Over the past 10 years, resources have been established for the genetic analysis of pearl millet, Pennisetum glaucum (L.) R. Br., an important staple crop of the semi-arid regions of India and Africa. Among these resources are detailed genetic maps containing both homologous and heterologous restriction fragment length polymorphism (RFLP) markers, and simple sequence repeats (SSRs). Genetic maps produced in four different crosses have been integrated to develop a consensus map of 353 RFLP and 65 SSR markers. Some 85% of the markers are clustered and occupy less than a third of the total map length. This phenomenon is independent of the cross. Our data suggest that extreme localization of recombination toward the chromosome ends, resulting in gaps on the genetic map of 30 cM or more in the distal regions, is typical for pearl millet. The unequal distribution of recombination has consequences for the transfer of genes controlling important agronomic traits from donor to elite pearl millet germplasm. The paper also describes the generation of 44 SSR markers from a (CA)n-enriched small-insert genomic library. Previously, pearl millet SSRs had been generated from BAC clones, and the relative merits of both methodologies are discussed.

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The effectiveness of early generation selection for grain yield in a cowpea [Vigna unguiculata (L.) Walp.] population was examined in the Guinea and Sudan savanna agroecologies of Ghana where genotype × location interaction is known to be large. A set of 131 F3:4 lines were developed from a cross between a local cultivar and an unadapted source of large grain size. Mild selection was practiced during line development at one location in the Guinea savanna zone to eliminate poorly adapted lines. Unreplicated F3 plant data were collected on all the lines at the one location during the development of the lines. Multilocation trials were conducted with lines formed by bulk harvest of F4 families to assess how effectively the early generation selection protocol was able to generate superior lines for the target agroecology. Genotypic correlation for grain yield between locations was high only between the two locations in the Guinea savanna zone. Narrow‐sense heritability estimates were low and not different from zero for grain yield, but heritability estimates for days to flowering and seed size were large. F4 lines derived from the highest 10% performing F3 individuals were no higher yielding than F4 lines derived from the remaining F3 individuals, indicating that early generation selection for yield was ineffective. Single‐seed descent (SSD) or bulk breeding methods will be more efficient than pedigree breeding for developing cowpea varieties with high yield potential for this agroecology.

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High seedling mortality during the establishment phase of cocoa has become a critical constraint to sustainable cocoa farming. The objective of this study was to develop varieties with higher seedling survival compared with cultivars currently recommended for planting in denuded regions. Thirty‐seven families composed into two sets of 18 (families from recommended clones) and 19 (families from recently introduced clones) were evaluated for field survival under full sunlight and dry weather conditions. Families differed significantly for vigour (increase in trunk cross‐sectional area, TCSA) and percentage of survived seedlings by the end of the dry weather conditions. Survival and vigour were significantly correlated in only one set of families. The contribution of general combining ability (GCA) to seedling survival was low to moderate. An Amelonado clone and clones from the Iquitos genetic group had negative GCA estimates for both increase in TCSA and survival. Clones of Scavina origin had the most positive contribution to survival. The key outcome of the study was that cocoa families with higher establishment success can be obtained from the currently available germplasm.

Pearl millet is one of the most important subsistence crops grown in India and sub-Saharan Africa. In many cereal crops, reduced height is a key trait for enhancing yield, and dwarf mutants have been extensively used in breeding to reduce yield loss due to lodging under intense management. In pearl millet, the recessive d2 dwarfing gene has been deployed widely in commercial germplasm grown in India, the United States, and Australia. Despite its importance, very little research has gone into determining the identity of the d2 gene. We used comparative information, genetic mapping in two F2 populations representing a total of some 1500 progeny, and haplotype analysis of three tall and three dwarf inbred lines to delineate the d2 region by two genetic markers that, in sorghum, define a region of 410 kb with 40 annotated genes. One of the sorghum genes annotated within this region is ABCB1, which encodes a P-glycoprotein involved in auxin transport. This gene had previously been shown to underlie the economically important dw3 dwarf mutation in sorghum. The cosegregation of ABCB1 with the d2 phenotype, its differential expression in the tall inbred ICMP 451 and the dwarf inbred Tift 23DB, and the similar phenotype of stacked lower internodes in the sorghum dw3 and pearl millet d2 mutants suggest that ABCB1 is a likely candidate for d2.

SU MMARYSelection for specific adaptation in a breeding programme may be required when significant genotyperenvironment (GrE) interaction occurs within a target production region. Knowledge of the key stress factors underlying the GrE interaction permits delineating a production region into homogeneous zones for the purpose of recommending specific varieties. Twenty-eight cowpea genotypes were evaluated in 2002 and 2003 at four locations in the Guinea and Sudan savannah zones of Ghana, to assess grain yield stability and its relationship with intensity of stress in each production environment. Data were analysed in a year-location cross-classification, producing eight environments. Significant variations were observed between the genotypes for grain yield and grain yield components, biomass production, days to maturity, and resistance to Striga, pod-sucking bugs, and bacterial blight severity. Across environments, stress intensity (in terms of grain yield) was negatively correlated with the genotypic and error variances but positively correlated with the phenotypic, genotypic and experimental coefficients of variation. Stress intensity was also negatively correlated with heritability for grain yield. Genotypic correlations for grain yield between locations were moderate to high. The b-value as a measure of yield stability showed positive correlations with mean yield over all environments, and also with average yield in each of the three most favourable locations. In the least favourable location, the b-value showed a weak inverse relationship (P=0 . 10) with grain yield. Although direct selection for grain yield was considered superior in the target environments, simultaneous selection for grain yield and tolerance to stress in the less favourable environments will identify genotypes broadly adapted to the major cowpea growing regions.

In the Sudano-Sahelian zones of West Africa, cultivated land is degraded due to reduced fallow periods and Striga hermonthica (Del.) Benth. infestation, and poses a serious threat to sustainable food production. To provide options for addressing these problems, we investigated the possibility of replacing the short fallow periods with food legume cultivation. We tested this in two farming communities within the Sudan savannah zone of Ghana. In the 1997 and 1998 cropping seasons, selected farmers within each community planted soybean [Glycine max. (L.) Merr.] or bambara groundnuts [Vigna subterranea (L.) Verdc.] on continuously cropped, Striga-infested farmlands, whereas lands that were maintained under two or three years of bush fallowing were planted to millet [Pennisetum glaucum (L.) R. Br.] or sorghum [Sorghum bicolor (L.) Moench]. Analysis of soil sampled from 0-15 cm (plough layer) revealed that contents of major plant nutrients were inadequate to support long-term crop production. Within each farming community, total nitrogen content at initial sampling showed significant negative correlation with the number of Striga seeds in the plough layer. Under legume cultivation, soil organic carbon content increased and the number of Striga seeds per square metre decreased in both communities from a mean value of 28 183 seeds m À2 to 8185 seeds m À2. Under cereal cultivation Striga seed density increased from 9383 seeds m À2 to 16 696 seeds m À2. The study provides evidence that introduction of food legume cultivation on farmlands with high S. hermonthica infestation as an alternative to bush fallow system could help achieve sustainable crop production in the Sudan savannah zones of sub-Saharan Africa.