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Inheritance of seed size and number per pod in cowpeas (Vigna unguiculata L. Walp.)
Field Crops Research, 11 (1985) 335--344
335
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
INHERITANCE
OF SEED SIZE AND NUMBER PER POD IN COWPEAS
(VIGNA UNGUICULATA L . W A L P . )
I. D R A B O
l ,T.A.O. LADEINDE
2 , R. REDDEN
3,4 a n d J.B. S M I T H S O N
3's
'National Cowpea Improvement Programme SAFGRAD-IITA, Ouagadougou (Upper
Volta) 2Department of Agricultural Biology, University of Ibadan, Oyo Road, Ibadan (Nigeria) 3International Institute of Tropical Agriculture (IITA), Oyo Road, PMB 5320, Ibadan (Nigeria) * Present address: Department of Primary Industries, Hermitage Research Station, Warwick, Qld. 4370 (Australia) Present address: 5 Byemoor Avenue, Great Ayton, County Cleveland, T59 6JP (Great Britain) (Accepted 11 January 1985)
ABSTRACT
Drabo, I., Ladeinde, T.A.O., Redden, R. and Smithson, J.B., 1985. Inheritance of seed size and number per pod in cowpeas (Vigna unguiculata L. Walp.). Field Crops Res., 11: 335--344. The inheritance of seed size and seed number per pod in cowpeas was examined in reciprocal crosses of two large-seeded lines with two small-seeded lines and four lines of intermediate seed size. For seed size, log transformations removed significant correlations among the means and variances. Additive-dominance models failed to explain differences among generation means but genetic mechanisms varied among crosses. F o r seed size, additive effects were most pronounced but epistatic effects were also involved. For seeds per pod, additive, dominance and epistatic effects were most important and of similar magnitude. Many seeds tended to be dominant over few seeds and, in two crosses, there was clear overdominance of many seeds per pod. Broad sense heritabilities were generally large but varied among crosses and correlations between the two characters were negative but small and usually not significantly different from zero. The implications of these results in breeding cowpeas are discussed.
INTRODUCTION C o w p e a (Vigna unguiculata L. W a l p . ) is a n i m p o r t a n t s o u r c e o f p l a n t p r o t e i n s i n W e s t A f r i c a , w h e r e i t is b y f a r t h e m a i n f o o d l e g u m e i n t e r m s of area and production. Traditional cultivars are well adapted to the low input situations in which they are generally cultivated, but have poor yield potential and are susceptible to the major diseases and pests. Large seeds a r e p r e f e r r e d b y t h e c o n s u m e r a n d s e e d n u m b e r p e r p o d is a n i m p o r t a n t c o m p o n e n t o f s e e d y i e l d , so i m p r o v e m e n t i n b o t h c h a r a c t e r s is a p r i n c i p a l
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© 1985 Elsevier Science Publishers B.V.
336 objective of cowpea breeding programmes (for example, Redden et al., 1984). Genetic studies of seed size and seeds per pod in cowpeas have been few and the results conflicting. Sene (1968), Areetey and Laing {1973) and Drabo et al. (1985) concluded that genetic variation in seed size was mainly additive. Areetey and Laing (1973), Leleji {1976) and Drabo et al. {1985) observed partial dominance of small seeds over large seeds. However, Brittingham {1950) and Agble {1972) f o u n d considerable heterosis to be involved. The inheritance of seed size appears to be controlled by six to ten pairs of genes and broad and narrow sense heritabilities are large (Sene, 1968; Areetey and Lalng, 1973; Leleji, 1976; Drabo et al., 1985). Areetey and Laing {1973) reported that m a n y seeds per pod was d o m i n a n t over few seeds but Leleji {1976) found the reverse expression. The two characters are considered to be negatively correlated (Rachie and Roberts, 1974). In this paper, we report studies of the inheritance of seed size and seed number in cowpea. This information is directly relevant to the breeding programme at the International Institute of Tropical Agriculture (IITA) at Ibadan in Nigeria, and involves both germplasm and breeding lines with small, medium, and large seed sizes. MATERIALS AND METHODS Two large-seeded cowpea lines (TVu 6203 and TVx 881-3G) were crossed reciprocally with two lines with small seeds (TVu 1977-0D and TVx 1201E) and four of intermediate seed size (TVu 4557, TVx 331J, TVx 289-4G and TVx 1193-10F) {Table 1). The TVx lines are derived from crosses among germplasm lines. The study therefore involved materials of very diverse origins and character. Reciprocal crosses were made in a glasshouse in 1978. The F1 's were selfed to obtain F2 seeds and simultaneously backcrossed to both parents. The eight parents, 24 F1 and F2 and 48 backcrosses generations were sown in the field on the research farm of IITA in May 1979. For ease of reference the eight parents will be designated P1 to P8, as shown in Table 1; BC1 will denote the populations derived from backcrosses to the large or manyseeded parents; and BC2 the backcrosses to the smaller or fewer-seeded parents. The experimental design was a randomised block with five replicates. The plot sizes were: ten plants each for the parents and F~'s; 20 to 30 plants for the backcrosses; and 30 to 40 plants for the F2 populations. The seeds were sown in rows 1 m apart, with 30 cm between positions within rows. The crop was hand-weeded and protected against insects and single plants were harvested separately at maturity. Estimates of seed size and seeds per pod were derived from the total undamaged seeds from 10 to 20 pods of each plant.
337 TABLE 1 Origins and characteristics of parents used in crosses to study the inheritance of seed size and number per pod in cowpeas Parents
Code number
Origin
Parentage
Weight Seed size of 100 category seeds (g)
TVu TVx TVu TVx TVx TVx TVu TVx
P1 P2 P3 P4 P5 P6 P7 P8
Zimbabwe Nigeria Nigeria Nigeria Nigeria Nigeria Senegal Nigeria
Germplasm accession TVu 2 X TVu 95 Germplasm accession TVu 37 X TVu 530 TVx 33-1B X TVx 23-2B TVu 1190 X TVu 76 Germplasm accession TVu 4557 X TVu 508
8.7 8.7 9.8 10.1 11.8 10.7 14.8 16.1
1977-OD 12-01E 4557 33-1J 289-4G 1193-10F 6203 881-3G
Small Small Medium Medium Medium Medium Large Large
T h e p o p u l a t i o n s w e r e r e d u c e d b y field erosion, d i f f e r e n t i a l g e r m i n a b i l i t y a n d m o r t a l i t y d u e t o diseases b u t w e r e s u f f i c i e n t l y large to p r o v i d e reliable e s t i m a t e s o f p o p u l a t i o n m e a n s . Variances w e r e c o m p u t e d f r o m individual p l a n t data. Since b l o c k e f f e c t s w e r e small, b e t w e e n a n d w i t h i n p l o t variances w e r e c o m b i n e d f o r analysis. F o r seed size a log10 t r a n s f o r m a t i o n was used since it r e m o v e d significant c o r r e l a t i o n s b e t w e e n t h e m e a n s a n d t h e s t a n d a r d errors (0.67 t o 0.79). F o r seeds p e r p o d , a suitable t r a n s f o r m a t i o n was n o t f o u n d , so t h e original scale was used. T h e significance o f r e c i p r o c a l d i f f e r e n c e s w e r e e x a m i n e d b y t w o - t a i l e d ' t ' tests. A d d i t i v e a n d d o m i n a n c e e f f e c t s were c o m p u t e d f r o m t h e g e n e r a t i o n m e a n s a c c o r d i n g to M a t h e r a n d J i n k s ( 1 9 7 1 ) a n d Cavalli ( 1 9 5 2 ) . B r o a d sense heritabilities w e r e estim a t e d b y t h e m e t h o d s o f Allard {1960) a n d L a w r e n c e and J i n k s ( 1 9 7 4 ) , and correlation coefficients between the two characters were calculated f r o m F2 single p l a n t data. RESULTS T h e d i f f e r e n c e s b e t w e e n t h e m e a n s o f t h e r e c i p r o c a l cross p r o g e n i e s w e r e small a n d m o s t w e r e non-significant. T h e m e a n s f r e q u e n t l y fell closer to t h e p a t e r n a l t h a n t o t h e m a t e r n a l p a r e n t values. T h e r e was t h u s n o evidence for the occurrence of cytoplasmic effects for either character and t h e d a t a f r o m r e c i p r o c a l crosses w e r e p o o l e d f o r s u b s e q u e n t analysis. Generation means
I n d i c a t i o n s o f t h e m o d e o f i n h e r i t a n c e o f t h e t w o c h a r a c t e r s m a y be o b t a i n e d f r o m an e x a m i n a t i o n o f t h e g e n e r a t i o n means. F o r seed size, in crosses o f P8, w i t h P2, P4, P5 a n d P6, a n d t h e cross o f P2 w i t h P7, d o m i n a n c e a p p e a r s a b s e n t as t h e F1 a n d F2 m e a n s are similar
338 to mid-parent (MP) values and the backcrosses tend to be intermediate between the MPs and their respective recurrent parents (Table 2). Partial dominance of large seed size is indicated in crosses of P3 with P7 and P8 as the F1 and F2 means are larger than those of the MPs and the backcrosses tend towards the larger seeded parents. Crosses of P1 with P7 and P8 show partial dominance of small seed size as the F~s and F2s are smaller seeded than the MPs (except the F1 of P1 × P7) and the backcrosses lie closer to the smaller seeded parent than would be expected in the absence of dominance. The F~s of the crosses of P4, P5 and P6 with P7 behave as though large seed is completely dominant, though the F2s and backcrosses to the large-seeded parent are smaller seeded than expected, and two of the backcrosses to the small seeded parent (P5 and P6 with P7) have larger seed than expected.
TABLE
2
M e a n s and standard deviations o f weight o f 1 0 0 s e e d s (g) t r a n s f o r m e d t o log10 for parents and F l , F2, BC1 a n d B C 2 generations crosses m a d e to s t u d y t h e inheritance o f s e e d size i n c o w p e a s
Cross
Mid parent
Generation C o m m o n parent
P1 P1 P2 P2 P3 P3 P4 P4 P5 P5 P6 P6
X × × X X X X X X X X ×
P7 a P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
1.055 1.074 1.076 1.095 1.054 1.073 1.086 1.105 1.121 1.140 1.098 1.117
0.941 ± 0.0037 0.983 ± 0.0051 0.989 ± 0.0075 1.003 ± 0.0038 1.073 ± 0.0050 1.028 + 0.0061
F 1 1.078 1.048 1.105 1.093 1.188 1.133 1.152 1.118 1.170 1.159 1.171 1.128
F2 ± + + + ± ± + + + ± + ±
0.0061 0.0040 0.0046 0.0037 0.0048 0.0061 0.0046 0.0044 0.0052 0.0046 0.0068 0.0039
1.022 1.041 1.063 1.076 1.083 1,091 1.082 1.099 1.115 1.131 1.121 1.116
BC1 ± 0.0048 ± 0.0040 i 0.0050 ± 0.0033 ± 0.0051 ± 0.0034 -~ 0 . 0 0 5 6 ± 0.0042 ± 0.0062 ± 0.0046 ± 0.0062 ± 0.0040
1.092 1.116 1.111 1.150 1.134 1.144 1.117 1.152 1.133 1.145 1.127 1,143
BC2 + 0.0058 -+ 0 . 0 0 5 0 + 0.0054 + 0.0031 + 0.0060 ± 0.0043 ± 0.0054 + 0.0039 + 0.0065 + 0.0052 + 0.0072 ± 0.0037
1.000 0.974 1.050 1.021 1.061 1.045 1.057 1.073 1.131 1.127 1.132 1.086
+ 2 + + + ± ± 2 + + ± ±
0.0032 0.0026 0.0036 0.0024 0.0039 0.0084 0.0028 0.0033 0.0037 0.0032 0.0043 0.0038
a p 7 = 1 . 1 6 8 +- 0 . 0 1 2 5 ; PS = 1 . 2 0 7 + 0 . 0 0 8 4 .
For seeds per pod, m a n y seeds appear to be generally d o m i n a n t over few seeds (Table 3). The exceptions are crosses of P5 and P6 with P8, where dominance appears very low in the Fls and inconsistent in other generations. In crosses of P1 and P2 with P8 and P5 and P6 with P7, m a n y seeds appear partially d o m i n a n t over few seeds per pod in the F~s, BCls and the BC2s, but not always in the F2s: whereas in P1, P2 and P3 with P7 and P4 with P8, the F~s and BC2s indicate complete dominance of m a n y seeds although the F2s and BCls suggest partial or no dominance. Crosses of P3 with P8 and P4 with P7 show clear overdominance of m a n y over few seeds per pod -- the Fls exceed the m a n y seeded parent as too do the F2 of P3 with P8, and BCls and the BC2s for both crosses are similar to the MP.
339 TABLE 3 Means a n d s t a n d a r d d e v i a t i o n s o f seed n u m b e r per p o d i n p a r e n t s a n d F~, F2, BC1 a n d BC2 generat i o n s of crosses m a d e t o s t u d y t h e i n h e r i t a n c e of s e e d size i n c o w p e a s Cross
P1 P1 P2 P2 P3 P3
X X X X X X
Mid parent
P7 a 1 0 . 6 6 P8 1 1 . 6 4 P7 9.02 P8 1 0 . 0 0 P7 6.96 P8 7.94
P4 X P7
P4 P5 P5 P6 P6
8.45
X P8
9.43 9.10 10.08 8.41 9.40
X P7
X P8 X P7 X P8
Generations Common parent
Fx
15.26 + 0.275
14.61 13.46 11.35 11.29 7.96 10.53 13.59 10.59 10.51 10.78 9.41 10.20
11.97 + 0.418 7.85 -+ 0 . 2 4 2 10.84
+ 0.350
12.13 + 0.215 10.77 + 0.349
F2
+ 0 . 1 8 6 1 2 . 2 1 -+ 0 . 1 6 0 +- 0 . 2 0 4 1 1 . 9 3 + 0 . 1 5 1 + 0.285 9.48 + 0.131 + 0.236 9.57 + 0 . 1 2 6 + 0.348 7.28 + 0 . 1 3 0 + 0.301 9 . 1 8 -+ 0 . 1 2 1 + 0.166 10.26 + 0.140 + 0.213 9.31 + 0 . 1 2 5 ± 0.203 8.92 ± 0 . 1 2 4 + 0.251 9 . 3 4 +- 0 . 1 1 9 + 0.286 8.35 ± 0 . 1 2 9 ± 0.187 9.50 ± 0.121
BC1
14.21 13.34 10.98 10.50 7.32 9.25 11.25 10.14 10.73 10.16 9.44 9.42
BC2
+ 0.111 9.77 -+ 0 . 1 6 2 + 0 . 1 3 3 1 1 . 4 1 +- 0 . 2 0 2 + 0.122 8.66 + 0 . 1 2 6 + 0.104 9.83 + 0.156 -+ 0 . 1 3 0 6.85 + 0.175 + 0.103 8.05 + 0.170 ± 0.125 8 . 0 9 +- 0 . 1 5 5 + 0.147 9.33 ± 0.143 ± 0.117 8.68 ± 0.145 ± 0.137 10.02 ± 0.169 + 0.154 8.00 + 0.121 + 0.114 9.82 + 0.156
a p 7 = 6 . 0 6 + 0 . 3 6 0 ; PS = 8 . 0 2 -+ 0 . 3 1 0 .
Scaling tests In all crosses, for both characters, at least one of the quantities, A, B or C, was significantly different from zero (Mather and Jinks, 1971) (Table 4) and X2 tests showed t h a t the c o m p u t e d m, d and h differed significantly TABLE
4
A, B and C values (Mather, 1949) for seed size and seeds per pod study the inheritance of seed size in cowpeas Cross
P1 X P1 X P2 x P2 x P3× P3 × P4 × P4 X P5 X P5 × P6 X P6 ×
P7 P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
from
crosses made to
Weight of 100 seeds (g) t r a n s f o r m e d t o log~0 • l 0 s
Seeds per pod
A
B
C
A
B
C
249 884** -211 740** -1724"* -749** 606** -599** -366* -408* -1278"* -315
1183"* 210 1201"* -133 331 824 1586"* 333 1418" 1295"* 1561" 786**
3495 1062 2440 -1106 -134 163 2960 831 2833 1407 1182 433
1.466"* 2.037** 1.371" 2.252** 1.174" 0.037 1.877"* 1.156" 1.186"* 2.128"* 1.288" 2.598**
1.131" -1.343" 0.102 -0.347 0.318 2.275** 3.395 -0.420 0.784 -1.420 0.534 -1.234"
1.736 2.498 2.813 4.272 0.709 0.228 2.874 2.812 3.527 1.207 2.235 4.353
*, **Significantly
different
from zero at 0.05 and 0.01 levels of probability.
340 f r o m e x p e c t e d v a l u e s ( T a b l e 5) ( C a v a l l i , 1 9 5 2 ) , so t h e a d d i t i v e - d o m i n a n c e model was inadequate to account for the data from this set of crosses. A model including interaction parameters (Jinks and Jones, 1958} was used b y D r a b o e t al. ( 1 9 8 5 ) w h e n t h e a d d i t i v e - d o m i n a n c e m o d e l w a s f o u n d t o b e i n a d e q u a t e . H o w e v e r , in t h i s s t u d y , r e c i p r o c a l e f f e c t s w e r e p r e s e n t ( t h o u g h a p p a r e n t l y n o n g e n e t i c ) a n d c o r r e l a t i o n s b e t w e e n m e a n s a n d variances were not removed for the seeds per pod data, therefore analysis was not taken beyond the additive-dominance model. TABLE 5 Estimates of m, d and h, their standard errors and x: values for tests of goodness of fit to expected values (joint scaling test of Cavalli, 1952) for seed size in cowpeas Cross P1 P1 P2 P2 P3 P3 P4 P4 P5 P5 P6 P6
× x X x x X X X X × X X
m P7 P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
1.032 1.064 1.060 1.084 1.057 1.065 1.059 1.098 1.101 1.127 1.084 1.108
d ± 0.0047** + 0.0036** ± 0.0051"* ± 0.0038** ± 0.0052** ± 0.0045** ± 0.0045** +- 0.0035** -+ 0.0050** ± 0.0040** -+ 0.0055** ± 0.0039**
0.094 0.134 0.070 0.122 0.092 0.116 0.064 0.091 0.023 0.046 0.038 0.073
h + 0.0044** + 0.0035** ± 0.0048** ± 0.0032** + 0.0050** ± 0.0039** ± 0.0043** ± 0.0033** ± 0.0048** ± 0.0038** ± 0.0052** ± 0.0037**
0.029 0.031 0.041 0.002 0.069 0.061 0.075 0.021 0.066 0.029 0.095 0.018
×2 ± 0.0078** ± 0.0057** ± 0.0079** ± 0.0063 ± 0.0082** ± 0.0039** + 0.0069** ± 0.0060** + 0.0078** ± 0.0065** ± 0.0093** -+ 0.0063**
46.51 25.07 38.18 49.72 32.42 57.64 34.68 26.03 23.60 21.45 45.91 20.66
*, **Significantly different from zero at 0.05 and 0.01 levels of probability. I n t h e c a s e o f s e e d size, e s t i m a t e s o f d a c c o r d i n g t o M a t h e r a n d J i n k s ( 1 9 7 1 ) w e r e all s i g n i f i c a n t l y g r e a t e r t h a n z e r o a n d u s u a l l y l a r g e r t h a n t h e e s t i m a t e s o f h ( T a b l e 5). E s t i m a t e s o f h a r e p o s i t i v e a n d s i g n i f i c a n t l y g r e a t e r t h a n z e r o in all c r o s s e s e x c e p t t h a t o f P2 w i t h P8. F o r s e e d n u m b e r , d w a s a l s o s i g n i f i c a n t l y g r e a t e r t h a n z e r o in all c r o s s e s , but estimates of h were similar to those for d, and were relatively larger t h a n w i t h s e e d size ( T a b l e 6). E s t i m a t e s o f h w e r e p o s i t i v e a n d s i g n i f i c a n t i n all e x c e p t o n e c r o s s ( P 6 X P S ) .
Heritability Narrow sense heritabilities were not computed because of the failure of the additive-dominance model. Broad sense heritabilities were larger for s e e d size t h a n f o r s e e d s p e r p o d ( T a b l e 7). I n t h e c a s e o f s e e d size t h e y ranged from 48.0 to 90.2% and for seeds per pod, from 20.8 to 81.5%. The broad sense heritabilities for seeds per pod were larger where P7 was a p a r e n t t h a n w i t h P8 a n d s m a l l e s t in c r o s s e s i n v o l v i n g P 2 , P3 a n d P 6 .
341 TABLE 6 E s t i m a t e s o f m, d a n d h , t h e i r s t a n d a r d e r r o r s a n d x 2 values for t e s t s o f g o o d n e s s o f fit t o e x p e c t e d values ( j o i n t scaling t e s t o f Cavalli, 1 9 5 2 ) f o r s e e d per p o d in c o w p e a s Cross P1 P1 P2 P2 P3 P3 P4 P4 P5 P5 P6 P6
X X x X x x × x x x x x
m P7 P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
10.148 11.355 8.577 9.207 6.888 7.824 7.260 9.020 8.934 9.445 8.161 9.350
d ± 0.1877"* ± 0.1642"* -+ 0 . 2 0 7 9 * * -+ 0 . 1 8 3 2 " * -+ 0 . 1 8 5 0 " * -+ 0 . 1 6 7 3 " * ± 0.1705"* -+ 0 . 1 7 3 8 " * -+ 0 . 1 5 6 5 " * ± 0.1795"* + 0.2005** + 0.1471"*
4.477 2.918 2.489 1.105 0.672 0.764 2.752 1.047 2.613 0.954 1.873 0.664
h ± ± ± ± ± ± ± ± ± ± ± ±
0.1470"* 0.1534"* 0.1478"* 0.1451"* 0.1521"* 0.1355"* 0.1553"* 0.1532"* 0.1371"* 0.1455"* 0.1695"* 0.1399"*
4.071 1.747 2.374 1.602 0.609 2.038 5.731 1.262 1.141 0.765 0.895 0.539
x2 ± ± ± ± ± + ± + ± + ± +
0.2668** 0.2879** 0.3948** 0.3423** 0.3628** 0.3259** 0.2830** 0.3126"* 0.2823** 0.3389** 0.3777** 0.2601
16.01 12.58 6.47 57.48 46.26 14.42 38.87 37.85 42.31 16.60 68.89 45.98
*, * * S i g n i f i c a n t l y d i f f e r e n t f r o m z e r o at 0 . 0 5 a n d 0.01 levels o f p r o b a b i l i t y .
TABLE 7 P e r c e n t a g e h e r i t a b i l i t y o f seed w e i g h t a n d s e e d n u m b e r per p o d in c o w p e a s Cross
P1 x P1 x P2 x P2 x P3 x P3 x P4 x P4x P5 × P5 x P6 x P6 x
Heritability
P7 P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
Seed w e i g h t
Seed n u m b e r per p o d
7 6 . 3 ± 4.8 7 9 . 9 -+ 3.7 8 4 . 9 ± 3.2 68.3 ± 10.8 7 9 . 9 + 4.8 4 8 . 0 + 17.7 9 0 . 2 + 2.0 81.9+ 3.3 6 0 . 4 +- 4.0 8 1 . 1 -+ 3.5 8 5 . 0 _+ 3.3 7 6 . 4 ± 4.6
8 1 . 5 + 3.7 6 7 . 8 +- 6.0 4 8 . 0 ± 11.1 4 1 . 5 +_ 1 1 . 4 45.4 ± 13.0 2 0 . 8 ± 15.3 7 0 . 2 ± 6.3 50.3+- 9.0 7 2 . 4 +- 5.5 5 6 . 6 ± 8.0 42.4 + 13.0 2 9 . 4 + 13.4
Correlations I n t h e F2 p o p u l a t i o n s , correlations between seed size and seeds per pod were negative but were small and differed significantly from zero in only about
half the crosses (Table 8).
342 TABLE 8 Correlation coefficients (R) between weight of 100 seeds (g) transformed to log,0 and seeds per pod in F 2 populations of cowpea crosses Cross
R
Cross
R
P1 X P7 P7 × P1 P2 × P7 P7 X P2 P3 X P7 P7 X P3 P4 × P7 P7 × P4 P5 X P7 P7 X P5 P6 × P7 P7 × P6
-0.02 -0.22* -0.31" -0.30* -0.23* -0.24* -0.05 -0.25* -0.09 -0.04 -0.13 -0.10
P1 X P8 P8 × P1 P2 X P8 P8 × P2 P3 X P8 P8 X P3 P4 X P8 P8 X P4 P5 × P8 P8 X P5 P6 X P8 P8 X P6
-0.26* -0.19" -0.25* -0.17 -0.17" -0.16 -0.31" -0.37* -0.33* -0.33* -0.11 -0.14
DISCUSSION
A variety of genetic mechanisms, ranging from small t o large additive effects and f r o m none to large dominance effects, plus epistatic interactions, were f o u n d to control the inheritance of seed size and seeds per pod in cowpeas. This is not surprising since o t h e r studies indicate t hat several genes are involved for inheritance o f seed size (Sene, 1968; Drabo et al., 1984); th a t f or both characters inheritance is quantitative (Leleji, 1976); and, in this study, different parents were used with m ore diverse origins and characteristics than above. The few observed reciprocal differences appeared to be random , reflecting environmental rather than a genetic, or a cytoplasmic, effect. Thus the data are in agreement with those of Drabo et al. (1985) with regard to seed size. T h e y also strongly suggest t hat the maternal effects on seed size r e p o r t e d by Rawal et al. {1976) in crosses of wild and cultivated types of V. unguiculata were n o t cytoplasmic in nature. The analysis o f generation means according to Mather and Jinks {1971) agreed with previous studies, showing that additive effects are m ost imp o r t a n t in the inheritance of b o t h characters but t hat dom i nance effects are also involved. Unlike previous studies of seed size, either dominance was negligible or large seed was d o m i n a n t over small seed size and only in crosses involving P1 were there indications of dominance of small seed. Dominance appeared especially i m p o r t a n t in the case o f seeds per pod, with m a n y seeds being d o m i n a n t over few. This observation agrees with th at o f Ar eet ey and Laing (1973) but differs from t hat o f Leleji (1976) for six o f his seven crosses. Also unlike previous studies, the additive-dominance model was inadequate in all crosses.
343
Although it appears likely t h a t epistasis was responsible for this, the possibility remains of some confounding with environmental effects. The dominance and overdominance effects in the seeds per pod data from crosses of P1 and P2 with P8, and P4, P5 and P6 with P7, in which there is lack of agreement in trends over generations, should be treated with caution. Similarly, there are reservations for the seed-size effects in the crosses of P5 and P6 with P7. However, there is general agreement in the data for both characters that both additive and dominance effects are important, with the levels of expression and the likely presence of genetic interactions differing amongst crosses. From the breeding point of view, the preponderance of additive effects the large broad sense heritabilities and the small correlations between seed size and seeds per pod, suggest that simultaneous improvement of the two characters can be achieved by o r t h o d o x selection methods. However, the variation in the genetic mechanisms among crosses stresses the need for care in the choice of parents and underlines the danger of using behaviour in one series of crosses to predict the o u t c o m e of others, even for the same characters in the same species. ACKNOWLEDGEMENTS
The data reported here formed part of a PhD study conducted by the senior author, assisted by funding provided by the International Development Research Centre (IDRC) of Canada which is gratefully acknowledged. The authors also gratefully acknowledge the technical assistance of staff of the Cowpea Breeding Section of IITA in glasshouse and field.
REFERENCES Agble, F., 1972. Seed size heterosis in cowpea (Vigna unguiculata L. Walp.). Ghana J. Sci., 12: 30--33. Allard, R.W., 1960. Principles of Plant Breeding. John Wiley, New York. Areetey, A.M. and Laing, E., 1973. Inheritance of yield components and their correlation with yields in cowpea (Vigna unguiculata L. Walp.). Euphytica, 22: 386--392. Brittingham, W.H., 1950. The inheritance of date of pod maturity, pod length, seed shape and seed size in the southern pea (Vigna sinensis). Proc. Am. Soc. Hort. Sci., 56: 381--388. Cavalli, L.L., 1952. An analysis of linkage in quantitative inheritance. In: E.C.R. Rieve and C.H. Waddington (Editors), Quantitative Inheritance. HMSO, London, pp. 135-144. Drabo, I., Redden, R., Smithson, J.B. and Aggarwall, V.D., 1985. Inheritance of seed size in cowpea (Vigna unguiculata L. Walp.). Euphytica, 33: 929--934. Jinks, J.L. and Jones, R.M., 1958. Estimation of the components of heterosis. Genetics, 43: 223--234. Lawrence, M.J. and Jinks, J.L., 1974. Quantitative genetics. In: P.H. Sheppard (Editor), Practical Genetics Blackwell Scientific Publications, London, pp. 88--129. Leleji, O., 1976. Inheritance of three agronomic characters of cowpea (Vigna unguiculata L. Walp.). Samaru Bull., 252.
344 Mather, K., 1949. Biometrical Genetics. Methuen, London, (first edition). Mather, K. and Jinks, J.L., 1971. Biometrical Genetics. Chapman and Hall Limited, London, 2nd edn. Rachie, K.O. and Roberts, L.M., 1974, Grain legumes of the lowland tropics. Adv. Agron., 26: 1--132. Rawal, K.M., Rachie, K.O. and Francowiak, J.D., 1976. Reduction in seed size in crosses between wild and cultivated cowpeas. Hered., 67: 253--254. Redden, R., Drabo, I. and Aggarwal, V.D., 1984. IITA programme of breeding cowpeas with acceptable seed types and disease resistance for West Africa. Field Crops Res., 8: 35--48. Sene, D., 1968. Heredite du poids de 100 graines chez Vigna unguiculata (L.) Walp. (Cowpea). Agron. Trop., 23: 1345--1351.
335
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
INHERITANCE
OF SEED SIZE AND NUMBER PER POD IN COWPEAS
(VIGNA UNGUICULATA L . W A L P . )
I. D R A B O
l ,T.A.O. LADEINDE
2 , R. REDDEN
3,4 a n d J.B. S M I T H S O N
3's
'National Cowpea Improvement Programme SAFGRAD-IITA, Ouagadougou (Upper
Volta) 2Department of Agricultural Biology, University of Ibadan, Oyo Road, Ibadan (Nigeria) 3International Institute of Tropical Agriculture (IITA), Oyo Road, PMB 5320, Ibadan (Nigeria) * Present address: Department of Primary Industries, Hermitage Research Station, Warwick, Qld. 4370 (Australia) Present address: 5 Byemoor Avenue, Great Ayton, County Cleveland, T59 6JP (Great Britain) (Accepted 11 January 1985)
ABSTRACT
Drabo, I., Ladeinde, T.A.O., Redden, R. and Smithson, J.B., 1985. Inheritance of seed size and number per pod in cowpeas (Vigna unguiculata L. Walp.). Field Crops Res., 11: 335--344. The inheritance of seed size and seed number per pod in cowpeas was examined in reciprocal crosses of two large-seeded lines with two small-seeded lines and four lines of intermediate seed size. For seed size, log transformations removed significant correlations among the means and variances. Additive-dominance models failed to explain differences among generation means but genetic mechanisms varied among crosses. F o r seed size, additive effects were most pronounced but epistatic effects were also involved. For seeds per pod, additive, dominance and epistatic effects were most important and of similar magnitude. Many seeds tended to be dominant over few seeds and, in two crosses, there was clear overdominance of many seeds per pod. Broad sense heritabilities were generally large but varied among crosses and correlations between the two characters were negative but small and usually not significantly different from zero. The implications of these results in breeding cowpeas are discussed.
INTRODUCTION C o w p e a (Vigna unguiculata L. W a l p . ) is a n i m p o r t a n t s o u r c e o f p l a n t p r o t e i n s i n W e s t A f r i c a , w h e r e i t is b y f a r t h e m a i n f o o d l e g u m e i n t e r m s of area and production. Traditional cultivars are well adapted to the low input situations in which they are generally cultivated, but have poor yield potential and are susceptible to the major diseases and pests. Large seeds a r e p r e f e r r e d b y t h e c o n s u m e r a n d s e e d n u m b e r p e r p o d is a n i m p o r t a n t c o m p o n e n t o f s e e d y i e l d , so i m p r o v e m e n t i n b o t h c h a r a c t e r s is a p r i n c i p a l
0378-4290/85/$03.30
© 1985 Elsevier Science Publishers B.V.
336 objective of cowpea breeding programmes (for example, Redden et al., 1984). Genetic studies of seed size and seeds per pod in cowpeas have been few and the results conflicting. Sene (1968), Areetey and Laing {1973) and Drabo et al. (1985) concluded that genetic variation in seed size was mainly additive. Areetey and Laing (1973), Leleji {1976) and Drabo et al. {1985) observed partial dominance of small seeds over large seeds. However, Brittingham {1950) and Agble {1972) f o u n d considerable heterosis to be involved. The inheritance of seed size appears to be controlled by six to ten pairs of genes and broad and narrow sense heritabilities are large (Sene, 1968; Areetey and Lalng, 1973; Leleji, 1976; Drabo et al., 1985). Areetey and Laing {1973) reported that m a n y seeds per pod was d o m i n a n t over few seeds but Leleji {1976) found the reverse expression. The two characters are considered to be negatively correlated (Rachie and Roberts, 1974). In this paper, we report studies of the inheritance of seed size and seed number in cowpea. This information is directly relevant to the breeding programme at the International Institute of Tropical Agriculture (IITA) at Ibadan in Nigeria, and involves both germplasm and breeding lines with small, medium, and large seed sizes. MATERIALS AND METHODS Two large-seeded cowpea lines (TVu 6203 and TVx 881-3G) were crossed reciprocally with two lines with small seeds (TVu 1977-0D and TVx 1201E) and four of intermediate seed size (TVu 4557, TVx 331J, TVx 289-4G and TVx 1193-10F) {Table 1). The TVx lines are derived from crosses among germplasm lines. The study therefore involved materials of very diverse origins and character. Reciprocal crosses were made in a glasshouse in 1978. The F1 's were selfed to obtain F2 seeds and simultaneously backcrossed to both parents. The eight parents, 24 F1 and F2 and 48 backcrosses generations were sown in the field on the research farm of IITA in May 1979. For ease of reference the eight parents will be designated P1 to P8, as shown in Table 1; BC1 will denote the populations derived from backcrosses to the large or manyseeded parents; and BC2 the backcrosses to the smaller or fewer-seeded parents. The experimental design was a randomised block with five replicates. The plot sizes were: ten plants each for the parents and F~'s; 20 to 30 plants for the backcrosses; and 30 to 40 plants for the F2 populations. The seeds were sown in rows 1 m apart, with 30 cm between positions within rows. The crop was hand-weeded and protected against insects and single plants were harvested separately at maturity. Estimates of seed size and seeds per pod were derived from the total undamaged seeds from 10 to 20 pods of each plant.
337 TABLE 1 Origins and characteristics of parents used in crosses to study the inheritance of seed size and number per pod in cowpeas Parents
Code number
Origin
Parentage
Weight Seed size of 100 category seeds (g)
TVu TVx TVu TVx TVx TVx TVu TVx
P1 P2 P3 P4 P5 P6 P7 P8
Zimbabwe Nigeria Nigeria Nigeria Nigeria Nigeria Senegal Nigeria
Germplasm accession TVu 2 X TVu 95 Germplasm accession TVu 37 X TVu 530 TVx 33-1B X TVx 23-2B TVu 1190 X TVu 76 Germplasm accession TVu 4557 X TVu 508
8.7 8.7 9.8 10.1 11.8 10.7 14.8 16.1
1977-OD 12-01E 4557 33-1J 289-4G 1193-10F 6203 881-3G
Small Small Medium Medium Medium Medium Large Large
T h e p o p u l a t i o n s w e r e r e d u c e d b y field erosion, d i f f e r e n t i a l g e r m i n a b i l i t y a n d m o r t a l i t y d u e t o diseases b u t w e r e s u f f i c i e n t l y large to p r o v i d e reliable e s t i m a t e s o f p o p u l a t i o n m e a n s . Variances w e r e c o m p u t e d f r o m individual p l a n t data. Since b l o c k e f f e c t s w e r e small, b e t w e e n a n d w i t h i n p l o t variances w e r e c o m b i n e d f o r analysis. F o r seed size a log10 t r a n s f o r m a t i o n was used since it r e m o v e d significant c o r r e l a t i o n s b e t w e e n t h e m e a n s a n d t h e s t a n d a r d errors (0.67 t o 0.79). F o r seeds p e r p o d , a suitable t r a n s f o r m a t i o n was n o t f o u n d , so t h e original scale was used. T h e significance o f r e c i p r o c a l d i f f e r e n c e s w e r e e x a m i n e d b y t w o - t a i l e d ' t ' tests. A d d i t i v e a n d d o m i n a n c e e f f e c t s were c o m p u t e d f r o m t h e g e n e r a t i o n m e a n s a c c o r d i n g to M a t h e r a n d J i n k s ( 1 9 7 1 ) a n d Cavalli ( 1 9 5 2 ) . B r o a d sense heritabilities w e r e estim a t e d b y t h e m e t h o d s o f Allard {1960) a n d L a w r e n c e and J i n k s ( 1 9 7 4 ) , and correlation coefficients between the two characters were calculated f r o m F2 single p l a n t data. RESULTS T h e d i f f e r e n c e s b e t w e e n t h e m e a n s o f t h e r e c i p r o c a l cross p r o g e n i e s w e r e small a n d m o s t w e r e non-significant. T h e m e a n s f r e q u e n t l y fell closer to t h e p a t e r n a l t h a n t o t h e m a t e r n a l p a r e n t values. T h e r e was t h u s n o evidence for the occurrence of cytoplasmic effects for either character and t h e d a t a f r o m r e c i p r o c a l crosses w e r e p o o l e d f o r s u b s e q u e n t analysis. Generation means
I n d i c a t i o n s o f t h e m o d e o f i n h e r i t a n c e o f t h e t w o c h a r a c t e r s m a y be o b t a i n e d f r o m an e x a m i n a t i o n o f t h e g e n e r a t i o n means. F o r seed size, in crosses o f P8, w i t h P2, P4, P5 a n d P6, a n d t h e cross o f P2 w i t h P7, d o m i n a n c e a p p e a r s a b s e n t as t h e F1 a n d F2 m e a n s are similar
338 to mid-parent (MP) values and the backcrosses tend to be intermediate between the MPs and their respective recurrent parents (Table 2). Partial dominance of large seed size is indicated in crosses of P3 with P7 and P8 as the F1 and F2 means are larger than those of the MPs and the backcrosses tend towards the larger seeded parents. Crosses of P1 with P7 and P8 show partial dominance of small seed size as the F~s and F2s are smaller seeded than the MPs (except the F1 of P1 × P7) and the backcrosses lie closer to the smaller seeded parent than would be expected in the absence of dominance. The F~s of the crosses of P4, P5 and P6 with P7 behave as though large seed is completely dominant, though the F2s and backcrosses to the large-seeded parent are smaller seeded than expected, and two of the backcrosses to the small seeded parent (P5 and P6 with P7) have larger seed than expected.
TABLE
2
M e a n s and standard deviations o f weight o f 1 0 0 s e e d s (g) t r a n s f o r m e d t o log10 for parents and F l , F2, BC1 a n d B C 2 generations crosses m a d e to s t u d y t h e inheritance o f s e e d size i n c o w p e a s
Cross
Mid parent
Generation C o m m o n parent
P1 P1 P2 P2 P3 P3 P4 P4 P5 P5 P6 P6
X × × X X X X X X X X ×
P7 a P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
1.055 1.074 1.076 1.095 1.054 1.073 1.086 1.105 1.121 1.140 1.098 1.117
0.941 ± 0.0037 0.983 ± 0.0051 0.989 ± 0.0075 1.003 ± 0.0038 1.073 ± 0.0050 1.028 + 0.0061
F 1 1.078 1.048 1.105 1.093 1.188 1.133 1.152 1.118 1.170 1.159 1.171 1.128
F2 ± + + + ± ± + + + ± + ±
0.0061 0.0040 0.0046 0.0037 0.0048 0.0061 0.0046 0.0044 0.0052 0.0046 0.0068 0.0039
1.022 1.041 1.063 1.076 1.083 1,091 1.082 1.099 1.115 1.131 1.121 1.116
BC1 ± 0.0048 ± 0.0040 i 0.0050 ± 0.0033 ± 0.0051 ± 0.0034 -~ 0 . 0 0 5 6 ± 0.0042 ± 0.0062 ± 0.0046 ± 0.0062 ± 0.0040
1.092 1.116 1.111 1.150 1.134 1.144 1.117 1.152 1.133 1.145 1.127 1,143
BC2 + 0.0058 -+ 0 . 0 0 5 0 + 0.0054 + 0.0031 + 0.0060 ± 0.0043 ± 0.0054 + 0.0039 + 0.0065 + 0.0052 + 0.0072 ± 0.0037
1.000 0.974 1.050 1.021 1.061 1.045 1.057 1.073 1.131 1.127 1.132 1.086
+ 2 + + + ± ± 2 + + ± ±
0.0032 0.0026 0.0036 0.0024 0.0039 0.0084 0.0028 0.0033 0.0037 0.0032 0.0043 0.0038
a p 7 = 1 . 1 6 8 +- 0 . 0 1 2 5 ; PS = 1 . 2 0 7 + 0 . 0 0 8 4 .
For seeds per pod, m a n y seeds appear to be generally d o m i n a n t over few seeds (Table 3). The exceptions are crosses of P5 and P6 with P8, where dominance appears very low in the Fls and inconsistent in other generations. In crosses of P1 and P2 with P8 and P5 and P6 with P7, m a n y seeds appear partially d o m i n a n t over few seeds per pod in the F~s, BCls and the BC2s, but not always in the F2s: whereas in P1, P2 and P3 with P7 and P4 with P8, the F~s and BC2s indicate complete dominance of m a n y seeds although the F2s and BCls suggest partial or no dominance. Crosses of P3 with P8 and P4 with P7 show clear overdominance of m a n y over few seeds per pod -- the Fls exceed the m a n y seeded parent as too do the F2 of P3 with P8, and BCls and the BC2s for both crosses are similar to the MP.
339 TABLE 3 Means a n d s t a n d a r d d e v i a t i o n s o f seed n u m b e r per p o d i n p a r e n t s a n d F~, F2, BC1 a n d BC2 generat i o n s of crosses m a d e t o s t u d y t h e i n h e r i t a n c e of s e e d size i n c o w p e a s Cross
P1 P1 P2 P2 P3 P3
X X X X X X
Mid parent
P7 a 1 0 . 6 6 P8 1 1 . 6 4 P7 9.02 P8 1 0 . 0 0 P7 6.96 P8 7.94
P4 X P7
P4 P5 P5 P6 P6
8.45
X P8
9.43 9.10 10.08 8.41 9.40
X P7
X P8 X P7 X P8
Generations Common parent
Fx
15.26 + 0.275
14.61 13.46 11.35 11.29 7.96 10.53 13.59 10.59 10.51 10.78 9.41 10.20
11.97 + 0.418 7.85 -+ 0 . 2 4 2 10.84
+ 0.350
12.13 + 0.215 10.77 + 0.349
F2
+ 0 . 1 8 6 1 2 . 2 1 -+ 0 . 1 6 0 +- 0 . 2 0 4 1 1 . 9 3 + 0 . 1 5 1 + 0.285 9.48 + 0.131 + 0.236 9.57 + 0 . 1 2 6 + 0.348 7.28 + 0 . 1 3 0 + 0.301 9 . 1 8 -+ 0 . 1 2 1 + 0.166 10.26 + 0.140 + 0.213 9.31 + 0 . 1 2 5 ± 0.203 8.92 ± 0 . 1 2 4 + 0.251 9 . 3 4 +- 0 . 1 1 9 + 0.286 8.35 ± 0 . 1 2 9 ± 0.187 9.50 ± 0.121
BC1
14.21 13.34 10.98 10.50 7.32 9.25 11.25 10.14 10.73 10.16 9.44 9.42
BC2
+ 0.111 9.77 -+ 0 . 1 6 2 + 0 . 1 3 3 1 1 . 4 1 +- 0 . 2 0 2 + 0.122 8.66 + 0 . 1 2 6 + 0.104 9.83 + 0.156 -+ 0 . 1 3 0 6.85 + 0.175 + 0.103 8.05 + 0.170 ± 0.125 8 . 0 9 +- 0 . 1 5 5 + 0.147 9.33 ± 0.143 ± 0.117 8.68 ± 0.145 ± 0.137 10.02 ± 0.169 + 0.154 8.00 + 0.121 + 0.114 9.82 + 0.156
a p 7 = 6 . 0 6 + 0 . 3 6 0 ; PS = 8 . 0 2 -+ 0 . 3 1 0 .
Scaling tests In all crosses, for both characters, at least one of the quantities, A, B or C, was significantly different from zero (Mather and Jinks, 1971) (Table 4) and X2 tests showed t h a t the c o m p u t e d m, d and h differed significantly TABLE
4
A, B and C values (Mather, 1949) for seed size and seeds per pod study the inheritance of seed size in cowpeas Cross
P1 X P1 X P2 x P2 x P3× P3 × P4 × P4 X P5 X P5 × P6 X P6 ×
P7 P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
from
crosses made to
Weight of 100 seeds (g) t r a n s f o r m e d t o log~0 • l 0 s
Seeds per pod
A
B
C
A
B
C
249 884** -211 740** -1724"* -749** 606** -599** -366* -408* -1278"* -315
1183"* 210 1201"* -133 331 824 1586"* 333 1418" 1295"* 1561" 786**
3495 1062 2440 -1106 -134 163 2960 831 2833 1407 1182 433
1.466"* 2.037** 1.371" 2.252** 1.174" 0.037 1.877"* 1.156" 1.186"* 2.128"* 1.288" 2.598**
1.131" -1.343" 0.102 -0.347 0.318 2.275** 3.395 -0.420 0.784 -1.420 0.534 -1.234"
1.736 2.498 2.813 4.272 0.709 0.228 2.874 2.812 3.527 1.207 2.235 4.353
*, **Significantly
different
from zero at 0.05 and 0.01 levels of probability.
340 f r o m e x p e c t e d v a l u e s ( T a b l e 5) ( C a v a l l i , 1 9 5 2 ) , so t h e a d d i t i v e - d o m i n a n c e model was inadequate to account for the data from this set of crosses. A model including interaction parameters (Jinks and Jones, 1958} was used b y D r a b o e t al. ( 1 9 8 5 ) w h e n t h e a d d i t i v e - d o m i n a n c e m o d e l w a s f o u n d t o b e i n a d e q u a t e . H o w e v e r , in t h i s s t u d y , r e c i p r o c a l e f f e c t s w e r e p r e s e n t ( t h o u g h a p p a r e n t l y n o n g e n e t i c ) a n d c o r r e l a t i o n s b e t w e e n m e a n s a n d variances were not removed for the seeds per pod data, therefore analysis was not taken beyond the additive-dominance model. TABLE 5 Estimates of m, d and h, their standard errors and x: values for tests of goodness of fit to expected values (joint scaling test of Cavalli, 1952) for seed size in cowpeas Cross P1 P1 P2 P2 P3 P3 P4 P4 P5 P5 P6 P6
× x X x x X X X X × X X
m P7 P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
1.032 1.064 1.060 1.084 1.057 1.065 1.059 1.098 1.101 1.127 1.084 1.108
d ± 0.0047** + 0.0036** ± 0.0051"* ± 0.0038** ± 0.0052** ± 0.0045** ± 0.0045** +- 0.0035** -+ 0.0050** ± 0.0040** -+ 0.0055** ± 0.0039**
0.094 0.134 0.070 0.122 0.092 0.116 0.064 0.091 0.023 0.046 0.038 0.073
h + 0.0044** + 0.0035** ± 0.0048** ± 0.0032** + 0.0050** ± 0.0039** ± 0.0043** ± 0.0033** ± 0.0048** ± 0.0038** ± 0.0052** ± 0.0037**
0.029 0.031 0.041 0.002 0.069 0.061 0.075 0.021 0.066 0.029 0.095 0.018
×2 ± 0.0078** ± 0.0057** ± 0.0079** ± 0.0063 ± 0.0082** ± 0.0039** + 0.0069** ± 0.0060** + 0.0078** ± 0.0065** ± 0.0093** -+ 0.0063**
46.51 25.07 38.18 49.72 32.42 57.64 34.68 26.03 23.60 21.45 45.91 20.66
*, **Significantly different from zero at 0.05 and 0.01 levels of probability. I n t h e c a s e o f s e e d size, e s t i m a t e s o f d a c c o r d i n g t o M a t h e r a n d J i n k s ( 1 9 7 1 ) w e r e all s i g n i f i c a n t l y g r e a t e r t h a n z e r o a n d u s u a l l y l a r g e r t h a n t h e e s t i m a t e s o f h ( T a b l e 5). E s t i m a t e s o f h a r e p o s i t i v e a n d s i g n i f i c a n t l y g r e a t e r t h a n z e r o in all c r o s s e s e x c e p t t h a t o f P2 w i t h P8. F o r s e e d n u m b e r , d w a s a l s o s i g n i f i c a n t l y g r e a t e r t h a n z e r o in all c r o s s e s , but estimates of h were similar to those for d, and were relatively larger t h a n w i t h s e e d size ( T a b l e 6). E s t i m a t e s o f h w e r e p o s i t i v e a n d s i g n i f i c a n t i n all e x c e p t o n e c r o s s ( P 6 X P S ) .
Heritability Narrow sense heritabilities were not computed because of the failure of the additive-dominance model. Broad sense heritabilities were larger for s e e d size t h a n f o r s e e d s p e r p o d ( T a b l e 7). I n t h e c a s e o f s e e d size t h e y ranged from 48.0 to 90.2% and for seeds per pod, from 20.8 to 81.5%. The broad sense heritabilities for seeds per pod were larger where P7 was a p a r e n t t h a n w i t h P8 a n d s m a l l e s t in c r o s s e s i n v o l v i n g P 2 , P3 a n d P 6 .
341 TABLE 6 E s t i m a t e s o f m, d a n d h , t h e i r s t a n d a r d e r r o r s a n d x 2 values for t e s t s o f g o o d n e s s o f fit t o e x p e c t e d values ( j o i n t scaling t e s t o f Cavalli, 1 9 5 2 ) f o r s e e d per p o d in c o w p e a s Cross P1 P1 P2 P2 P3 P3 P4 P4 P5 P5 P6 P6
X X x X x x × x x x x x
m P7 P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
10.148 11.355 8.577 9.207 6.888 7.824 7.260 9.020 8.934 9.445 8.161 9.350
d ± 0.1877"* ± 0.1642"* -+ 0 . 2 0 7 9 * * -+ 0 . 1 8 3 2 " * -+ 0 . 1 8 5 0 " * -+ 0 . 1 6 7 3 " * ± 0.1705"* -+ 0 . 1 7 3 8 " * -+ 0 . 1 5 6 5 " * ± 0.1795"* + 0.2005** + 0.1471"*
4.477 2.918 2.489 1.105 0.672 0.764 2.752 1.047 2.613 0.954 1.873 0.664
h ± ± ± ± ± ± ± ± ± ± ± ±
0.1470"* 0.1534"* 0.1478"* 0.1451"* 0.1521"* 0.1355"* 0.1553"* 0.1532"* 0.1371"* 0.1455"* 0.1695"* 0.1399"*
4.071 1.747 2.374 1.602 0.609 2.038 5.731 1.262 1.141 0.765 0.895 0.539
x2 ± ± ± ± ± + ± + ± + ± +
0.2668** 0.2879** 0.3948** 0.3423** 0.3628** 0.3259** 0.2830** 0.3126"* 0.2823** 0.3389** 0.3777** 0.2601
16.01 12.58 6.47 57.48 46.26 14.42 38.87 37.85 42.31 16.60 68.89 45.98
*, * * S i g n i f i c a n t l y d i f f e r e n t f r o m z e r o at 0 . 0 5 a n d 0.01 levels o f p r o b a b i l i t y .
TABLE 7 P e r c e n t a g e h e r i t a b i l i t y o f seed w e i g h t a n d s e e d n u m b e r per p o d in c o w p e a s Cross
P1 x P1 x P2 x P2 x P3 x P3 x P4 x P4x P5 × P5 x P6 x P6 x
Heritability
P7 P8 P7 P8 P7 P8 P7 P8 P7 P8 P7 P8
Seed w e i g h t
Seed n u m b e r per p o d
7 6 . 3 ± 4.8 7 9 . 9 -+ 3.7 8 4 . 9 ± 3.2 68.3 ± 10.8 7 9 . 9 + 4.8 4 8 . 0 + 17.7 9 0 . 2 + 2.0 81.9+ 3.3 6 0 . 4 +- 4.0 8 1 . 1 -+ 3.5 8 5 . 0 _+ 3.3 7 6 . 4 ± 4.6
8 1 . 5 + 3.7 6 7 . 8 +- 6.0 4 8 . 0 ± 11.1 4 1 . 5 +_ 1 1 . 4 45.4 ± 13.0 2 0 . 8 ± 15.3 7 0 . 2 ± 6.3 50.3+- 9.0 7 2 . 4 +- 5.5 5 6 . 6 ± 8.0 42.4 + 13.0 2 9 . 4 + 13.4
Correlations I n t h e F2 p o p u l a t i o n s , correlations between seed size and seeds per pod were negative but were small and differed significantly from zero in only about
half the crosses (Table 8).
342 TABLE 8 Correlation coefficients (R) between weight of 100 seeds (g) transformed to log,0 and seeds per pod in F 2 populations of cowpea crosses Cross
R
Cross
R
P1 X P7 P7 × P1 P2 × P7 P7 X P2 P3 X P7 P7 X P3 P4 × P7 P7 × P4 P5 X P7 P7 X P5 P6 × P7 P7 × P6
-0.02 -0.22* -0.31" -0.30* -0.23* -0.24* -0.05 -0.25* -0.09 -0.04 -0.13 -0.10
P1 X P8 P8 × P1 P2 X P8 P8 × P2 P3 X P8 P8 X P3 P4 X P8 P8 X P4 P5 × P8 P8 X P5 P6 X P8 P8 X P6
-0.26* -0.19" -0.25* -0.17 -0.17" -0.16 -0.31" -0.37* -0.33* -0.33* -0.11 -0.14
DISCUSSION
A variety of genetic mechanisms, ranging from small t o large additive effects and f r o m none to large dominance effects, plus epistatic interactions, were f o u n d to control the inheritance of seed size and seeds per pod in cowpeas. This is not surprising since o t h e r studies indicate t hat several genes are involved for inheritance o f seed size (Sene, 1968; Drabo et al., 1984); th a t f or both characters inheritance is quantitative (Leleji, 1976); and, in this study, different parents were used with m ore diverse origins and characteristics than above. The few observed reciprocal differences appeared to be random , reflecting environmental rather than a genetic, or a cytoplasmic, effect. Thus the data are in agreement with those of Drabo et al. (1985) with regard to seed size. T h e y also strongly suggest t hat the maternal effects on seed size r e p o r t e d by Rawal et al. {1976) in crosses of wild and cultivated types of V. unguiculata were n o t cytoplasmic in nature. The analysis o f generation means according to Mather and Jinks {1971) agreed with previous studies, showing that additive effects are m ost imp o r t a n t in the inheritance of b o t h characters but t hat dom i nance effects are also involved. Unlike previous studies of seed size, either dominance was negligible or large seed was d o m i n a n t over small seed size and only in crosses involving P1 were there indications of dominance of small seed. Dominance appeared especially i m p o r t a n t in the case o f seeds per pod, with m a n y seeds being d o m i n a n t over few. This observation agrees with th at o f Ar eet ey and Laing (1973) but differs from t hat o f Leleji (1976) for six o f his seven crosses. Also unlike previous studies, the additive-dominance model was inadequate in all crosses.
343
Although it appears likely t h a t epistasis was responsible for this, the possibility remains of some confounding with environmental effects. The dominance and overdominance effects in the seeds per pod data from crosses of P1 and P2 with P8, and P4, P5 and P6 with P7, in which there is lack of agreement in trends over generations, should be treated with caution. Similarly, there are reservations for the seed-size effects in the crosses of P5 and P6 with P7. However, there is general agreement in the data for both characters that both additive and dominance effects are important, with the levels of expression and the likely presence of genetic interactions differing amongst crosses. From the breeding point of view, the preponderance of additive effects the large broad sense heritabilities and the small correlations between seed size and seeds per pod, suggest that simultaneous improvement of the two characters can be achieved by o r t h o d o x selection methods. However, the variation in the genetic mechanisms among crosses stresses the need for care in the choice of parents and underlines the danger of using behaviour in one series of crosses to predict the o u t c o m e of others, even for the same characters in the same species. ACKNOWLEDGEMENTS
The data reported here formed part of a PhD study conducted by the senior author, assisted by funding provided by the International Development Research Centre (IDRC) of Canada which is gratefully acknowledged. The authors also gratefully acknowledge the technical assistance of staff of the Cowpea Breeding Section of IITA in glasshouse and field.
REFERENCES Agble, F., 1972. Seed size heterosis in cowpea (Vigna unguiculata L. Walp.). Ghana J. Sci., 12: 30--33. Allard, R.W., 1960. Principles of Plant Breeding. John Wiley, New York. Areetey, A.M. and Laing, E., 1973. Inheritance of yield components and their correlation with yields in cowpea (Vigna unguiculata L. Walp.). Euphytica, 22: 386--392. Brittingham, W.H., 1950. The inheritance of date of pod maturity, pod length, seed shape and seed size in the southern pea (Vigna sinensis). Proc. Am. Soc. Hort. Sci., 56: 381--388. Cavalli, L.L., 1952. An analysis of linkage in quantitative inheritance. In: E.C.R. Rieve and C.H. Waddington (Editors), Quantitative Inheritance. HMSO, London, pp. 135-144. Drabo, I., Redden, R., Smithson, J.B. and Aggarwall, V.D., 1985. Inheritance of seed size in cowpea (Vigna unguiculata L. Walp.). Euphytica, 33: 929--934. Jinks, J.L. and Jones, R.M., 1958. Estimation of the components of heterosis. Genetics, 43: 223--234. Lawrence, M.J. and Jinks, J.L., 1974. Quantitative genetics. In: P.H. Sheppard (Editor), Practical Genetics Blackwell Scientific Publications, London, pp. 88--129. Leleji, O., 1976. Inheritance of three agronomic characters of cowpea (Vigna unguiculata L. Walp.). Samaru Bull., 252.
344 Mather, K., 1949. Biometrical Genetics. Methuen, London, (first edition). Mather, K. and Jinks, J.L., 1971. Biometrical Genetics. Chapman and Hall Limited, London, 2nd edn. Rachie, K.O. and Roberts, L.M., 1974, Grain legumes of the lowland tropics. Adv. Agron., 26: 1--132. Rawal, K.M., Rachie, K.O. and Francowiak, J.D., 1976. Reduction in seed size in crosses between wild and cultivated cowpeas. Hered., 67: 253--254. Redden, R., Drabo, I. and Aggarwal, V.D., 1984. IITA programme of breeding cowpeas with acceptable seed types and disease resistance for West Africa. Field Crops Res., 8: 35--48. Sene, D., 1968. Heredite du poids de 100 graines chez Vigna unguiculata (L.) Walp. (Cowpea). Agron. Trop., 23: 1345--1351.