Ann. appl. Biol. (1974), 77, 61-65 Printed in Great Britain

61

Heritability of bulb shape in some north European onion varieties

BY B. D. DOWKER AND J. F. M. FENNELL National Vegetable Research Station, Wellesbourne, Wamick

(Accepted 16 November 1973)

S U M M A R Y

Heritabilities for shape index (the ratio of bulb height to diameter), based on parent-offspring regressions, were calculated for north European onion cultivars and inbred lines derived from them. Heritability estimates of 0.46 and 0.47 respectively were obtained for the two groups, So parent bulbs giving S, progenies, and S, parent bulbs giving S, progenies.

Within each offspring progeny, the regression of bulb shape index on log, bulb weight was significant. The regressions were used to estimate the mean shape indices of the progenies at the same mean bulb weight as their parents, and a series of ‘corrected’ progeny shape means thus obtained. Recalculation of the heritabilities using these ‘ corrected ’ progeny means gave increased estimates (0.78 and 0.84).

By using this regression approach, the breeder can achieve high herita- bilities when selecting for a specific mean shape index.

I N T R O D U C T I O N

Bulb shape is an important characteristic of onion cultivars and in the U.K. a globe shape is generally preferred (Anon., 1965). Magruder et al. (1941) described onion bulb shape in terms of the ratio of diameter to height, measuring vertical height from the junction of the base plate with the scales to the neck constriction at the point where curvature changed from convex to concave, and diameter as the greatest bulb diameter in the horizontal plane. Other workers, e.g. Nakamura (1959), McCollum (1966), have used the same basic measurements but calculated the reciprocal ratio, i.e. height/diameter, as an index of shape, and this method has been used here. Either ratio fails adequately to describe subtle variations bdt is nevertheless useful as a primary indicator of shape. Heritabilities of shape index have been reported by Nakamura (1959) and by McCollum (1966). Nakamura obtained heritability estimates for the cultivar Senshuki in the range 0.17-0’40; McCollum reported estimates of 0-34-0-68 for populations of the Sweet Spanish type. This paper describes the esti- mation of heritabilities of shape index, defined as the ratio of bulb height to diameter, in some north European cultivars, from parent-offspring regressions, with and without allowance for certain environmental or genotype-environmental effects.

62 B. D. DOWKER AND J. F. M. FENNELL

M A T E R I A L S A N D M E T H O D S

In 1966 unreplicated variety and inbred evaluation trials were grown at Welles- bourne. The variety trial included Rijnsburger (Elsoms), Early Maincrop (Elsoms), Bola (Vreekens Zaden), Famo (Sluis), Wijbo (Sluis en Groot) and Goudkogel (Huizer), and the inbred trial contained S, lines originating from cvs Rijnsburger (Elsoms), Giant Zittau (Elsoms), Bedfordshire Champion (Elsoms) and Rawska (Poland).

Bulbs were chosen from these cultivars and S, lines for subsequent seeding. Selection of these parent bulbs was done at the beginning of March 1967, after a five-month storage period, and was biased towards bulbs of good appearance, which had neither rotted nor sprouted in store.

Table I . Means and ranges of shape indices and bulb weights for parents and ofspring

Shape (H/d) index A

3 Bulb weight No. of Uncorrected Corrected (9) parent (-*-, & -1

bulbs Mean Range Mean Range Mean Range

So parent (1966) 37 1.11 0.95-1.34 85 34-14 S , offspring (1968) 1.27 098-1.81 1.16 089-1.48 45 38-55 S, parent (1966) 24 091 0.66-1.15 108 25-201 S I offspring (1968) 1'20 1'02-1.39 1 ' 0 0 0'72-1'27 62 40-104

Selected bulbs were potted up in March and subsequently flowered under glass in July. Each plant was self-pollinated by enclosing the inflorescence in a cellophane bag and introducing blowflies as pollinators. The selfed seed obtained was sown in the field at Wellesbourne in March 1968, as unreplicated S, and S, progeny plots.

After harvest and subsequent storage, the inbreds were evaluated for yield, bulb appearance and storage quality, and 61 (37 S,, 24 S,) lines were selected on the basis of their agronomic performance. All bulbs in each of these plots were recorded individually for bulb weight, height and diameter. The mean number of bulbs recorded per plot was 58 for the S, lines and 118 for the S, lines. Means and ranges of the shape indices and bulb weights of these progenies together with corresponding values for their parent bulbs are given in Table I .

R E L A T I O N S H I P BETWEEN B U L B S H A P E A N D W E I G H T

McCollum (1966) reported high negative correlations, both phenotypic and environmental, between bulb weight and shape index. He obtained phenotypic correlation coefficients ranging from - 0.33 to - 0.57, and environmental correlation coefficients from -0.49 to -0.72. Salem (1966) also reported that bulb size was negatively correlated with the ratios determining bulb shape. Dowker & Bowman (1968) obtained highly significant negative linear regressions of shape index on bulb weight (both measured as plot means), though the range of shape indices was relatively small.

Heritability of bulb shape in some onion varieties 63 In the current experiments, investigation of the effect of bulb weight on shape

within a progeny plot showed that this linear relationship also held to some extent. However, the linear regressions were not significant for some of the progenies, and hence an appropriate transformation for the bulb weight variate was sought. The log, transformation proved to be the most satisfactory, and highly significant (negative) regressions of individual bulb shape on log, individual bulb weight were obtained for each of the 61 progenies. On average, the regression sums of squares accounted for 64 yo of the total variability. This compared with 49 yo for the corresponding regressions of shape index on untransformed bulb weight. For regression on log, weight, the mean regression coefficient for all progenies was -0.19, with a range of -0.11 to -0.30.

H E R I T A B I L I T Y E S T I M A T E S

Heritability, in the narrow sense, may be defined as the ratio of additive genetic variance to the total phenotypic variance, and given certain assumptions, may be estimated as the slope of the regression of offspring on parent (Falconer, 1960). Such

Table 2 . Heritability estimates Heritabilities (HN) estimated as regression coefficients from :

No. of Data ‘ corrected’ ‘ Corrected’ data Group parent bulbs Raw data by regression rescaled

s o - + s1 37 0.46 Ifi 024 0 8 5 +012 0.78 0.1 I

s1 -+ s, 24 0.47 k 0.16 1.19ko.16 084&o.r2

heritability estimates for shape index were obtained by regression of the progeny means on their respective individual parent bulb values. These heritabilities were estimated separately for the two groups, namely So parent bulbs giving S, progenies, S, parent bulbs giving S, progenies. These estimates are presented in Table 2. The values obtained, 0.46 and 0.47 respectively, were within the range quoted for popu- lations of the Sweet Spanish type by McCollum (1966).

The mean weights of the parent bulbs and their offspring were often dissimilar (Table I), and because of the relation between bulb weight and shape, this must increase the variance due to ‘ environment ’. A more useful estimate of heritability would be obtained if this environmental contribution due to the difference in mean bulb weight between parent and offspring was reduced. The regressions of shape on log, bulb weight for the progenies provide a method for achieving this. For each progeny, this regression was used to estimate its shape ratio at the same mean bulb weight as its parent bulb, and hence a series of ‘corrected’ progeny means was obtained (Table I). No extrapolation from the data was necessary as parental shape indices were within the range of progeny indices. The heritabilities were then re- calculated by regression of ‘corrected’ progeny means on parent bulb values. As expected, the heritability estimates were increased (Table 2). Although some of the estimates exceeded unity none was significantly greater than unity. Nevertheless, such estimates cannot be used for predicting response to selection. When heritabilities are calculated from the values for parents grown in one season and offspring in another

64 B. D. DOWKER AND J. F. M. FENNELL season, as in this example, inflated estimates due to genotype-environmental effects are not uncommon. Some of these effects may arise from changes of the scale of measurement from season to season. The range of shape index for parents and offspring in 1966 and 1968 respectively (Table I) suggests some expansion and shift of the scale from season to season which may have led to heritability estimates in excess of unity.

Frey & Horner (1957) have suggested that such data required rescaling to stan- dardized deviation units (this is equivalent to correlation analysis of the original data). A result of this method is that the ceiling for heritability estimates is unity and genotype-environmental effects due to scaling are reduced. Thus further rescaling has been done for the ‘corrected’ data in this example and the results are presented in Table 2.

D I S C U S S I O N

Hanson (1963) has argued that, to be of value to plant breeders, heritability should be considered in terms of selection concepts, and hence requires a flexible definition to cover its range of use in plant selection. Also, any heritability statement should be prefaced by a statement of the material and selection units upon which the heritability is based. In the experiment considered, the type of response to selection required by the onion breeder is that the bulb shape of the offspring should vary as little as possible from that of the chosen parent bulb. The material used, though somewhat limited in range of shape index, nevertheless represents the type of cultivar and family within a cultivar which is likely to be used in a U.K. onion breeding programme. The conclusions presented, however, should not be extrapolated beyond this material and method of selection.

The regression approach used here to increase the estimates of heritability can be adopted by the plant breeder. It has been demonstrated that if a specific mean shape is required in an offspring family, the breeder must consider both the shape and weight of the selected parent bulb. If, for example, the offspring family is to be grown at the standard plant density of 80 plants/m2 and is expected to give a yield of 40 t/ha, the expected mean bulb weight will be 50 g. Thus, assuming that a mean shape index of 1.0 is required, the parent bulb should be chosen such that its estimated shape index at a weight of 50 g is 1.0. This can readily be calculated for any given weight of parent bulb, using the regression coefficient from the shape/log, weight relationship for that cultivar. With genotypes similar to those in the experiments described it can be predicted that the correlation between bulb shape of offspring and parent will be in the region of -to%

Rescaling of the heritabilities in terms of standardized deviation units is sensible when estimating expected response to selection. However, the genotype-environ- mental effects causing inflated heritability estimates remain, leading to reduced heritabilities in practice. At present we have no evidence of any variety with a low sensitivity to seasonal environment, which would tend to eliminate this effect of genotype-environment interaction.

The heritability considerations have been confined to progeny means, but there remains considerable variation within progenies in bulb weight and hence in bulb shape. Any way of reducing variation in bulb weight within a population is expected

Heritability of bulb shape in some onion varieties 65 to reduce variation in shape between individual bulbs in that population. Reduction of variability in shape within a population might be achieved by selecting for a reduced response to the effect of bulb weight, i.e. low genotypic values of the regression coefficients from the intra-population regression of shape on log, weight. Considera- tion of the progeny regression coefficients and their standard errors suggested that such genotypic differences exist.

R E F E R E N C E S

ANON. (1965). Onions and related crops. Bulletin No. 69, Ministry of Agriculture, Fisheries and

DOWKER, B. D. & BOWMAN, A. R. A. (1968). Onions. Report of the National Vegetable Research

FALCONER, D. S. (1960). In Introduction to quantitative Genetics. Edinburgh, London: Oliver

FEY, K. J. & HORNER, T. (1957). Heritability in standard units. Agronomy Journal 49, 59-62. HANSON, W. D. (1963). Heritability. In Statistical Genetics and Plant Breeding, pp. 125-140.

Ed. W. D. Hanson & H. F. Robinson. Publication 982, National Academy of Sciences- National Research Council, Washington, D.C.

MCCOLLUM, G. D. (1966). Heritability and genetic correlations of some onion bulb traits. Journal of Heredity 57, 105-110.

MAGRUDER, R., WEBSTER, R. E., JONES, H. A., RANDALL, T. E., SNYDER, G. B., BROWN, H. D., HAWTHORN, L. R. & WILSON, A. L. (1941). Descriptions of types of principal American varieties of onions. United States Department of Agriculture Miscellaneous Publications, 435.

NAKAMURA, N. (1959). Studies on the breeding of Allium cepa L. I. Estimating heritability. Japanese Journal of Breeding 8 , 255-260.

SALEM, I. A. (1966). The inheritance of onion bulb shape and its component measurements. Dissertation Abstracts Section B 27, 1373.

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