Content uploaded by Fernando Lopez Anido

Author content

All content in this area was uploaded by Fernando Lopez Anido

Content may be subject to copyright.

Heritability and expected selection response in asparagus

67

Genetics and Molecular Research 4 (1): 67-73 (2005) www.funpecrp.com.br

Heritability and expected selection response

for yield traits in blanched asparagus

Ileana Gatti

1

, Fernando López Anido

2

, Vanina Cravero

1

, Pablo Asprelli

2

and Enrique Cointry

2

1

Conicet, Facultad de Ciencias Agrarias, Universidad Nacional de Rosario,

CC 14, S2125ZAA Zavalla, Argentina

2

Cátedra de Genética, Facultad de Ciencias Agrarias,

Universidad Nacional de Rosario, CC 14, S2125ZAA Zavalla, Argentina

Corresponding author: F. López Anido

E-mail: felopez@fcagr.unr.edu.ar

Genet. Mol. Res. 4 (1): 67-73 (2005)

Received October 26, 2004

Accepted January 21, 2005

Published March 14, 2005

ABSTRACT. Despite the continuous breeding that has been conducted

with asparagus (Asparagus officinalis L.) since the beginning of the

last century, there is little information on parameters for predicting direct

and indirect selection response. Yield traits for blanched asparagus pro-

duction were studied along a two-year period in a half-sib family popula-

tion planted in Zavalla, Argentina. Half-sib family mean heritability val-

ues were low for total yield and marketable spear number (0.31 and

0.35), intermediate for marketable yield and total spear number (0.55

and 0.64), and relatively high for spear diameter and spear weight (0.75

and 0.74). An average increase in marketable yield of 15.9% is ex-

pected after each cycle of selection of the top 5% of the families. Total

yield failed to express significant genetic correlations with any of the

yield components; meanwhile marketable yield showed highly signifi-

cant relations with market spear number (0.96) and spear weight (0.89).

Indirect selection response over yield components (CRx) failed to be

advantageous over direct selection (Rx), since the ratio CRx/Rx was

always equal or below unity.

Key words: Asparagus officinalis, Genetic correlations, Genetic gain

Genetics and Molecular Research 4 (1): 67-73 (2005)

FUNPEC-RP www.funpecrp.com.br

I. Gatti et al.

68

Genetics and Molecular Research 4 (1): 67-73 (2005) www.funpecrp.com.br

INTRODUCTION

Asparagus (Asparagus officinalis L.) is a dioecious plant reproduced mainly by seed.

Despite its perennial nature, two years of harvesting have proven to give a reliable evaluation of

yield performance (Bussell et al., 1987). Different approaches in breeding programs with the

aim to increase yield and uniformity have led to the release of different kinds of materials since

the beginning of the last century, i.e., populations improved by mass selection, and single, double

and clonal hybrids with an average yield increase of 75% over unselected materials (Ellison,

1986). Sexual dimorphism, in which staminate plants produce a higher number of thinner spears

than pistillate plants, has been reported for this species (Robbins and Jones, 1926). During the

last 40 years, emphasis has been given to all-male hybrids, the progeny of a supermale (YY)

and a pistillate (XX) plant. However, its convenience has been argued (Corriols, 1984). Results

from the Second International Asparagus Cultivar Trial (Benson, 1999) had ranked all-male

hybrids on average at 14th in comparison with dioecious hybrids at the 9th position. From the

selection point of view, all-male materials are dead-ends. The recombination of selected

andromonoecious plants (sources of supermales) can also lead to an increased expression of

andromonoecism, which restores the advantage of all-male materials (Sneep, 1953).

Breeding programs require stages of selection for parents and progenies. The selection

criteria of elite plants and progenies depend on the variability in the base population and the relative

magnitude of the genetic components determining the phenotypic expression of the traits. Individual

plant selection can be effective only if the variables under selection have high heritability values.

Broad-sense heritability estimates on a one-replication basis were presented for asparagus by Legg

et al. (1968) and López Anido et al. (1997). The estimates of heritability were low for most yield

traits, suggesting the influence of the micro-environmental conditions upon the phenotypic expres-

sion of a single plant. Narrow-sense heritability estimates based on means of asparagus families

suggested the use of recurrent selection in the breeding programs (López Anido et al., 1999).

Traits under selection are often associated with each other in a very complex way.

Correlations between characters have been studied to identify those of easy measurement for

indirect selection for yield. Currence and Richardson (1937), Ellison et al. (1960) and Ellison

(1986) found that spear diameter and spear number were highly positively correlated with yield,

and negatively correlated with each other (Ellison and Scheer, 1959). Cointry et al. (2000) found

that spear number and mean spear weight were the principal components of yield. All these

studies were based on phenotypic correlations. For yield components to be useful as secondary

traits in indirect selection, additive genetic correlations should be considered instead. Heritability

and expected selection response for yield and its components would aid in decision making in

the breeding programs of this species.

MATERIAL AND METHODS

Plant material and evaluation

The open pollinated cultivar Argenteüil was a parent from which many of the modern

varieties and clonal hybrids were developed (Knaflewski, 1996). Populations grown from this

cultivar (cv.) are regarded as representative for the estimation of genetic parameters. In the

summer of 1994 a four-year-old field of cv. Argenteüil was left for open pollination. One hun-

Heritability and expected selection response in asparagus

69

Genetics and Molecular Research 4 (1): 67-73 (2005) www.funpecrp.com.br

dred fruits were collected from 32 plants in the fall. This constituted a base population of 32

half-sib families for our study. One-year-old crowns from these families were planted in August

1996 at the Rosario National University experimental fields located at Zavalla (33° 01’ S, 60°

53’ W), Santa Fe Province, Argentina. A randomized complete block design with four replicates

of 20 plants per family was used in a normal planting grid for white asparagus (2.1 m between

rows and 0.45 m among plants in the row).

During the spring seasons of 1997 and 1998 the following traits of white asparagus

production were evaluated on a plot basis: total spear number, mean spear weight (g), mean

spear diameter (mm), measured at the base of the upper third of the spear, total yield (g),

marketable yield (g) or weight of spears with diameter equal or greater than 12 mm, and number

of marketable spears. All observations were conducted during a forty-day period after the

harvest of the first spear of each plant. Harvests were made three times a week and spear

length was standardized to 15 cm prior to weighing with a digital scale.

Data analysis

Data were subjected to an ANOVA using PROC GLM (SAS Institute, 1996) along the

following linear model for perennial plants proposed by Nyquist (1991), but considering one

location:

P

jkm

= µ + R

j

+ F

k

+ a

(jk)

+ Y

m

+ b

(jm)

+ FY

(km)

+ c

(jkm)

where P

jkm

is the phenotypic plot value of the j

th

replicate of the k

th

family in the m

th

year, µ is the

general mean, R

j

is the effect of the j

th

replicate, F

k

is the effect of the k

th

family, Y

m

is the effect

of the m

th

year, FY

(km)

is the interaction effect of the k

th

family with the m

th

year, and a

(jk)

, b

(jm)

and c

(jkm)

are error terms.

The expected mean squares (MS) were deduced considering all effects randomly (Table

1). When the family by year MS was not significant, the family MS was tested against error (a)

MS. Narrow sense heritability in a half-sib family mean basis was calculated as proposed by

Nyquist (1991):

σ

2

F

h

2

fl

=

σ

2

F

+ σ

2

FY

/y + σ

2

a

/r + σ

2

c

/yr

where σ

2

F

, σ

2

FY

, σ

2

a

and

σ

2

c

are the variance components of family, family by year, error (a) and

error (c) respectively, y is the number of years and r is the number of replicates. σ

2

F

represents

1/4 of additive plus 1/16 additive-by-additive variance.

Confidence intervals for heritability were obtained following Knapp et al. (1985).

The expected selection response (R) was estimated considering the half-sib family

selection method described by Nyquist (1991):

R

x

= i h

2

fl

σ

x

where i is the selection intensity (2.06 for the top 5%), and σ

x

is the square root of the pheno-

I. Gatti et al.

70

Genetics and Molecular Research 4 (1): 67-73 (2005) www.funpecrp.com.br

typic variance among families. Confidence intervals for the selection response were estimated

as proposed by Tai (1989).

The cross product sum matrix for each of the elements of the model was generated by

the PROC MANOVA procedure (SAS Institute, 1996) and the genetic co-variance between

traits solved. Genetic correlations were calculated as proposed by Searle (1961):

r

g

= Cov

g

xy/(σ

g

x . σ

g

y)

where Cov

g

xy is the genetic co-variance between characters x and y and σ

g

x and σ

g

y are the

genetic standard deviations. The standard error (SE) of the genetic correlation was obtained

following Becker (1967). The statistical significance of estimated genetic correlations was as-

sessed by the test proposed by Hébert et al. (1994), in which the inferior limit of the confidence

interval of genetic correlation is:

r

g(min)

= r

g

- t

[0.975, (f-2)]

SE(r

g

)

where t is the Student test value and f the number of families. The r

g(min)

value was compared to

the critical absolute values of the correlation coefficients that were significant at a confidence

level of 95 and 99%. For our sample size these critical values were 0.34 and 0.44, respectively

(Snedecor, 1956). If the inferior limit was greater than these critical values then the genetic correla-

tion was considered to be significantly different from zero for the corresponding confidence level.

The merit of indirect selection upon yield components considering equal intensity of

selection in the primary and secondary traits was tested with the ratio proposed by Falconer and

Mackay (1996):

CRx r

g

h

y

=

Rx h

x

where CRx is the correlated response of character X resulting from selection applied to the

secondary character Y, Rx is the direct response of selecting the desirable primary character X,

h

y

and h

x

are the square roots of the heritability of characters x and y.

Sources of variation d.f. Mean Expected mean square Direct and approximate

square F test

Replicates r-1

Families (F) f-1 M

1

σ

2

c

+ yσ

2

a

+ rσ

2

FY

+ ryσ

2

F

(M

1

+ M

4

)/(M

2

+ M

3

)

Error (a) (r-1)(f-1) M

2

σ

2

c

+ yσ

2

a

Years (y-1)

Error (b) (r-1)(y-1)

Families by years (FY) (f-1)(y-1) M

3

σ

2

c

+ rσ

2

FY

M

3

/M

4

Error (c) (r-1)(f-1)(y-1) M

4

σ

2

c

Table 1. Expected mean squares from the analysis of variance. σ

2

F

, σ

2

FY

, σ

2

a

, σ

2

c

, f, y, and r are the variance

components of family, family by year, error (a), error (c) and number of families, years and replicates, respectively,

in asparagus breeding.

Heritability and expected selection response in asparagus

71

Genetics and Molecular Research 4 (1): 67-73 (2005) www.funpecrp.com.br

RESULTS AND DISCUSSION

The ANOVA layout is presented in Table 2. Differences among families were highly

significant (P < 0.01) for marketable yield, total spear number, spear diameter, and spear weight.

The interaction family by year was significant (P < 0.05) for total yield and highly significant for

total spear number and marketable spear number. The general mean, half-sib family mean

heritability, square root of the phenotypic variance among families, and expected selection re-

sponse of the top 5% were determined (Table 3). The heritability values were low for total yield

and marketable spear number, intermediate for marketable yield and total spear number, and

relatively high for spear diameter and spear weight.

*,** = significant at the 0.05 and 0.01 probability level, respectively; n.s. = not significant; d.f. = degrees of freedom.

Sources of variation d.f. Mean squares

Total Marketable Total spear Marketable Spear Spear

yield yield number spear number diameter weight

Replicates 3 404.59 10006.39 1.0938 40.2987 0.4145 1.6551

Families 31 121.07

n.s.

265.47 ** 10.2865** 8.7181

n.s.

0.1633** 0.5785 **

Error (a) 93 69.04 118.62 2.5558 4.3333 0.0400 0.1469

Years 1 3538.15 13.11 674.99 0.1744 3.4204 7.6016

Error (b) 3 9.21 24.50 0.5607 0.0740 0.0184 0.0527

Years by Families 31 30.44 * 55.08

n.s.

1.9741** 2.4369** 0.0171

n.s.

0.0609

n.s.

Error (c) 93 16.56 41.53 0.9215 1.1172 0.0157 0.0444

Table 2. Means squares of the ANOVA for the different asparagus traits.

Traits General mean Lower h

2

fl

Upper σ

x

Lower Rx Upper

limit limit limit limit

Total yield (kg/ha) 1700 ± 40 -0.03 0.31 0.51 7.26 1.65 4.72 14.14

Marketable yield (kg/ha) 1001 ± 65 0.29 0.55 0.71 14.03 9.84 15.99 36.48

Total spear number (thousand/ha) 116 ± 3.2 0.34 0.64 0.73 8.10 10.83 10.96 22.69

Market spear number (thousand/ha) 46 ± 2.2 0.01 0.35 0.54 11.88 6.04 8.61 25.11

Spear diameter (mm) 11 ± 0.05 0.61 0.75 0.84 4.30 6.36 6.69 13.28

Spear weight (g) 14.9 ± 0.19 0.60 0.74 0.84 6.94 10.02 10.67 21.27

Table 3. General mean, half-sib family mean heritability (h

2

fl

) with its 90% confidence interval, square root of the

phenotypic variance among families (σ

x

), and expected selection response of the top 5% (Rx) with its 90%

confidence interval; these two latest parameters are expressed as percentage of the mean in asparagus breeding,

planted at a density of 11,900 plants/hectare.

Comparing the two yield traits, total yield and marketable yield, the latter proved to be

more appropriate for selection schemes, since the variation among family means was highly

significant with a concomitant higher heritability value than the former. An important fraction of

the variance among families was of an additive nature, thus susceptible to be used to attain

selection response. From the economic point of view it would be also more appropriate to focus

I. Gatti et al.

72

Genetics and Molecular Research 4 (1): 67-73 (2005) www.funpecrp.com.br

exclusively on marketable yield, since even when the rejected fresh marketable production is

sold for processing industry, it demands an increased labor and handling cost per harvested kg

(Van den Broek and Boonen, 1990). Spear diameter and spear weight gave the highest herita-

bility estimates. This is in concordance with the close relation between spear diameter of paren-

tal plants and offsprings found by Currence (1947) in cv. ‘Mary Washington’. High heritabilities

for marketable yield and spear diameter (0.72 and 0.60, respectively) in a full-sib family basis

were also found by López Anido et al. (1999) in a study of cv. ‘Argenteüil’ under green and

blanched production.

Based on our results, after each cycle of selection of the top 5% of the families an

average increase of 15.9% in marketable yield is expected. Due to the higher square root of the

phenotypic variance among families, the selection response was higher for marketable yield

than for the rest of the traits. If we consider a five-year-selection cycle, three years from seed

to end of the second harvest plus two more years for recombination and conformation of the

next generation, the annual increase would be on the order of 3.2%. Analyzing the genealogy of

modern asparagus cultivars (Knaflewski, 1996), we can observe that only three or at most four

cycles of selection and recombination were made. If continuous recurrent family selection

schemes had been maintained since 1919, when the first improved cultivar ‘Mary Washington’

was released by Norton (1919), the average yield of actual asparagus breeding stocks would be

much greater.

The genetic correlations and the merit of indirect selection upon yield components were

calculated (Table 4). Total yield failed to express significant associations with any of the charac-

ters; meanwhile marketable yield showed a highly significant relation with market spear number

and spear weight. When studying the correlations among yield components, we found that total

spear number was significantly negatively associated with spear diameter and spear weight;

while marketable spear number was positively associated with these two components. Thus, if

selection were conducted on marketable yield, contrary to what can be inferred when consider-

ing phenotypic correlations (Ellison and Scheer, 1959), it would be possible to attain increases in

both marketable spear number and spear diameter.

Indirect selection failed to be advantageous over direct selection, since the ratio CRx/

Rx was always equal or below unity (Table 4).

*,** = significant at the 0.05 and 0.01 probability level, respectively.

Total Marketable Total spear Market spear Spear Spear

yield yield number number diameter weight

Total yield 0.68 0.73 0.03 0.39

Marketable yield 0.48 ± 0.30 -0.53 0.77 0.70 1.03

Total spear number 0.47 ± 0.28 -0.49 ± 0.32

Market spear number 0.69 ± 0.25 0.96 ± 0.03** -0.42 ± 0.46

Spear diameter 0.02 ± 0.26 0.57 ± 0.10* -0.61 ± 0.13* 0.62 ± 0.13*

Spear weight 0.25 ± 0.32 0.89 ± 0.07** -0.70 ± 0.18* 0.92 ± 0.13** 0.70 ± 0.20

Table 4. Genetic additive correlations among traits (below diagonal) and merit of indirect selection on yield compo-

nents (total spear number, market spear number, spear diameter, and spear weight) in relation to direct selection

upon total yield and marketable yield (CRx/Rx) (above diagonal) in asparagus breeding.

Heritability and expected selection response in asparagus

73

Genetics and Molecular Research 4 (1): 67-73 (2005) www.funpecrp.com.br

CONCLUSIONS

Substantial additive variation for marketable yield is available to be exploited by family

mean selection schemes. When selecting for this trait, no negative effect on marketable spear

number or spear diameter will be produced. Indirect selection for yield components is not ad-

vantageous over direct selection.

REFERENCES

Becker, W.A. (1967). Manual of Procedures in Quantitative Genetics. Washington State University

Press, Pullman, WA, USA.

Benson, B.L. (1999). Second international asparagus cultivar trial. Acta Hortic. 479: 143-148.

Bussell, W.T., Falloon, P.G. and Nikoloff, A.S. (1987). Evaluation of asparagus yield performance after two

years’ harvesting. N.Z.J. Exp. Agric. 15: 205-208.

Cointry, E., López Anido, F.S., Gatti, I., Cravero, V.P., Firpo, I.T. and García, S.M. (2000). Early selection of

elite plants in asparagus. Bragantia 59: 21-26.

Corriols, L. (1984). Are all-male hybrids attractive? Asparagus Res. Newsl. 2: 16-19.

Currence, T.M. (1947). Progeny tests of asparagus plants. J. Agric. Res. 74: 65-76.

Currence, T.M. and Richardson, A.L. (1937). Asparagus breeding studies. Proc. Am. Soc. Hortic. Sci. 35:

554-557.

Ellison, J.H. (1986). Asparagus breeding. In: Breeding Vegetables Crops (Bassett, M.J., ed.). AVI Publish-

ing Company, Westport, CT, USA, pp. 521-569.

Ellison, J.H. and Scheer, D.F. (1959). Yield related to brush vigor in asparagus. Proc. Am. Soc. Hortic. Sci.

73: 339-344.

Ellison, J.H., Scheer, D.F. and Wagner, J.J. (1960). Asparagus yield as related to plant vigor, earliness and

sex. Proc. Am. Soc. Hortic. Sci. 75: 411-415.

Falconer, D.S. and Mackay, T.F.C. (1996). Introduction to Quantitative Genetics. 4th edn. Longman,

Essex, England.

Hébert, D., Fauré, S. and Olivieri, I. (1994). Genetic, phenotypic, and environmental correlations in blck

medic, Medicago lupulina L., grown in three environments. Theor. Appl. Genet. 88: 604-613.

Knaflewski, M. (1996). Genealogy of asparagus cultivars. Acta Hortic. 415: 87-91.

Knapp, S.J., Stroup, W.W. and Ross, V.M. (1985). Exact confidence intervals for heritability on a progeny

mean basis. Crop Sci. 25: 192-194.

Legg, P.D., Souther, R. and Takatori, F.H. (1968). Estimates of heritability in Asparagus officinalis from

replicated clonal material. Proc. Am. Soc. Hortic. Sci. 92: 410-417.

López Anido, F.S., Cointry, E.L., Picardi, L. and Camadro, E. (1997). Genetic variability of productive and

vegetative characters in Asparagus officinalis L. - Estimates of heritability and genetic correlations.

Braz. J. Genet. 20: 275-281.

López Anido, F.S., Cointry, E.L., Firpo, I.T. and García, S.M. (1999). Heritability of green and white

asparagus yield. In: Eucarpia Leafy Vegetables’ 99 (Lebeda, A. and Kristková, E., eds.). Palacký

University Press, Olomouc, Czech Republic, pp. 277-280.

Norton, J.B. (1919). Washington asparagus: Information and suggestions for growers of new pedigreed

rust-resistance strains. U.S. Dep. Agric. Bur. Plant Ind. Circ. 7.

Nyquist, W.E. (1991). Estimation of heritability and prediction of selection response in plant populations.

Crit. Rev. Plant Sci. 10: 235-322.

Robbins, W.W. and Jones, H.A. (1926). Sex as a factor in growing asparagus. Proc. Am. Soc. Hortic. Sci.

25: 13-16.

SAS Institute (1996). SAS User’s Guide: Statistics. SAS Inst. Inc., Cary, NC, USA.

Searle, S.R. (1961). Phenotypic, genotypic and environmental correlations. Biometrics 17: 474-480.

Snedecor, G.W. (1956). Statistical Methods. 4th edn. Iowa State University Press, Ames, IA, USA.

Sneep, J. (1953). The significance of andromonoecy for the breeding of Asparagus officinalis L. II.

Euphytica 2: 224-228.

Tai, G.C.C. (1989). A new procedure to construct confidence intervals for genotypic variance and ex-

pected response to selection. Genome 32: 307-308.

Van den Broek, J.H. and Boonen, P.H. (1990). Today’s asparagus breeding in the Netherlands. Acta

Hortic. 271: 33-38.