sample of 36 and 47 bulbs wereintermated from BP9335 and BP9243, respectively, toform unselected half-sib family populations 9335-Uand 9243-U, which were used as unselected (parental)populations to calculate realized heritability estimates,and were also used to derive narrow-sense heritabilityestimates.

Seed was collected from individual bulbs withinisolation cages to form maternal half-sib families.Field experiments were located at the Fabian GarciaScience Center, in Las Cruces, New Mexico, from1993 to 1996. Plots were sown both years during lateOctober in a randomized complete block design with6 replications. Plots were 1.1 m long with 4 evenly-spaced rows planted on a 76 cm wide bed, and with

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Table 2. Population parameters, mean squares for half-sib family effects and residual error mean squares for fourdehydrator onion breeding populations

Breeding Year Previous SSC Number half- Total sample Mean square Residual error

population selection cycles sib families size family effect mean square

Parentalz 9335-U 1996 1 36 2160 22.90∗ 0.98

Progenyy 9335-S 1996 2 29 1740 17.49∗ 1.54

Parental 9243-U 1995 3 47 2820 25.66∗ 2.49

Progeny 9243-S 1995 4 21 1260 56.66∗ 1.78

∗ Probability> F-statistic≤0.001.z denotes population in which original selection was performed.y denotes population derived from intermated selected high SSC bulbs.

Table 3. Mean soluble solids content (SSC) of parental andprogeny populations, gain in SSC from one cycle of selection,and heritability estimates for four dehydrator onion breedingpopulations

Breeding Mean SSCz SSC Realized Narrow-sense

population (%) gain heritability heritability

Parental 9335-Uy 11.65±0.25 – – – – 0.58±0.05

Progeny 9335-Sx 12.47±0.32 6.6% 0.36 0.40±0.03

Parental 9243-U 15.56±0.34 – – – – 0.30±0.03

Progeny 9243-S 16.60±0.49 6.3% 0.64 0.63±0.23

z SSC means between parental and progeny generations are signif-icantly different, as determined by t-tests, Probability> |t|≤ 0.01.y denotes population in which original selection was performed.x denotes population derived from intermated selected high SSCbulbs.

a 1 m row spacing. Plots were thinned to leave 6 cmbetween plants (67 plants·m2−1). The crop was man-aged according to standard practices for fall plantedonions in southern New Mexico (Bailey & Corgan,1986; Corgan & Kedar, 1990), and water and ni-trogen were applied using drip irrigation. Plots wereharvested when∼80% of the tops had fallen. Fifteenbulbs were randomly harvested from within the middle0.9 m of each plot, so that diseased or damaged bulbscould be discarded, and a 10 bulb sub-set from eachreplication was then used for SSC testing.

Soluble solids evaluation

Bulbs were washed to remove dirt and loose outerscales, and then air-dried. Bulbs were cut in half lat-erally, and scraped with a serrated knife across theentire cut surface of the top-half to express onionjuice. Juice was applied to a hand-held refractometerfor SSC assay in 0.5% increments, to test for SSC.The bottom-half of selected bulbs were marked with ahalf-sib family number and SSC was noted. Selected

bulb-bottoms were kept at∼24 C with forced air cir-culation to deter pathogens during 6 to 8 weeks ofstorage, prior to re-planting in field isolation cages tointermate selected bulbs.

Response to selection

Realized heritability estimates were obtained by di-viding the response to selection (R) by the selectionintensity differential (S). Response to selection (R)is the difference in mean SSC between unselected-parental and selected-progeny populations, comparedconcurrently. The selection differential was the meandifference between selected and unselected bulbs thatwere originally screened (Falconer, 1981).

Narrow-sense heritability estimates from half-sibfamily analysis

Narrow-sense heritability estimates were derived fromhalf-sib family analysis according to the methods ofHallauer & Miranda (1988) and were based on parti-tioning variance due to crosses among half-sib fam-ilies, variance due to interactions between half-sibfamilies within replications, and variance within half-sib families. Narrow-sense heritability is calculated bydividing additive genetic variance by total phenotypicvariance, according to the model in Table 1.

Results and discussion

Narrow-sense heritability estimates from half-sibfamily analysis

Significant F-statistics for half-sib family effects inboth breeding populations indicate that there are ge-netic factors that control inheritance of the SSC trait

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Table 4. Range in soluble solids content within mean separation groups derived from the Waller-Duncan k-ratiot-tests for two selected dehydrator onion breeding populations

Population 9335-S Population 9243-S

Mean Range of SSC in Number of families Mean Range of SSC in Number of families

separation mean separation in mean separation separation mean separation in mean separation

groupz group (%) groupy groupz group (%) groupx

a 13.17–13.58 3 a (18.24) 1

b 12.83–13.19 10 b 17.47–17.80 3

c 12.63–13.00 10 c 17.24–17.51 5

d 12.48–12.88 5 d 16.88–17.31 4

e 12.43–12.83 5 e 16.86–17.24 4

f 12.28–12.63 7 f 16.53–16.93 6

g 12.16–12.48 9 g 16.33–16.71 6

h 12.01–12.37 8 h 16.16–16.57 6

i 11.93–12.28 7 i (15.72) 1

j 11.85–12.20 7 j 14.77–15.07 2

k 11.76–12.16 5 k 14.47–14.77 2

l 11.63–12.01 6

m 11.49–11.85 5

z Mean separation groups are not significantly different at k-ratio=100 which∼ Probability> Fstatistic≤ 0.05.y 29 half-sib family means from 9335-S are contained in subsets of 13 mean separation groups. Minimum significantdifference = 0.412% SSC for half-sib family means in the 9335-S population.x 21 half-sib family means from 9243-S are contained in subsets of 11 mean separation groups. Minimum significantdifference = 0.429% SSC for half-sib family means in the 9243-S population.

in these populations (Table 2). This implies that re-current selection will be effective in improving SSCin these populations, although the mid-range heritabil-ity estimates of 0.30 to 0.63 suggests that progressin raising SSC using phenotypic recurrent selectionwill likely be incremental, and require a long termbreeding strategy (Table 3). A large number of half-sibfamilies and a large sample size are required to mini-mize residual error mean squares and standard errorsof heritability estimates (Tables 2 and 3). Repetitionof experiments in multiple years and locations will benecessary to detect half-sib family× year× locationeffects, if present. The data presented from these ex-periments may mask the influence of environment. Ifyear or location interactions are present, or if a sig-nificant amount of selfing occurred in isolation cages,the stated narrow-sense heritability estimates may beinflated (Hallauer & Miranda, 1988).

Heritability estimates from response to selection

Advances were made after a cycle of selection inboth populations, and t-tests indicate that mean SSCgains were statistically significant (Table 3). Selectionand intermating of parental phenotypes was effec-tive in raising mean SSC of progeny. Both realized

and narrow-sense heritability estimates demonstratea moderate degree of heritability (0.30–0.64) for theSSC trait, although heritability estimates were higherin BP9243, which had greater advancement for theSSC trait. Further testing will be required to determineif these narrow-sense heritability estimates includemasked year or location effects, or non-additive geneeffects, that could inflate estimates.

This experiment provides a static estimate of real-ized and narrow-sense heritability. Ideally, heritabilityof SSC should be studied over several generations ofselection to demonstrate changes in inheritance pat-terns during advancing cycles of recurrent selection.Gains in SSC were significant for the populations inthis experiment that are in the early to middle phase ofdevelopment as dehydrator onions. This suggests thatSSC potential has not been reached and further gainscan be anticipated.

Half-sib family progeny testing

Significant differences between half-sib families in theadvanced populations, 9335-S and 9243-S, demon-strate that progeny testing was effective for evaluatingSSC of phenotypic selections (Table 4). Families thatperform well in progeny tests could be intermated

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Table 5. Range in soluble solids content means and min-imum and maximum soluble solids content values of fourdehydrator onion breeding populations

Breeding Range in mean Minimum Maximum

population SSC (%) SSC value (%) SSC value (%)

9335-Uz 10.35–13.17 6.5 18.5

9335-Sy 11.49–13.58 9.0 17.0

9243-U 14.03–17.14 9.0 21.0

9243-S 14.47–18.24 10.5 21.5

z denotes population in which original selection was per-formed.y denotes population derived from intermated selected highSSC bulbs.

from remnant seed, or from bulbs saved from progenytests. Differences between half-sib family means wereincremental and many mean separation groups wereobserved (Table 4).

These data suggest that selection of high perform-ing families is likely to improve the frequency ofdesirable alleles in recurrent selection programs, rel-ative to simple phenotypic selection without progenyevaluation. SSC ranges and minimum values were alsoraised in both populations by one cycle of selection(Table 5).

Conclusions

Phenotypic selection was effective in raising meanSSC of dehydrator onion breeding populations. Dif-ferences in the mean SSC of maternal half-sib familiesdeveloped from selected high-SSC bulbs demonstratethat progeny evaluation was effective for evaluatingphenotypic selections. Narrow-sense heritability esti-mates for SSC in this study concur with other studiesthat report additive gene action for SSC (Lin et al.,1995; Simon, 1995; Havey & Randle, 1996), and her-itability estimates in the 0.4 to 0.6 range (McCollum,1968; Kadams & Nwasike, 1986). Greater progressin improving the SSC trait can be expected at the be-ginning and intermediate stages of dehydrator onionbreeding programs. Longer term recurrent selection islikely to raise the average SSC of these populations tothe≥18% dry mass content required for commercialdehydrator onion cultivars.

Acknowledgements

This research was supported by the New MexicoDry Onion Commission, the New Mexico Agricul-tural Experiment Station and the Jose Fernandez Chairfor Crop Production research at N.M.S.U. We thankM.R. Doyle, Jose Luis Mendoza, Helen Redden andMelodie Borden for their technical support.

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