1. Loss of Materials at different stages in cassava
breeding program
Olasanmi B., 1,2,3 M.O. Akoroda1, E. Okogbenin2,3, C. Egesi3 and M. Fregene2,4 ­­
1Department of Agronomy, University of Ibadan, Nigeria; 2International Centre for Tropical Agriculture (CIAT), Cali, Colombia;
3National Root Crops Research Institute (NRCRI), Umudike, Nigeria, 4Donald Danforth Plant Science Center, St Louis, MO 63132
Abstract
Table 2: Loss of materials (%) at different stages of development and evaluation of genotypes in nine cassava hybrid populations at Umudike, Nigeria
Stages in cassava breeding program include hybridization, raising of seedlings and subsequent evaluation in respect to the breeder’s interest. Materials (genotypes) are lost at each stage which may render a new population inadequate for the intended breeding program. Loss of materials at different stages of cassava breeding program was investigated in Nigeria using nine hybrid populations. Seedlings were raised from true seeds and the plants were evaluated at seedling, clonal and preliminary evaluation stages. Loss of materials ranged between 91.2 and 97.4% with average of 94.1% among the populations. The loss was very high at the early stages (74.3 and 60.7% during hybridization and raising of seedlings respectively) but drastically reduced as the study progressed (7.9 and 5.3% during clonal and preliminary evaluation trials respectively). This shows that large population sizes are critical in identification of rare recombinants in the development of improved cassava varieties. High loss of materials observed at early stages indicates that hybridization and germination of resultant hybrid seeds are more critical stages in cassava breeding programs, hence, the need for generation of large populations so that breeders can have enough materials from which to select the desirable genotypes. A
Populations
Hybridization
Raising of seedlings
Seedling evaluation stage
Clonal Evaluation stage
Preliminary evaluation stage
COB-1
79.2
48.7
7.2
1.9
0.0
COB-2
80.0
50.6
11.8
4.2
2.3
COB-3
72.4
60.1
14.5
3.9
4.5
COB-4
80.1
64.6
18.2
5.4
3.2
COB-5
78.4
64.5
5.3
2.4
COB-6
75.6
74.3
17.8
29.7
8.3
COB-7
67.0
61.8
20.2
11.5
6.0
COB-8
71.1
60.0
28.1
7.6
12.2
COB-9
61.9
17.5
4.6
Mean
60.7
15.6
7.9 B C
Keywords::Hybridization, breeding program, hybrid populations, seed germination, genotypes, percentage loss
Preamble
Cassava (Manihot esculenta Crantz) is the most important of the root crops in the tropics and ranks fourth after rice, sugarcane and maize as a source of calorie for human needs (CIAT, 1992; IITA, 2000). Cassava is a highly heterozygous species and has extremely high inbreeding depression (CIAT, 1974) which may result in loss of vigour to the extent that it may be very difficult or impossible to advance some progeny beyond early evaluation stages in a breeding programme. Sexual seeds are used mostly in cassava breeding programmes to generate new genetic variation. However, botanical seeds have also been used occasionally in commercial propagation schemes (Rajendran et al., 2000). One or two seeds are obtained per pollination on average in cassava (Ceballos et al., 2004). Stages in breeding program includes identification, isolation, variation, evaluation and selection. Steps taken in most cassava breeding programmes are: hybridization, raising of seedlings and subsequent evaluation in respect to the breeder’s interest. Materials (genotypes) are lost at each of these stages which may render a new hybrid population inadequate for the intended breeding program or result in loss of promising genotypes in relation to the trait under investigation. The aim of this study was therefore to assess loss of materials at different stages of cassava breeding programme and suggest ways to prevent or reduce the loss.
Materials and Methods
Loss of materials at different stages of cassava breeding program was investigated in Nigeria using nine hybrid populations developed from crosses among six cassava varieties (Table 1). Seedlings were raised from the true seeds generated in each population and the plants were evaluated at seedling, clonal and preliminary evaluation stages using stem cuttings. Loss of materials (%) was estimated as a ratio of number of expected genotypes or number of genotypes at the beginning of a stage to the number of generated or number of genotypes at the end of the stage. The observations made and the precautions taken at each stage of this study were used to proffer possible solutions to some of the problems that may cause loss of materials in a cassava breeding programme.
Conclusion and Recommendations
High percentage loss which culminated to about 94% after preliminary evaluation trial means that cassava breeding programme requires generation of large populations so that breeders can have enough materials from which to select the best desirable genotypes. The following recommendations are therefore made to reduce loss of materials in cassava breeding programmes and ensure generation of large population needed in routine cassava breeding programme.
Use of prolific varieties as parents in cassava breeding program
Pollination at the peak of flowering
Adhering strictly to the best method of pollination
Provision of good remuneration and welfare packages as well as conducive working environment for members of the pollination team
Breeders should employ any technique that can improve germination of the seeds e.g. covering of beds in the nursery using black polythene after sowing to increase the soil temperature, embryo rescuing etc.
Good agronomic practices during evaluation of new genotypes on the field
Backup of the new genotypes in-vitro and good maintenance of germplasm (in-vivo and in-vitro)
Use of competent workers in handling breeding programs and training/retraining of the workers to update their knowledge and skills
Table 1: Nine cassava hybrid populations developed from six parents in Nigeria
Table 1: Nine cassava hybrid populations developed from six parents in Nigeria
Population
Female parent
Male parent
Number of seeds generated and sowed
Number of emerged seedlings
COB-1
TMS 30572
TMS 30555
460
236
COB-2
NR 8212
326
161
COB-3
NR 8083
331
132
COB-4
TMS 97/2205
426
151
COB-5
383
136
COB-6
408
105
COB-7
TMS 98/0505
730
279
COB-8
500
200
COB-9
570
217
Total
4134
1617
Average
459.3
179.7
Acknowledgement
Results and Discussion
This study was conducted with funding support from Kirkhouse Trust Foundation, UK and with support from University of Ibadan, Nigeria; International Center for Tropical Agriculture (CIAT), Cali, Colombia; and National Root Crops Research Institute (NRCRI), Umudike. The fund for attending this conference was provided by Bill and Melinda Gates Foundation through the organizers of GCP21.
Average loss of materials was very high at the early stage of this study (74.3 and 60.7% at hybridization and raising of seedlings stages respectively) but drastically reduced as the study progressed (7.9 and 5.3% during clonal and preliminary evaluation trials respectively) (Table 2). Total loss of materials ranged between 91.2% and 97.4 with average of 94.1% among the nine hybrid populations (Fig. 1). The total loss is significantly higher in population COB-6 than in all other populations except COB-4 and in population COB-4 than in populations COB-1, COB-3, COB-7 and COB-9. The total loss is also significantly higher in populations COB-1, COB-2, COB-8 and COB-5 than in population COB-9 (Fig. 1). High loss of materials observed at early stages of this study indicates that there are many factors militating against successful hybridization among cassava varieties and germination of resultant seeds, hence, these two stages are more critical in a cassava breeding program. About 6% of the materials started with at hybridization stage in this study survived until the end of preliminary evaluation trial. This shows that large population sizes are critical in identification of rare recombinants in the development of improved cassava varieties.
References
Ceballos, H., C. A. Iglesias, J. C. Perez and A. G. O. Dixon (2004). Cassava breeding: opportunities and challenges, Plant Molecular Biology 56:
CIAT, Centro Internacional de Agricultura Tropical. Annual Report for Cali, Colombia.
CIAT, Cassava Program Annual Report for Cali, Colombia.
IITA Starting a cassava farm: IPM Field Guide for Extension Agents. 4-10
Rajendran, P.G., Ravindran, C.S., Nair, S.G. and Nayar, T.V.R True cassava seeds (TCS) for rapid spread of the crop in non-traditional areas. Thiruvananthapuram, Kerala, India: Central Tuber Crops Research Institute (Indian Council of Agricultural Research)
Poster presented at GCP-21-II Conference in Utebbe, Uganda from 18th - 22nd June, 2012.
National Root Crops Research Institute, Umudike, PMB 7006, Umuahia, Nigeria.