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Drought is a devastating environmental stress in agriculture and hence a common target of plant breeding. A review of breeding progress on drought tolerance shows that, to a certain extent, selection for high yield in stress-free conditions indirectly improves yield in water-limiting conditions. The objectives of this study were to (i) assess the genotype × environment (GE) interaction for grain yield (GY) and other agronomic traits for maize (Zea mays L.) across East African agro-ecologies; and (ii) evaluate agronomic performance and stability in Uganda and Tanzania under optimum and random drought conditions. Data were recorded for major agronomic traits. Genotype main effect plus GE (GGE) biplot analysis was used to assess the stability of varieties within various environments and across environments. Combined analysis of variance across optimum moisture and random drought environments indicated that locations, mean-squares for genotypes and GE were significant for most measured traits. The best hybrids, CKDHH1097 and CKDHH1090, gave GY advantages of 23% and 43%, respectively, over the commercial hybrid varieties under both optimum-moisture and random-drought conditions. Across environments, genotypic variance was less than the GE variance for GY. The hybrids derived from doubled-haploid inbred lines produced higher GY and possessed acceptable agronomic traits compared with the commercial hybrids. Hybrid CKDHH1098 ranked second-best under optimum-moisture and drought-stress environments and was the most stable with broad adaptation to both environments. Use of the best doubled-haploids lines in testcross hybrids make-up, well targeted to the production environments, could boost maize production among farmers in East Africa.

In Tanzania, the Lethal Decline (LDT), lethal yellowing-type disease, is the most destructive disease of the coconut palm. It is due to one phytoplasma (Mpunami et al., Plant Pathol., 48, 109-114. 1999) belonging to the 16S-IV group. Although the disease spreads the full coastal belt of Tanzania, its incidence varies between the north where it is low and the south of the country where the incidence is very high. To evaluate the LDT phytoplasma diversity, the 16S rRNA gene and the 16S-23S rRNA spacer region have been amplified using the P1/P7 primer pair (Smart et al., Appl. Environ. Microbiol., 62, 2988-2993. 1996), and the PCR product sequenced for 15 samples of diseased coconut palms collected in the different coastal region of Tanzania. Comparison of the sequences revealed 5 different genotypes named TZ-I to TZ-V and distributed from the north up to the south of the country. The genotype TZ-I and TZ-II are observed in the north of Tanzania only, whereas the genotype TZ-III and TZ-IV are present in the central regions and the genotype TZ-V in the south exclusively. The distribution of the different genotypes could explain the differences in the incidence of the disease, and could be associated with the history of the coconut introduction in Tanzania also. (Texte intégral)

Different DNA marker types were used to construct linkage maps in coconut (Cocos nucifera L.; 2n = 32). In this study, we present a reference map in a mapping population from con-trolled cross involving the genotype EAT0707 representing an East African Tall ecotype. More-over, five other satellite maps involving different "Tall" and "Dwarf" coconut ecotypes such as EAT 1011, RLT0710, LAGT07, MYD20, RIT(PO2664) are also available. The EAT0707 map contains actually a total of 707 markers and has a length of 2083 cM. The 16 linkage groups corresponding to the 16 chromosomes have between 22 and 77 markers each and vary between 99 and 130 cM in length. The other five linkage maps (except RLT0710), are composed of 16 linkage groups and contain between 39 and 253 markers. The lengths of these maps vary between 243 and 2478 cM. Furthermore, several QTL related to growth and other characters of interest have been identified in the different populations.

The coconut palm (Cocos nucifera L.; 2n = 32) and the oil palm (Elaeis guineensis JACQ.; 2n = 32) are the two most important perennial oil crops of the tropics with the oil being the basis for both food production and industrial processing. The general objectives of a multinational project funded by the EC under the INCO-DEV programme are (i) to provide the methodological basis and molecular tools for improving the breeding efficiency in coconut and oil palm; and (ii) to develop DNA marker-based breeding strategies in collaboration with the most important countries in coconut and oil palm production and to directly transfer to developing countries small-scale technological solutions for the genetic improvement of these tropical oil crops. This is being achieved through the construction of high density molecular linkage maps in coconut and oil palm by developing SSR, AFLP and SNP markers, by performing comparative QTL analyses, by developing user-friendly molecular marker sets for future practical applications, by the generation of cosmid libraries for coconut and oil palm parents of the reference populations and physical mapping by SNP, and by synteny studies for coconut and oil palm in terms of comparative genome (microsequencing, SSR synteny) and QTL analyses. In the frame of these activities, cosmid libraries for coconut and oil palm representing some 4-5 genome equivalents were individualized, spotted onto filters and used in the isolation of important genes (oil biosynthesis, putative resistance genes). Different PCR-based DNA marker types were established to construct linkage maps for both coconut and oil palm. Polymorphic AFLP markers presented the majority of dominant DNA markers mapped. Currently, co-dominant SSR and SNP markers are being evaluated to serve as anchor markers for the integration of the available individual genotype-specific maps into general reference maps for coconut and oil palm. Mapping of QTLs related to flower morphology, nut production and oil content will be undertaken in the near future. The co-segregation of DNA markers with these QTLs will provide opportunities to breeders for marker-assisted selection in breeding programmes. (Texte intégral)

Cassava brown streak disease (CBSD) was described for the first time in Tanganyika (now Tanzania) about seven decades ago. Tanganyika (now Tanzania) about seven decades ago. It was endemic in the lowland areas of East Africa and inland parts of Malawi and caused by Cassava brown streak virus (CBSV; genus Ipomovirus; Potyviridae). However, in 1990s CBSD was observed at high altitude areas in Uganda. The causes for spread to new locations were not known.The present work was thus initiated to generate information on genetic variability, clarify the taxonomy of the virus or viruses associated with CBSD in Eastern Africa as well as to understand the evolutionary forces acting on their genes. It also sought to develop a molecular based diagnostic tool for detection of CBSD-associated virus isolates. Comparison of the CP-encoding sequences of CBSD-associated virus isolates collected from Uganda and north-western Tanzania in 2007 and the partial sequences available in Genbank revealed occurrence of two genetically distinct groups of isolates. Two isolates were selected to represent the two groups. The complete genomes of isolates MLB3 (TZ:Mlb3:07) and Kor6 (TZ:Kor6:08) obtained from North-Western (Kagera) and North-Eastern (Tanga) Tanzania, respectively, were sequenced. The genomes were 9069 and 8995 nucleotides (nt), respectively. They translated into polyproteins that were predicted to yield ten mature proteins after cleavage. Nine proteins were typical in the family Potyviridae, namely P1, P3, 6K1, CI, 6K2, VPg, NIa-Pro, NIb and CP, but the viruses did not contain HC-Pro. Interestingly, genomes of both isolates contained a Maf/HAM1-like sequence (HAM1h; 678 nucleotides, 25 kDa) recombined between the NIb and CP domains in the 3’-proximal part of the genomes. HAM1h was also identified in Euphorbia ringspot virus (EuRSV) whose sequence was in GenBank. The HAM1 gene is widely spread in both prokaryotes and eukaryotes. In yeast (Saccharomyces cerevisiae) it is known to be a nucleoside triphosphate (NTP) pyrophosphatase. Novel information was obtained on the structural variation at the N-termini of polyproteins of viruses in the genus Ipomovirus. Cucumber vein yellowing virus (CVYV) and Squash vein yellowing virus (SqVYV) contain a duplicated P1 (P1a and P1b) but lack the HC-Pro. On the other hand, Sweet potato mild mottle virus (SPMMV), has a single but large P1 and has HC-Pro. Both virus isolates (TZ:Mlb3:07 & TZ:Kor6:08) characterized in this study contained a single P1 and lacked the HC-Pro which indicates unique evolution in the family Potyviridae. Comparison of 12 complete genomes of CBSD-associated viruses which included two genomes characterized in this study, revealed genetic identity of 69.0–70.3% (nt) and amino acid (aa) identities of 73.6–74.4% at polyprotein level. Comparison was also made among 68 complete CP sequences, which indicated 69.0-70.3 and 73.6-74.4 % identity at nt and aa levels, respectively. The genetic variation was large enough for dermacation of CBSD-associated virus isolates into two distinct species. The name CBSV was retained for isolates that were related to CBSV isolates available in database whereas the new virus described for the first time in this study was named Ugandan cassava brown streak virus (UCBSV) by the International Committee on Virus Taxonomy (ICTV). The isolates TZ:Mlb3:07 and TZ:Kor6:08 belong to UCBSV and CBSV, respectively. The isolates of CBSV and UCBSV were 79.3-95.5% and 86.3-99.3 % identitical at nt level, respectively, suggesting more variation amongst CBSV isolates. The main sources of variation in plant viruses are mutations and recombination. Signals for recombination events were detected in 50% of isolates of each virus. Recombination events were detected in coding and non-coding (3’-UTR) sequences except in the 5’UTR and P3. There was no evidence for recombination between isolates of CBSV and UCBSV. The non-synonomous (dN) to synonomous (dS) nucleotide substitution ratio (ω) for the HAM1h and CP domains of both viruses were ≤ 0.184 suggesting that most sites of these proteins were evolving under strong purifying selection. However, there were individual amino acid sites that were submitted to adaptive evolution. For instance, adaptive evolution was detected in the HAM1h of UCBSV (n=15) where 12 aa sites were under positive selection (P< 0.05) but not in CBSV (n=12). The CP of CBSV (n=23) contained 12 aa sites (p