African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 9, Num. 3, 2001, pp. 463-470
African Crop Science Journal, Vol. 9. No. 3, pp. 463-470
SEGREGATION FOR SEED WEIGHT, POD LENGTH AND DAYS TO FLOWERING
FOLLOWING A COWPEA CROSS
B. EWA UBI,* H. MIGNOUNA1 and G. OBIGBESAN2
International Institute of Tropical Agriculture (IITA), PMB 520, Ibadan,
1Department of Crop Science, University of Calabar, Calabar, Nigeria
2Department of Agronomy, University of Ibadan, Ibadan, Nigeria
Received 11 October, 1999
Accepted 31 March, 2001
Code Number: cs01065
Field studies were conducted to evaluate the segregation of the F3
(early generation) and F6 (late
generation) families for seed weight, pod length and days to flowering among
cowpea inter-sub-specific crosses. A wide range of segregants were provided
in this cross and families were highly significantly different in the three
agronomic traits studied. The continuous distributions observed for these traits
studied in both generations confirms the quantitative nature of inheritance
for these traits. Broad sense heritability estimates ranged from 47.8 to 91.1%.
Estimates of genetic advance ranged from high to low and were consistent in
both generations for all the traits. The F3 and F6
generations were not significantly different in all the three agronomic traits.
Intergeneration correlations ranging from 0.35 to 0.49 also revealed strong
associations between traits measured in the two generations. A no significant
drop was observed between F3 mean and the corresponding
F6 mean. This suggests the existence of a good measure of additive
and possibly of additive x additive components of variance (which alone are
fixable through subsequent inbreeding) although some amount of dominance and
duplicate epistasis (which are non-fixable) may also be operative. The results
of this study indicate that selection in early generations for superior types
Key Words: Agronomic traits, early generation, late generation, Vigna
Des études en champs ont été conduites pour évaluer
la ségrégation des familles en F3
(jeune génération) et F6
(génération avancée) quant au poids des graines, la longueur
de la gousse et le nombre de jours à la floraison chez des sous-espèces
de niébé croisées. Une large gamme de lignées ségrégantes
était introduite dans ces croisements et les familles utilisées
étaient sensiblement différentes pour les trois caractères
agronomiques étudiés. Les distributions continues observées
pour ces caractères étudiés au sein des deux générations
confirment la nature quantitative du patrimoine héréditaire pour
ces caractères. Les estimations générales d'héritabilité
se rangeaient entre 47,8 et 91,1%. Les estimations de transferts génétiques
allaient des plus basses aux plus élevées et elles étaient
constantes au sein des deux générations pour tous les caractères
étudiés. Les corrélations entre générations
s'est rangées entre 0.35 et 0.49 et montrées aussi les fortes
associations entre les caractères mesurés pour les deux générations.
Un déclin non significatif était observé entre la moyenne
de F3 et son correspondant de F6. Ceci suggère
l'existence d'une bonne mesure additive et peut-être de l'
additive x additive composante de la variance (fixable pendant le croisement
ultérieur) même si certains traits de dominance et de double épistasies
(non fixables) pourraient être opérationnels. Les résultats
de cette étude montre qu'une sélection parmi les jeunes
générations de races supérieures est faisable.
Mots Clés: Caractéristiques agronomiques, jeune génération,
Cowpea (Vigna unguiculata L.Walp) is a highly self-pollinated crop
and the procedures in use for cultivar development have followed the conventional
methods of individual plant selection in naturally occuring or hybridisation-induced
genetic variability, following the pedigree method of breeding (Allard, 1960).
An important assumption underlying early generation selection generally adopted
for self-pollinated species is that selection for a character in the early
generation (F2 or F3) would be as effective as when
practised in the later generations assuming high heritability (Bartley and
Weber, 1952; Allard, 1960). The use of early generation testing to identify
superior crosses and eliminate large amounts of material from a cultivar development
programme may increase breeding efficiency (Knauft and Wynne, 1995). An early
and accurate appraisal of segregates has been of vital interest to most breeders
of self-pollinated species including soybean, barley, peanut, oats and wheat;
and early generation testing studied extensively in some allogamous species,
has suggested the possibility of a similar application in autogamous species
(Mahmud and Kramer, 1951; Knauft and Wynne, 1995). More precisely, Mahmud
and Kramer (1951) have indicated that the two closely related problems are
first, a selection of those crosses which are most likely to give the highest
proportion of superior segregates, and second, an evaluation of the potentialities
of the segregates from those crosses. Selection of crosses has been attempted
on the basis of their performance in tests of bulk hybrid populations. It
has been shown that replicated tests of segregating populations in the F2
or F3 generation would provide the average yield performance of
the different crosses (Fehr, 1987).
Seed weight, pod length (Nakawuka and Adipala, 1998) and days to flowering
are important agronomic traits in cowpea. Insight into the genetic segregation
of these traits could be of benefit in the choice of methods to enhance selection
efficiency. Several published quantitative studies on the genetics of these
traits have been reported (Fery, 1985; Fery and Singh, 1997). Pedigree selection
is commonly used in cowpea breeding programmes. However, comparisons among
the various methods for selection in early generations and advancement to
near-homozygous lines have not been made in cowpea.
The present study was conducted to determine the relationship of F3
lines to individual F6 lines derived by single-seed descent from
the F3 lines of an inter-sub-specific cowpea cross. An attempt
was made to avoid a possible interaction of seasons and generations by testing
the F3 and F6 lines in the same year.
MATERIALS AND METHODS
A cross was made between an improved cowpea line, IT84s-2246-4 (Vigna
unguiculata ssp. unguiculata) and a wild relative, TVNu110-3A (Vigna
unguiculata ssp. dekindtiana var. Pubescens), and advanced
to the F2 generation. A plant from each F3 family was
advanced to F5 by single-seed descent (SSD) method to produce the
F6 seeds. The two parental genotypes and their F3 and
F6 families were established in a field trial that was conducted
to evaluate the performance of the F3 and F6 generations
at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria
(7o30/ N; 3o54/ E) in late August,
1996. The IITA, is located at about 243 m above sea level with a mean annual
temperature of 22.03°C (min.) and 30.33°C (max), mean
seasonal rainfall of 1600 mm, and relative humidity of 88% during the late
growing season (late August to late November) (IITA, unpublished).
The F3 and F6 plants and parents were grown in three
replicates in a randomised complete block (RCB) design. Single-row plots were
used for each entry per replicate. Each entry consisted of 16 plants maintained
at 1 plant/hill (after thinning) at a spacing of 0.5 m within and 1.0 m between
the rows. A total of 100 F3 families derived from the 100 F2
plants, and 94 F6 families derived by SSD were used for this study.
Necessary agronomic practices were carried out to ensure optimum crop growth.
Data on three important agronomic traits were recorded for each F3,
F6 and the parental lines, as follows:
(i) Days to 50 percent flowering: the number of days from planting to when
50 percent of the plants had flowered.
(ii) Pod length: the length of fully mature pods, indicated by the change
in pod colour, measured to the nearest centimetre. The average length of 40
pods was used as the score for each family in each replication.
(iii) 100-seed weight: weight in grams of 100 seeds after oven drying to a
uniform moisture level of ca.13.5%. The average weight determined from four
replicate samples was used as the score for each family in each replication.
Lines, generations, and replications were considered random effects in the
combined analysis of variance over generations. A variance components (VARCOMP)
procedure of the statistical analysis system (SAS) was used to calculate the
maximum likelihood (ML) estimates of broad-sense heritability (hb)
for the agronomic traits. Environmental variance was estimated as the mean
square for the replicate x family interaction (Anderson et al., 1991).
Genetic advance (GA) was calculated according to the formula used by Johnson
et al. (1955):
GA (%) = ____ X 100
where k is the selection differential. It was given a value of 2.06, which
is its expectation at 5% selection intensity from a normally distributed population,
σ2g is the genotypic variance, x is the family mean, and σp
is the phenotypic standard deviation.
The phenotypic coefficient of variation (PCV) and genotypic coefficient of
variation (GCV) were calculated according to the formula used by Empig et
al. (1970) as follows:
v σ2 p v σ2 g
PCV = ____ x 100 and GCV = ____ x 100 x x
where σ2p and σ2g are phenotypic and genotypic
variances, respectively, and x is the family mean.
High variability was observed in the parents and segregating populations with
respect to days to flowering, pod length and 100-seed weight. The mean squares
from the combined analysis of variance over generations for these agronomic
traits are presented in Table 1. Families
were highly significantly (P<0.01) different for all three characters studied.
The generations x families interaction, which is primarily a measure of environmental
variation, was highly significant (P<0.01). No significant difference was
observed between the F3 and F6 generations among all
the three characters.
The frequency distributions for each of the three characters studied in
this cross showed continuous variation, suggesting polygenic inheritance for
all the traits. Histograms showing seed weight, pod length and days to flowering
in the F3 and F6 generations are shown in Figures
1, 2 and 3,
respectively. The population means and the mid-parental values are also shown
for each trait in both generations. Days to flowering ranged from 46 to 63
and 44 to 65 days; pod length ranged from 6.3 to 10.0 and 5.5 to 12.0 cm;
and seed weight ranged from 1.9 to 7.9 and 2.4 to 7.4 g/100 seeds in the F3
and F6 generations, respectively. The F6 progenies did
not differ in their mean values from the F3 lines in all the traits
studied. None of the segregants had seeds as heavy as the heavier seed-producing
parent (IT84S-2246-4) in all generations (Fig.
1). Individual segregants exhibiting early flowering equal to that of
the early flowering parent (IT84S-2246-4) were observed in all generations;
but segregants with late flowering equal to that of the late flowering parent
(TVNu110-3A) were not observed (Fig. 2).
The population means were closer to the early flowering parent (IT84S-2246-4)
in all generations (Fig. 2), suggesting
partial dominance of early flowering. No individual segregant had pods as
long as the longer podded-parent (IT84S-2246-4). However, individual segregants
that were shorter than the short podded-parent (TVNu110-3A) were observed
(Fig. 3). In both generations, the population
means were closer to the length of the short podded-parent (TVNu110-3A) (Fig.
3), suggesting partial dominance of short pod. The values in all generations
were nearer those of the parent with lower seed weight, suggesting partial
dominance of small seed weight.
The means, standard errors and broad sense heritability estimates for the
characters studied in this cross are shown in Table
2. Broad sense heritability estimates were moderate to high depending
on the trait with values ranging from 47.8%, 74.9% and 91.1% for days to flowering,
pod length and seed weight, respectively.
Estimates of phenotypic coefficient of variation (PCV), genotypic coefficient
of variation (GCV) and genetic advance (GA) expressed in percent for traits
in the F3 and F6 generations are shown in Table
3. The PCV and GCV values ranged from 10.6 to 46.1% and 5.4 to 25.5%,
respectively, for characters in the F3 generation and were close
to the estimated values in the F6 generation. The estimated GA
(%) was consistently high for 100-seed weight in both generations; and low
for days to flowering. A fairly high GA was observed for pod length. Associations
between performance of F3 and F6 progenies were determined
by intergeneration correlation coefficients and are presented in Table
4. All coefficients were significant at 1% probability level.
The cross used in this study provided a large range of genotypes and each
differed in extent of variation for all attributes. Families were highly
significantly different for all the traits studied. No significant differences
are observed between the F3 and F6 generations in
all the three characters studied. Inter-generation correlation coefficients
for all the traits studied in the F3 and F6 generations
were highly significant. Thus, there appears to be no important genetic
reason why early generations should not provide good estimates of average
yield potentialities of segregates from segregating populations when genetic
shifts and interactions with environmental factors are controlled.
Analysis of the traits studied in the F3 and F6
generations of this cross showed continuous distributions, indicative of
quantitative patterns of inheritance. The mean values of both generations
for days to flowering were closer to the value of the early flowering parent,
suggesting partial dominance of the alleles for early flowering. Adu-Dapaah
et al. (1988) have observed a tendency for dominance of early flowering
in cowpea. The mean values for pod length and seed weight in the four generations
were closer to the short-podded and small-seeded parents, respectively,
suggesting partial dominance of the alleles for these traits. This observation
on seed-weight is consistent with previous reports in several plant species
including cowpea (Drabo et al., 1984; Fatokun et al., 1992;
Maughan et al., 1996). Nienhuis et al. (1987) reported a similarly
skewed distribution for seed-weight that favoured the alleles of the wild
parent in an interspecific cross of tomato.
Effective selection for desired traits when conditioned by quantitative
inheritance is usually difficult in segregating populations and real progress
from selection may be difficult to achieve. Hence, it may become desirable
to determine genotypic variance in a segregating population in order to
know the magnitude of the heritable fraction. A knowledge of such genetic
parameters as heritability, phenotypic coefficient of variability (PCV),
genotypic coefficient of variability (GCV), genetic advance (GA) and association
among quantitative traits should help improve the efficiency of selection.
The extent of variability in these agronomically important traits has
been well demonstrated by phenotypic and genotypic coefficients of variation.
The highest GCV was observed for 100-seed weight in both generations while
the lowest was observed for days to 50% flowering. This can be explained
by the fact that segregating progeny were scored for days to 50% flowering
on a progeny-row basis. Consequently, late flowering segregates obscured
the early segregates in the row with a resultant bias toward lateness. This
fact may also account for the low heritability estimates obtained for this
Permanent gain from selection depends on the degree of relationship between
genotype and phenotype. The correspondence between genotype and phenotype
of a trait is expressed by heritability of the trait, which largely reflects
the extent to which genetic segregation is expected in later generations
of a cross for the trait in question (Mahmud and Kramer, 1951). Lush, 1948
(cited in Weber and Moorthy, 1952) defined heritability in two ways, broad
sense heritability and narrow sense heritability. In the broad sense, heritability
refers to the ratio of heritable variance to total variance. In the narrow
sense, heritability is defined as the ratio of additive genetic variance
to total variance. Broad sense heritability estimates in this study ranged
from moderate to high. The heritability values obtained in this cross are
within the values reported from several recently published studies in cowpea
(Fery, 1985; Fery & Singh, 1997; Nakawuka & Adipala, 1998) and these
estimates are encouraging for selection of families for the traits studied.
Heritability indicates the effectiveness with which selection of genotypes
can be based on phenotypic performance. If heritability were 100% (i.e.,
σ2g=σ2p) then phenotypic performance would
be a perfect indication of genotypic value; but even in such a hypothetical
situation, the heritability value in itself provides no indication of the
amount of genetic progress that would result from selecting the best individuals.
According to Johnson et al. (1955) heritability estimates along with
estimates of genetic advance (GA) were more useful than heritability alone
in predicting the resultant effect for the selection of the best individuals
from segregating populations. Very high heritability and GA (%) observed
for 100-seed weight indicates that additive genetic variance is important
for this trait and improvement can be achieved for this trait by phenotypic
selection. The estimated GA (%) was consistently high in the F3
and F6 generations, indicating that the superiority of lines
selected on the basis of F3 data may be retained in the later
generations. Moderate heritability coupled with low GA (%) observed for
days to 50% flowering indicates that little progress can be made in the
improvement of this trait by phenotypic selection. Though high heritability
was observed for pod length, estimates of genetic advance were only fairly
high. No significant drop was observed between F3 mean and the
corresponding F6 mean which may suggest the existence of a good
measure of additive and possibly of additive x additive components of variance
(which alone are fixable through subsequent inbreeding) although some amount
of dominance and duplicate epistasis (which are non-fixable) may also be
operative. The results of this study indicate that selection in early generations
in cowpeas for superior types is feasible.
We thank Dr.C.A.Fatokun, and Prof. M.E. Aken'Ova and Dr. I .Fawole
for their immense contributions.
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