1. ANALYSIS OF INBREEDING DEPRESSION IN FIVE S1 CASSAVA FAMILIES OF AFRICAN VARIETIES
Lydia Ezenwaka1, Emmanuel Okogbenin1*, Olalekan Akinbo1, Chiedozie Egesi1, Ugochukwu Ikeogu1, Eunice Ekaette1, Kenneth Eluwa1, Alex Ogbonna1 and Favour Ewa1
1 National Roots Crops Research Institute (NRCRI), Umudike, Nigeria.
corresponding author address:
RESULTS AND DISCUSSION
Data on plant height, vigour, fresh root yield (FRY), fresh foliage yield (FFY), harvest index (HI) and dry matter content (DMC) for the five S1 families is presented in Table 2. The range of ID for each family is shown in Table 3. The average performance of the S0 progenitors, S1 clones and ID values for each family and trait are also provided (Table 2). For plant height, average ID was about -25%. The depression was lowest in plant height when compared to other traits evaluated. This implies that the performance of the S1 progenies was close to that of the S0 progenitor indicating no inbreeding depression for this trait at S1. The actual effect of selecting plants that could produce enough planting material was most likely negligible (Rojas, et al 2009). The range of ID among the different S1 families ranged from % (TME 419) to 4.00% (TMS 98/0505). Data for fresh root yield indicated that the mean of the S1 progeny was lower than that of the S0 progenitor in all the families. On average, lowest FRY (2.76 kg plant-1) was observed in family TMS 98/0505 (Fig. 1). ID associated with FRY ranged from 74.38% in TMS 01/1371 to 91.84% in TME 419. Inbreeding depression for FRY was, as expected, higher, with an average of 86.55%. In other words, S1 genotypes yielded only 13.45% of the yield. Average IDs for other traits were 66.38% for FFY (ranging from 42.50% in TMS 01/1371 to 90.63% in TMS 98/0505), 0.55% for HI (ranging from 0.55% in TMS 01/1371 to 0.75% in TMS 98/0505) and 14.32% for dry matter content (ranging from 4.81% in TMS 01/1371 to 20.28% in TMS 30572). For vigour, it was observed that all the families had their S1 mean lower than that of the non-inbred parents.TMS 01/1371 had the lowest ID value (-3.03%) which implies that the depression was lowest in TMS 01/1371 (Fig. 2). The highest mean value (3.63) was observed in family TMS 419, while the lowest (2.78) was found in TMS family. Reduced heterozygosity associated with selfing (S1) appeared to have strong effects on both FRY and FFY thus underscoring the relative importance of non-additive genetic effects in their inheritance. For each trait, results identified individual S1 genotypes that substantially outperformed the non-inbred parents likely representing transgressive segregants for favorable allele combinations. This was observed mainly for plant height, DMC and vigour where ID values were lower.
ABSTRACT
Cassava is an out-crossing, highly heterozygous plant and is reported to suffer from inbreeding depression. The effects of inbreeding in cassava are still under-quantified. The need to unravel recessive traits and explore additive genes in cassava would in the minimum require some limited level of inbreeding in genetic improvement of cassava. A study to assess the performance of S1 progenies of five African cassava varieties (TMS 30572, TME 419, TMS 98/0505, TMS 01/1371 and TMS 98/0002) for six agronomic traits was conducted. The results of the average inbreeding depression (ID) levels for the traits are as follows: fresh root yield (86.55%), fresh foliage yield (66.38%), harvest index (25.34%), root dry matter content (14.32%), plant height (-25.49%) and vigour (20.83%). The average ID obtained indicate that harvest index, dry matter content, and vigour were not severely affected in performance. The average trait value for plant height in the S1 was not reduced implying no inbreeding depression for this trait. The effects of ID was highest in fresh root yield and fresh foliage yield which are relatively much more complex traits than others. Reduced heterozygosity associated with selfing (S1) appeared to have strong effects on both traits thus underscoring the relative importance of non-additive genetic effects in their inheritance. For each trait, results identified individual S1 genotypes that substantially outperformed the non-inbred parents likely representing transgressive segregants for favorable allele combinations. This was observed mainly for plant height, DMC and vigour where ID values were lower. These results demonstrate that inbreeding may be strategically explored in breeding to increase genetic gain and identify recessive traits. Advanced selfed generations are currently in development to screen for inbreeding tolerance in cassava to enhance genetic improvement of the crop.
 INTRODUCTION
Cassava (Manihot esculenta Crantz) is among the most important sources of energy in the diet of most tropical countries of the world. It is also becoming an important source of raw material for different industries. Therefore, it is vital to develop information about the genetic structure and the inheritance of relevant traits. Inbreeding is the reproduction from the mating of two genetically related parents, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is called inbreeding depression. Cassava, as a typical cross-pollinated species, shows severe inbreeding depression (Rojas et al., 2009). Nonetheless, it is encouraging to observe that interest in cassava inbreeding is beginning to gain momentum (Ceballos et al., 2004, 2007; Rojas et al., 2009). Certainly, it would be prudent in cassava to tap into the benefits of inbreeding as witnessed in maize. Inbreeding provides an opportunity to exploit both additive and non-additive effects (Rojas et al., 2009). It is envisaged that inbreeding in cassava will provide several advantages including: reduction of genetic load which limits attainment of sustainable genetic progress, increased probability of increasing the expression of useful recessive traits, facilitation of the implementation of mutation breeding and allows implementation of backcross scheme (Ceballos et al., 2004). We report results obtained from quantifying inbreeding depression in cassava.
MATERIALS AND METHODS
Seed production
Five of the African cassava varieties (TMS 30572, TME 419, TMS 98/0505, TMS 01/1371 and TMS 98/0002) were obtained from the germplasm of International Institute of Tropical Agriculture, (IITA) Ibadan. These five african cassava varieties were used as progenitors (So) to generate S1 progenies. For each variety, 20 stem cuttings were planted at spacing of 1m x 1m. Self pollinations were obtained from each of these elite clones. The pollinated flowers were bagged with pollination bags to protect them against bees carrying foreign pollens; the bags were removed a day later. The seeds matured about 70 to 90 days after pollination. The mature fruits were carefully harvested, placed in labeled pollination bags and were left to shatter naturally (IITA, 1990).
Seed germination and transplanting
Harvested S1 botanical seeds were allowed a two month dormancy break down period before being established in nurseries under screen house conditions. Seeds germinated quickly at optimal soil temperatures (300C to 350C) and moisture regimes. They were irrigated twice daily, in the mornings and evenings. The seeds germinated from 10 to 30 days after planting and were ready for transplanting when they were from 15 to 20 cm high. After two months in the nursery, S1 seedlings were transplanted to a well – prepared field. The S1 seedlings were planted at a spacing of 0.5m on 1m ridges. Planting was done in family basis (Table 1). Fertilizer was applied at eight weeks after planting. The fields were weeded regularly.
 Field evaluations
Harvesting was done at 12 months after planting. Variables collected and analyzed were plant height (cm); vigour; fresh root yield (kg plant-1); fresh foliage yield (kg plant-1); harvest index (0-1) and dry matter content (%),determined by the specific gravity methodology as suggested by Kawano et al., (1987). Inbreeding depression (ID) was estimated for each variable as a percentage of the S1 average:
ID = [(So mean –S1 mean) / S0 mean] x 100.
Therefore, the lower the ID value, the lower the depression, which implies that the performance of the S1 progenies is close to that of the S0 progenitors.
Figure 1: Low yield due to the effect of inbreeding depression in TMS 98/0505.
Figure 2: TMS 01/1371 showing good vigour.
CONCLUSION
The results demonstrate that inbreeding may be strategically explored in breeding to increase genetic gain and identify recessive traits. Advanced selfed generations are currently in development to screen for inbreeding tolerance in cassava to enhance genetic improvement of the crop.
ACKNOWLEDGEMENTS
This study was conducted with funding support of the Generation Challenge Programme (GCP) and National Root Crops Research Institute (NRCRI), Umudike.
REFERENCES
Ceballos, H., C.A. Iglesias, J.C., Perez and A. Dixon, A. (2004). Cassava breeding: Opportunities and challenges. Plant Mol. Biol. 56:503 – 516.
Ceballos, H., M. Fregene, J.C., Perez, N. Morante, and F. Calle. (2007). Cassava genetic improvement. P. 965 – In Breeding major food staples.
IITA (1990). Cassava in tropical Africa. A reference manual. International Institute of Tropical Agriculture, Ibadan, Nigeria.
Rojas M.C., Perez, J.C., Cellabos, H., Baena, D., Morante, N. and Calle, F Analysis of inbreeding depression in eight S1cassava families. Crop Science 49:
Kawano, K.F., W.M. Goncalves Fukuda, and U. Cednpukdee (1987). Genetics and environmental effects on dry matter content of cassava root. Crop Sci. 27:69-74.
Kawuki, R.S., Nuwamaya E., Labuschange, M.T., Herselman, L. and Ferguson, M.E (2011). Segregation of selected agronomic traits in six S1 cassava families. Journal of Plant Breeding and Crop Science 3(8):
Table 1: Origin of elite clone and size of their respective S1 families
Family
Female
Male
No of S1 genotypes evaluated
GCO 228
TMS 30572 90
GCO 223
TMS 01/1371 63
GCO 264
TMS 98/0002 44
GCO 219
TME 419 33
GCO 274
TMS 98/0505 25
Table 2: Inbreeding Depression (ID, as a percentage of the performance from the S0 generation) measured in five S1 Cassava Families based on the yield data.
Family
Plant height (cm)
Root Yield (kg plant-1)
Foliage Yield (kg plant-1)
Harvest Index (0 – 1)
DMC (%)
Vigour (1-5) S0 S1 ID
TME 419
170.00
303.50
-78.53
40.80
3.33
91.84
11.8
4.28
63.73
0.78
0.46
40.70
33.18
29.39
11.41
5.00
3.63
27.50
TMS 01/1371
210.00
220.45
-4.98
17.80
4.56
74.38
6.00
3.45
42.5
0.75
0.55
26.46
25.93
24.68
4.81
3.00
3.09
-3.03
TMS 30572
200.00
213.33
-6.67
39.20
4.19
89.31
18.90
5.07
73.17
0.67
0.48
28.86
33.69
26.86
20.28
2.78
44.44
TMS 98/0002
146.50
207.00
-41.30
23.60
2.76
88.31
7.55
2.88
61.85
0.76
0.49
35.32
29.88
24.43
18.24
4.00
3.40
15.00
TMS 98/0505
250.00
240.00
48.60
5.40
88.89
19.20
1.80
90.63
0.72
-4.63
28.15
23.41
16.85
3.19
20.25
Average
195.30
236.86
-25.49
34.00
4.05
86.55
12.69
3.50
66.38
0.73
25.34
30.17
25.75
14.32
4.20
3.22
20.83
Table 3: Ranges for Inbreeding Depression (ID, as a percentage of the performance from the S0 generation) measured in five S1 cassava families.
Family
Plant height (cm)
Root Yield
Foliage Yield
Harvest Index
DMC (%)
Vigour (0-1)
(kg plant-1)
(0-1)
Max
Min
TME 419
-47.06
91.84
89.22
84.75
22.03
40.7
9.75
26.81
-14.21
60.00
0.00
TMS 01/1371
2.38
-14.29
74.38
19.66
76.67
-26.67
26.46
1.06
33.43
-35.03
33.33
-66.67
TMS 30572
65.00
-95.00
89.31
75.77
90.48
19.57
28.86
9.59
34.29
8.38
20.00
TMS 98/0002
-22.87
-91.13
88.31
80.51
86.75
28.47
35.32
11.57
26.23
18.24
50.00
-25.00
TMS 98/0505
56.00
-12.00
88.89
94.79
90.62
-4.63
23.24
16.85
Average
10.69
-68.36
86.55
70.81
86.69
25.34
5.47
28.80
-1.15
50.67
-19.33