Bettina I. G. Haussmann

Improved methodologies for breeding striga-resistant sorghums.

Parasitic flowering weeds of the genus Striga (Scrophulariaceae) cause substantial losses in sorghum [Sorghum bicolor (L.) Moench] production in sub-Saharan Africa. Striga-resistant sorghum cultivars could be a major component of integrated striga management, if resistance was available in adapted, productive germplasm. In this paper we review methodologies for breeding striga-resistant sorghums. The agar-gel assay is an excellent tool to screen host genotypes in the laboratory for low production of the striga seed germination stimulant. Further laboratory assays are needed which allow the non-destructive, rapid and inexpensive evaluation of individual plants for additional resistance mechanisms. Field screening for striga resistance is hampered by high microvariability in African soils, heterogeneity of natural infestations, and concomitant large environmental effects on striga emergence. An improved field testing methodology should include one or several of the following practices: field inoculation with striga seeds; appropriate experimental design including elevated replication number; specific plot layout; use of appropriate susceptible and resistant checks; evaluation in adjacent infested and uninfested plots; and the use of selection indices derived from emerged striga counts, striga vigor, and grain yield or a host plant damage score. Due to the extreme variability of the parasite and significant genotype×environment interaction effects, multi-locational screening is recommended to obtain materials with stable performance. Additional strategies include: careful definition of the target environments; determination of the most important selection traits in each target environment; characterization of crop germplasm and improvement of available sources of resistance for better agronomic performance; transfer and pyramiding of resistance genes into adapted, farmer-selected cultivars; development of striga-resistant parent lines for hybrid or synthetic cultivars; and development of random-mating populations with multiple sources of resistance. The development of marker-assisted selection techniques for broad-based, polygenic striga resistance is underway. This approach is particularly promising because striga resistance tests are difficult, expensive, and sometimes unreliable; the parasite is quarantined; and some resistance genes are recessive. Transgenic, herbicide-tolerant sorghums could contribute to an immediate, cost-effective control of striga by herbicides, but such cultivars are not yet available. The selection of sorghum cultivars with specific adaptation to integrated striga management approaches could contribute to sustainable sorghum production in striga-infested areas of sub-Saharan Africa

Evolutionary History of Pearl Millet (Pennisetum glaucum [L.] R. Br.) and Selection on Flowering Genes since Its Domestication

The plant domestication process is associated with considerable modifications of plant phenotype. The identification of the genetic basis of this adaptation is of great interest for evolutionary biology. One of the methods used to identify such genes is the detection of signatures of selection. However, domestication is generally associated with major demographic effects. It is therefore crucial to disentangle the effects of demography and selection on diversity. In this study, we investigated selection in a flowering time pathway during domestication of pearl millet. We first used a random set of 20 genes to model pearl millet domestication using approximate Bayesian computation. This analysis showed that a model with exponential growth and wild-cultivated gene flow was well supported by our data set. Under this model, the domestication date of pearl millet is estimated at around 4,800 years ago. We assessed selection in 15 pearl millet DNA sequences homologous to flowering time genes and showed that these genes underwent selection more frequently than expected. We highlighted significant signatures of selection in six pearl millet flowering time genes associated with domestication or improvement of pearl millet. Moreover, higher deviations from neutrality were found for circadian clock-associated genes. Our study provides new insights into the domestication process of pearl millet and shows that a category of genes of the flowering pathway were preferentially selected during pearl millet domestication.

Quantitative-genetic parameters of sorghum [Sorghum bicolor (L.) Moench] grown in semi-arid areas of Kenya

Low and erratic rainfall constitutes a major constraint to sorghum production, and impedes sorghum improvement in semi-arid tropics. To estimate quantitative-genetic parameters for sorghum under variable stress conditions, three sets of factorial crosses between four by four lines each were grown with parents and a local cultivar in eight macro-environments in semi-arid areas of Kenya. Fourteen traits were recorded including grain yield, above-ground drymatter, harvest index, days to anthesis, leaf rolling score, and stay-green. Environmental means for grain yield ranged from 167 to 595 g m-2. Mean hybrid superiority over mid-parent values was 47, 31, and 9% for grain yield, above-ground drymatter, and harvest index, respectively. Differences among both lines and hybrids were highly significant for all traits. Genotype × environment interaction variances were larger than genotypic variances for grain yield, above-ground drymatter, and harvest index. Corresponding heritabilities ranged between 0.72 and 0.84. Variation among hybrids was determined by GCA and SCA effects for most characters. Predominance of additive-genetic effects was found for grain yield components, plant height, and leaf rolling score. Lack of variation in GCA was noted among female lines for major performance traits. While low leaf rolling score was correlated with high grain yield, there was no such association for stay-green. Hybrid breeding could contribute to sorghum improvement for semi-arid areas of Kenya. To increase selection progress for major performance traits, genetic variation among female lines should be enhanced. Importance of genotype × environment interaction underlines the necessity of evaluating breeding materials under a broad range of dryland conditions.

Analysis of resistance to Striga hermonthica in diallel crosses of sorghum

Parasitic flowering weeds of the genus Striga aremajor biotic constraints to sorghum (Sorghum bicolor(L.) Moench) production in sub-Saharan Africa. The agar-gel assay was used to evaluate stimulation ofStriga hermonthica (Del.) Benth. seed germination by a complete F1 diallel involving nine sorghum cultivars and inbred lines. Striga populations from Mali and Niger were employed. The same genetic materials were planted in pot trials in both countries to observe striga plant emergence. Variation in hybrid performance was determined by general (GCA) and specific combining ability (SCA) effects, with preponderance of GCA, for both germination distance in the agar-gel assay and number of emerged striga. Reciprocal effects were significant only in the agar-gel assay and were unstable across striga populations. For lines and hybrids, estimates of broad-sense heritabilities were 0.97 and 0.91 for germination distance, and 0.38 and 0.58 for emerged striga, respectively. Only a weak positive relationship existed between in vitro germination distance and emerged striga number in the pot trial. Although selection for low germination distance has merit, valuable material with resistance mechanisms other than low stimulant production may be lost if these traits are not additionally assessed. Laboratory assays which allow a non-destructive, quick and economical screening for resistance mechanisms other than the low stimulant character are likely to increase the efficiency of breeding programs for striga resistance. The significant contribution of SCA effects indicates that thorough screening of testcrosses is indispensable for selection in hybrid sorghum breeding programs.

Phosphorus uptake and use efficiency of diverse West and Central African sorghum genotypes under field conditions in Mali

AimsSorghum [Sorghum bicolor (L.) Moench], a staple crop in West and Central Africa (WCA), is mostly cultivated on soils with low phosphorus (P) availability and thus adaptation to those conditions is vital for food security. Assessment of genotypic variation of WCA sorghum for P uptake and P use efficiency is undertaken to understand the diversity available and opportunities for its use.MethodWe assessed mature plant yield, P uptake and P use efficiency traits of 70 diverse WCA sorghum genotypes under –P (no P fertilization) and + P field conditions in Mali in 2010, to discover differences among all genotypes tested and between and within specific genotype groups.ResultsLarge significant genotypic variation for P uptake and P use efficiency traits were observed for all genotypes among and within landrace and researcher bred pools under –P conditions. P uptake traits had a larger genotypic variation than P use efficiency traits. Landrace genotypes showed generally higher P uptake and grain P concentration while formally bred genotypes exhibited a higher P use efficiency. Photoperiod sensitivity was related to higher P uptake.ConclusionGenotypic selection for P uptake and P use efficiency traits to improve adaptation to low P soils is possible in sorghum. Use and further study of WCA sorghums for adaptation to low P availability is appropriate as this germplasm shows large variation for P uptake and use efficiency and higher levels of P use efficiency than other important cereals.

Impact of farmers' practices and seed systems on the genetic structure of common sorghum varieties in Kenya and Sudan

To understand the effect of different farming systems on the dynamics of diversity of sorghum (Sorghum bicolor (L.) Moench) crop, genetic structure of widely used landraces and modern varieties collected from two contrasting agroecosystems, in eastern Sudan and western Kenya, were analysed with 16 polymorphic microsatellite markers. A total of 1104 accessions, grouped into 46 samples from individual farmers, were genotyped. Cluster analysis of the samples from the two countries displayed contrasting patterns. Most strikingly, differently named landraces from western Kenya formed widely overlapping clusters, indicating weak genetic differentiation, while those from eastern Sudan formed clearly distinguishable groups. Similarly, samples of the modern variety from Sudan displayed high homogeneity, whereas the most common modern variety from western Kenya was very heterogeneous. The high degree of fragmentation of farmlands of western Kenya, coupled with planting of different sorghum varieties in the same fields, increases the likelihood of inter-variety gene flow. This may explain the low genetic differentiation between the differently named landraces and heterogeneity of the modern variety from western Kenya. This study highlights the important role of farmers in shaping the genetic variation of their crops and provides population parameter estimates allowing forecasting of the fate of ‘modern’ germplasm (conventional or genetically modified) when introduced into subsistence farming systems.

Identification of geographical gaps in the pearl millet germplasm conserved at ICRISAT genebank from West and Central Africa

The International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) genebank in India holds the world’s largest collection of 21,594 pearl millet germplasm accessions including 18,447 landraces from 50 countries. West and Central Africa (WCA) region, which is considered as the centre of diversity for pearl millet, is also an important pearl millet germplasm source for resistance to biotic and abiotic stresses. A total of 7372 landraces were assembled from WCA countries. Out of which, 6434 landraces have the georeference data. The geographic origins of these landraces were analyzed using geographic information system tools to identify gaps in the collection. Geographical distribution of existing collections, type of vegetation, land cover and the high probability (.70%) for the occurrence of pearl millet estimated using the FloraMap software in different countries show that 62 districts in 13 provinces of Nigeria, 50 districts in 16 provinces of Burkina Faso, 9 districts in 6 provinces each of Mali and Mauritania, 8 districts in 8 provinces of Chad and 7 districts in 3 provinces of Ghana as the major geographical gaps in the pearl millet collection at the ICRISAT genebank. In view of this, we suggest that the final areas for exploration in these districts should be decided prior to the launch of the collection missions in consultation with local government officials and extension officers, who have the knowledge of pearl millet cultivation in the districts identified.

Overcoming Phosphorus Deficiency in West African Pearl Millet and Sorghum Production Systems: Promising Options for Crop Improvement.

West Africa (WA) is among the most food insecure regions. Rapid human population growth and stagnating crop yields greatly contribute to this fact. Poor soil fertility, especially low plant available phosphorus (P) is constraining food production in the region. P-fertilizer use in WA is among the lowest in the world due to inaccessibility and high prices, often unaffordable to resource-poor subsistence farmers. This article provides an overview of soil P-deficiency in WA and opportunities to overcome it by exploiting sorghum and pearl millet genetic diversity. The topic is examined from the perspectives of plant breeding, soil science, plant physiology, plant nutrition, and agronomy, thereby referring to recent results obtained in a joint interdisciplinary research project, and reported literature. Specific objectives are to summarize: (1) The global problem of P scarcity and how it will affect WA farmers; (2) Soil P dynamics in WA soils; (3) Plant responses to P deficiency; (4) Opportunities to breed for improved crop adaptation to P-limited conditions; (5) Challenges and trade-offs for improving sorghum and pearl millet adaptation to low-P conditions in WA; and (6) Systems approaches to address soil P-deficiency in WA. Sorghum and pearl millet in WA exhibit highly significant genetic variation for P-uptake efficiency, P-utilization efficiency, and grain yield under P-limited conditions indicating the possibility of breeding P-efficient varieties. Direct selection under P-limited conditions was more efficient than indirect selection under high-P conditions. Combining P-uptake and P-utilization efficiency is recommendable for WA to avoid further soil mining. Genomic regions responsible for P-uptake, P-utilization efficiency, and grain yield under low-P have been identified in WA sorghum and pearl millet, and marker-assisted selection could be possible once these genomic regions are validated. Developing P-efficient genotypes may not, however, be a sustainable solution in itself in the long-term without replenishing the P removed from the system in harvested produce. We therefore propose the use of integrated soil fertility management and systems-oriented management such as enhanced crop-tree-livestock integration in combination with P-use-efficiency-improved varieties. Recycling P from animal bones, human excreta and urine are also possible approaches toward a partially closed and efficient P cycle in WA.

Diallel analysis of sooty stripe resistance in sorghum

Sooty stripe [Ramulispora sorghi (Ellis and Everhart) Olive and Lefebre] is a widespread foliar disease of sorghum [Sorghum bicolor (L.) Moench] in West Africa, responsible for grain yield losses up to 46%. We studied the inheritance of sooty stripe resistance in a 9 × 9 sorghum F2-population diallel grown together with parent lines and checks in1996 under natural disease pressure at two locations in Mali. The percentage of infected leaf area was determined twice over a two-week interval during the season. At the second evaluation, the mean sooty stripe severity amounted to 13% infected leaf area at Samanko and 12% at Cinzana. The frequency distribution of the entries was approximately normal for the mean disease severity, averaged across assessment dates and locations, pointing to the involvement of multiple genes. With the data combined across the two locations, genetic differences among lines and among F2 populations were highly significant. Genotype × location interaction variances were also significant but much smaller than the genetic variances. Broad-sense heritability estimates were 0.92 for lines and 0.94 for the F2 populations, for the mean percentage infected leaf area across the two assessment dates. General combining ability effects (GCA) determined most of the differences among the F2 populations. Specific combining ability effects (SCA), and the interactions of GCA or SCA with locations were also significant but less important. Line performance per se was highly correlated with GCA. Because of the high heritability and predominance of additive effects, prospects are good for the genetic improvement of resistance to sooty stripe in sorghum in Mali, using simple pedigree or recurrent selection procedures.

Farmer participatory varietal selection in pearl millet: Experience across some states of Northern Nigeria

Farmers participation in the process of on-farm research does not only enrich the speed up of information gathering, but also result in large scale adoption of the product of research. A small farmer deals with a variable environment and has multiple production objectives that will affect his or her choice of crops and selection of genotypes. In areas where farmers are unfamiliar with available improved varieties, there is need of conducting effective variety evaluations with farmers. The usefulness of the participatory approach for identifying cultivars for harsh environments, which are difficult to replicate in research stations, has been recognized by the crop breeders. Participatory plant breeding/selection has shown success in identifying more number of preferred varieties by farmers in shorter time (than the conventional system), in accelerating their dissemination and increasing cultivar diversity. This paper describes how plant breeders and farmers worked together to test and selected farmers preferred pearl millet varieties; PE05684 and PE05532 from a diverse pearl millet accessions in a participatory varietal selection program conducted across some states of Northern Nigeria.