Asian Journal of Agricultural Sciences 4(4): 236-241, 2012
ISSN: 2041-3890
© Maxwell Scientific Organization, 2012
Submitted: January 14, 2012
Accepted: March 02, 2012
Published: July 15, 2012
Crossability Studies in Cassava in the Coastal Savanna Zone of Ghana
1
W. Nunekpeku, 1,2H.M. Amoatey, 1V. Oduro and 2G.Y.P. Klu
Biotechnology and Nuclear Agricultural Research Institute, Ghana Atomic Energy Commission,
P.O. Box LG 80, Legon, Accra, Ghana
2
School of Nuclear and Allied Sciences, Ghana Atomic Energy Commission, P.O. Box AE 1,
Atomic Energy, Accra, Ghana
1
Abstract: Nine cassava (Manihot esculenta Crantz) accessions were artificially crossed in a pair-wise fashion,
in the coastal savanna agro-ecological zone in Ghana, to determine level of crossability and seed set as a guide
to future breeding programmer. Crossability among the accessions ranged from 0% (in AN x HO-008, LA x
HO-008 and AF x ME crosses) to 88% (in AS x AF crosses). For any pair of parents used, reciprocal crosses
exhibited significant (p#0.05) differences in percent crossability. Seed set and embryo formation among crosses
varied from 0 to 100% and 40 to 100% respectively. High crossability did not necessarily translate into high
seed set or embryo formation as no direct relationship could be established between these variables. Variations
in crossability among the accessions are attributed to inherent genetic differences among the accessions and
the effect of reciprocal crosses.
Keywords: Cassava, crossability, embryo formation, Manihot esculenta Crantz, seed set
was, therefore, to estimate hybridization success and
subsequent seed set as well as embryo development
among nine cassava accessions selected for various traits
such as earliness, tolerance to ACMV, starch content or
tuber yield, thereby proving baseline information for
future breeding programmes.
INTRODUCTION
Cassava (Manihot esculenta Crantz) is an important
component of agricultural systems throughout much of the
tropical regions of the world and constitutes a staple for
about 800 million people worldwide (Hahn and Keyser,
1985). The crop is grown mainly for its starchy, tuberous
roots which contain about 30-40% dry matter of which
85% is made of starch. After rice and maize, cassava is
the most important source of dietary calories in the tropics
(FAO, 2004). Apart from its use as a source of food,
cassava also has a great use as raw material for livestock
feed and several industrial products (Nweke, 2003).
Despite the numerous advantages cassava offers,
increased production in Ghana is constrained by low yield
arising from a combination of factors including incidence
of diseases particularly, African Cassava Mosaic Virus
(ACMV) disease and Cassava Bacterial Blight (CBB)
(Bocks and Wood, 1983; Fargette et al., 1985). A remedy
to the above-mentioned problem is the introduction of
genes for resistance and other agronomically desirable
traits such as high tuber yield and high starch content into
locally adapted cultivars through artificial sexual
hybridization to produce segregating populations from
which recurrent selection can be carried out.
However, in cassava, the outcome of hybridization
varies greatly between parents used, especially if these
parents are distantly related. The purpose of this study
MATERIALS AND METHODS
Two cassava cultivars (Afisiafi (AF) and Ankra
(AN)) and seven accessions (Bamboo Akwetey (BA),
Asare (AS), Larbi (LA), BNARI Selection 1 (BS-1), HO008 (HO), Megyewontem (ME) and Security (SE) with
desirable agronomic characteristics, were selected from
the field gene bank at the Research Farm of the
Biotechnology and Nuclear Agriculture Research Institute
(BNARI) of the Ghana Atomic Energy Commission at
Kwabenya near Accra. The experimental site lies within
the Coastal Savanna agro-ecological zone and receives
less than 1000 mm of annual rainfall. The predominant
soil type found in the area is a well-drained Savanna
Ochrosol, derived from quartzite schist.
For the sake of simplicity the two cultivars are also
referred to as accessions in this work. Crossing blocks
were established by planting nodal cuttings, each bearing
five to eight nodes, prepared from the mid-section of
healthy stems of each accession. Forty cuttings were
planted per accession in the crossing block comprising
Corresponding Author: W. Nunekpeku, Biotechnology and Nuclear Agricultural Research Institute, Ghana Atomic Energy
Commission, P.O. Box LG 80, Legon, Accra, Ghana
236
Asian J. Agric. Sci., 4(4): 236-241, 2012
from the earlier source. Paper bags were removed 3-5
days after pollination to ensure proper development of the
young fruits. After successful fertilization, the number of
fruits formed per cross was counted. Crossability was
computed as follows:
Percent crossability = NFF/NFP×100
where, NFF: number of fruits formed; NFP: number of
flowers pollinated
Variations among crosses for number of seeds formed
per fruit as well as percent embryo formation were
analysed using Chi-square in Minitab Statistical Software
(version 15.0).
RESULTS
Percent crossability among parental accessions:
Crossability among the cassava accessions is shown in
Table 1(a-i). Percent crossability between ME as female
and other accessions as male parents was very low but
varied significantly (p#0.05) among the various crosses.
Seed set
Embryo formation
% seed set/embriyoformation
100
80
60
40
80
60
40
20
BA x AS
BA x LA
BA x SE
BA x ANF
BA x BS-1
BA x AN
BA x ME
Crosses
(b) BA as female parent
Seed set
Embryo formation
% seed set/embriyoformation
100
80
60
40
20
120
Seed set
Embryo formation
100
80
60
40
20
(c) AN as female parent
(d) BS -1 as female parent
Fig. 1: Variation in seed set and embryo formation in crosses using ME, BA, AN and BS -1as female parents
237
BS-1 x HO-800
Crosses
Crosses
BS-1 x AS
BS-1 x LA
BS-1 x SE
BS-1 x BS-1
AN x AS
AN x LA
AN x SE
AN x AF
AN x BS-1
AN x AN
BS-1 x AF
0
0
BS-1 x AN
% seed set/embriyoformation
100
Crosses
(a) ME as female parent
120
Seed set
Embryo formation
0
ME x LA
ME x BS-1
0
ME x SE
20
120
BS-1 x ME
120
ME x FA
% seed set/embriyoformation
five rows of eight hills each, with inter-row and the intrarow spacing of 1.5 and 1 m, respectively. Individual
blocks were separated by a distance of 2 m.
At flowering, ten healthy plants from each accession
were randomly selected and used as parents for the
crosses between September and December, 2008. Each
accession was crossed with all others in the entry.
Reciprocal crosses (Table 1a) were also made. For each
cross or the reciprocal, a minimum of 100 normal female
flowers were pollinated using pollen of normal male
flowers from an appropriate source. Emasculation of
female parents was done prior to an thesis. All crosses
were made in the mornings between 08:00 and 12:00 h
GMT when flowers open for natural pollination. Using a
pair of scissors the ready-to-open male flowers were
excised from the inflorescence on the paternal parents and
used immediately for the pollination.
After pollination, each female flower was bagged to
prevent contamination from external pollen. Between
successive crosses involving different parents, tools used
for crossing as well as hands were wiped with cotton wool
soaked in 70% ethanol to avoid contamination with pollen
Asian J. Agric. Sci., 4(4): 236-241, 2012
Table 1: Estimates of hybridization success in crosses using ME, BA, AN, BS-1, AF, SE, LA, AS and HO-08 as female parents
Male parent
ME
BA
AN
BS-1
AF
SE
LA
AS
(a): ME as female parent
Number of flowers pollinated
*
**
*
143
106
101
100
*
Number of successful crosses
2
10
10
5
% Crossability
1.4
9.4
9.9
5.0
(b): BA as female parent
Number of flowers pollinated
100
**
100
100
219
100
100
100
Number of successful crosses
7
34
82
104
55
65
72
% Crossability
7.0
34.0
82.0
47.5
55.0
65.0
72.0
(c): AN as female parent
Number of flowers pollinated
*
**
100
100
100
100
100
100
Number of successful crosses
60
19
29
29
6
23
% Crossability
60.0
19.0
29.0
29.0
6.0
23.0
(d): BS-1 as female parent
Number of flowers pollinated
100
**
100
100
191
100
100
100
Number of successful crosses
9
43
74
138
51
74
71
% Crossability
9.0
43.0
74.0
72.3
51.0
74.0
71.0
(e): AF as female parent
Number of flowers pollinated
133
**
100
246
100
117
100
100
Number of successful crosses
0
24
104
65
51
64
44
% Crossability
0.0
24.0
42.3
65.0
43.6
64.0
44.0
(f): SE as female parent
Number of flowers pollinated
126
**
100
100
171
100
121
115
Number of successful crosses
13
57
67
109
68
93
59
% Crossability
10.3
57.0
67.0
63.7
68.0
76.9
51.3
(g): LA as female parent
Number of flowers pollinated
121
**
100
100
100
100
100
100
Number of successful crosses
5
47
75
40
27
55
47
% Crossability
4.13
47.0
75.0
40.0
27.0
55.0
47.0
(h): AS as female parent
Number of flowers pollinated
*
**
*
*
100
*
*
*
Number of successful crosses
88
% Crossability
88.0
(i): HO-008 as female parent
Number of flowers pollinated
*
**
*
*
*
100
*
*
Number of successful crosses
50
% Crossability
50.0
*: Crosses were not carried out; **: Accession did not produce pollen
It ranged from 1.4% in ME x BS-1 to 9.9% in ME x SE.
However, ME x AN, ME x AS and ME x HO-008 crosses
were not carried out due to non- synchronization of
flowering. In addition, BA could not be used as a male
parent because it produced dysfunctional anthers which
lacked pollen.
For crosses involving BA female and other
accessions as males, very high percent crossabilities were
obtained with BA x BS-1 crosses recording the highest
success rate of 82.0%. In contrast, BA x ME produced
only 7.0% success which was the lowest. Crossability
varied significantly (p#0.05) among the crosses
(Table 1b). BA x HO-008 crosses were not carried out
due to constraints with synchronization of flowering.
Variation in crossability among crosses involving AN
female and other parents as males are shown in
Table 1c. Differences observed in crossability among
these crosses were statistically significant (p#0.05) and
ranged from 0% in AN x HO-008 crosses to 60.0% in AN
x AN selfs. AN x ME crosses were not carried out due to
problems encountered with synchronization of flowering.
When BS-1 was used as female parent in crosses with
ME, AN, AF, SE, LA, AS and HO-008, as males, BS-1 x
BS-1 selfs and BS-1 x LA crosses produced the highest
HO-008
*
*
100
0
0.0
110
5
4.5
*
100
3
3.0
100
0
0.0
*
*
crossability of 74.0% each, while the lowest crossability
of 4.5% was observed in BS-1 x HO-008 crosses.
Variation in crossability among these crosses was
statistically significant (p#0.05) (Table 1d). In AF
(female) crosses with ME, BS-1, SE, LA, AN and AS (all
as male parents (Table 1e) crossability ranged from 0%
(in AF x ME crosses) to 65.0% (in AF x AF selfs),
varying significantly among different crosses (p#0.05).
AF x ME and AF x HO-008 crosses were not carried out
due to problems with synchronization of flowering.
When SE was used as a female parent and was
crossed with ME, AN, BS-1, AF, LA, AS and HO-008 as
male parents, significant variation in crossability (p#0.05)
was observed among the crosses (Table 1f). SE x LA
crosses recorded the highest crossability of 76.9% while
SE x HO-008 crosses recorded the least (3.0%). Similarly,
in LA (female) crosses with ME, AN, BS-1, AF, SE, AS
and HO-008 (as male parents), crossability ranged from
0% (in LA x HO-008) to 75.0% (in LA x BS-1).
Variations in crossability among these crosses (Table 1g)
were statistically significant (p#0.05).
For crosses involving AS and HO-08 (females) and
the other accessions as males, only AS x AF and HO-008
x SE crosses were carried out. Crossabilities in these
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Asian J. Agric. Sci., 4(4): 236-241, 2012
Seed set
Embryo formation
% seed set/embriyoformation
120
100
80
60
40
20
Seed set
Embryo formation
100
80
60
40
20
SE x AS
(b) SE as female parent
120
Seed set
Embryo formation
100
80
60
40
LA x AS
LA x SE
LA x AF
LA x AN
LA x LA
0
LA x BS-1
20
LA x ME
% seed set/embriyoformation
(a) AF as female parent
SE x LA
SE x AF
Crosses
SE x HO-800
Crosses
SE x BBS-1
SE x AN
SE x M E
SE x SE
AF x AS
AF x AN
AF x LA
AF x SE
0
AF x BS-1
0
AF x AF
% seed set/embriyoformation
120
Crosses
(c) LA as female parent
Fig. 2: Variation in seed set and embryo formation in crosses using AF, SE and LA as female parents
crosses were 88.0% (in AS x AF) and 50.0% (in HO-008
x SE) (Tables 1 (h) and Fig. 1 (i)).
various crosses (Fig. 1b). BA x AN crosses produced the
highest proportion of embryos (93.8%) while BA x ME
yielded the least of 80.0%. Again there were no
significant differences (p#0.05) among the crosses.
In crosses which utilised AN as female parent against
the other accessions as males, AN x BS-1 crosses
recorded the highest seed set of 100% while AN x AF
crosses yielded the least (71.43%) Differences among the
crosses in terms of seed set (Fig. 1c) as well as embryo
formation were significant (p#0.05). AN x AN
autohybrids and AN x LA crosses recorded the highest
percent embryo formation (100%) while AN x AS crosses
recorded the least (66.7%).
Figure 1d shows seed set and embryo formation in
crosses involving BS-1 female and other parents as males.
BS-1 x AS crosses had the highest seed set (71.8%) while
BS-1 x AF recorded the least of 35.4%. Percent embryo
formation was highest for BS-1 x AF crosses (94.1%)
while BS-1 x ME recorded the lowest (40.0%). Variations
in seed set as well as embryo formation among the crosses
were significant (p#0.05)
Seed set and embryo formation among crosses:
Percent seed set and embryo formation in crosses
using ME, BA, AN and BS-1 as female parents:
Crosses between ME as female parent and BS-1, AF, SE
and LA as males did not produce significant differences
(p#0.05) in seed set (Fig. 1a). However, ME x AF and
ME x LA crosses yielded the highest percent seed set of
88.9% each, while ME x SE crosses recorded the least
seed set of 77.8%. No significant differences (p#0.05)
were observed. While ME x SE crosses recorded the
highest percentage of embryos formed (85.7%), ME x BS1 crosses yielded the least (60.0%). Differences among
crosses were not significant.
Among crosses involving BA as female and AN, ME,
SE, AS, AF, BS-1 and LA as males percent seed set
ranged from 66.7 to 88.9%. No significant differences
(p#0.05) were found among values obtained for the
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Asian J. Agric. Sci., 4(4): 236-241, 2012
Since no major incompatibility system is known to
completely prevent crossing among species of the
Manihot genus (Asiedu et al., 1991), the variations
observed in percent crossability among the accessions
may be attributed to inherent genetic differences among
the accessions utilized for the crosses and the effect of
reciprocal crosses involving different female parents.
Indeed, in cassava, fertility rates of cultivars after
pollination have been found to be very low and the
genotype of the female appears to play a more important
role in determining success of the crosses than the pollen
source (Hershey, 1981). However, Ma et al. (1996)
suggested that in such manual crosses environmental
factors cannot be completely ignored.
Seed set and embryo formation among the crosses
varied from 0 to 100% and 40.0 to 100% respectively.
High crossability did not necessarily translate into high
seed-set and therefore high embryo formation. For
instance, with the exception of LA x AN crosses which
had the highest seed-set of 87.88% and at the same time
recorded the highest embryo formation of 100%, all the
other crosses had variable crossabilities, seed-set and
embryo formation. These results are comparable to those
obtained by other workers (Hershey, 1981; Ogburia and
Okele, 2001; Fukuda et al., 2002).
Percent seed set and embryo formation in crosses
using AF, SE, LA, AS and HO-008 as female parents:
In the case of AF (female) crosses with other accessions
as male parents, AF x AN crosses yielded the highest seed
set of 95.8% as against the lowest of 66.7% recorded in
AF x BS-1 crosses. Variations observed in seed set among
these crosses (Fig. 2a) were statistically significant
(p#0.05). However, no significant differences (p#0.05)
were noticed in terms of variation in embryo formation
among the crosses. AF x BS-1 crosses recorded the
highest percent embryo formation (100%), while the
lowest percent embryo formation (89.7%) was observed
in AF x AS crosses.
When SE (female) was crossed with other accessions
as male parents, significant differences (p#0.05) were
observed in terms of seed set (Fig. 2b), which ranged
from 0 to 75.4% in SE x ME and SE x AF crosses
respectively. With regard to embryo formation SE x BS-1
crosses produced the highest percentage of embryos
formed (85.9%) while SE x AS recorded the least
(52.6%). Differences in embryo formation among the
crosses were significant (p#0.05).
For crosses involving LA (female) and other
accessions as male parents LA X AN recorded the highest
seed set (87.9%). LA x ME crosses on the other hand
produced the least seed set (53.3%). Variation observed
among the crosses (Fig. 2c) was statistically significant
(p#0.05). With regard to embryo formation in the same
crosses LA x AN crosses again recorded the highest rate
of 100% while LA x AS produced 60.9% which was the
lowest. Variations observed in percent embryo formation
among the crosses were statistically significant (p#0.05).
While 93.3 and 95.2% were recorded in ASxAF
crosses for seed set and embryo formation respectively,
HO-008 x SE crosses produced a seed set of 73.3 and
90.9% embryo formation.
CONCLUSION
Hybridization among the nine accessions in a pairwise fashion produced crossabilities which ranged from
0 to 88.0%. Seed set ranged from 0 to 100% whereas
embryo formation rangedfrom 40.0 to 100%. For any pair
of parents used, reciprocal crosses exhibited significant
(p#0.05) differences with respect to crossability. The
accession Bamboo Akwetey (BA) did not produce any
pollen. Hence, it could only be used as maternal parent in
hybridization with the other accessions. The other
accessions were cross compatible, both as male or female
parents.
DISCUSSION
Hybridization, seed set and embryo formation among
accessions: Pair-wise crosses among the accessions
showed that percent crossability ranged from 0% (in AN
x HO-008, LA x HO-008 and AF x ME crosses) to 88.0%
(in AS x AF crosses). Reciprocal crosses yielded variable
results, without a clear trend. For example, the cross ME
x AF gave percent crossability of 9.4% while the
reciprocal cross was not successful. On the other hand, the
cross ME x SE gave percent crossability of 9.9% while
the reciprocal cross gave 10.3%. By contrast percent
crossability for SE x HO-008 was 3.0% compared to
50.0% for the reciprocal cross. In addition, the accession
BA produced no pollen and therefore could only serve as
female parent but not as male parent against any of the
other parents.
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