Heritability and expected selection response for yield traits in blanched asparagus

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 production were studied along a two-year period in a half-sib family population planted in Zavalla, Argentina. Half-sib family mean heritability values 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 expected 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 significant 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.

Full text

Heritability and expected selection response in asparagus
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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)
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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-

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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-

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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.

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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

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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.

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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.
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