M. T. Labuschagne

Characterization and genetic distance analysis of cassava (Manihot esculenta Crantz) germplasm from Mozambique using RAPD fingerprinting

Twenty-eight cassava genotypes from Mozambique, along with seven genotypes from Angola, Madagascar, Nigeria, Togo, Columbia, and Thailand for comparison, were fingerprinted using random amplified polymorphic DNA (RAPD) analysis. The Mozambican material represented a wide range of landraces. A total of 311 scored RAPD loci were used to calculate genetic distances between the genotypes. This revealed an average genetic distance of 3.1% between all the germplasm. The average genetic distance between the Mozambiquen genotypes was 2.7%, whilst the seven accessions from the other countries showed an average distance of 3.4%. Neighbor-joining (NJ) method cluster analysis of the genetic distance yielded a tree that did not indicate a relationship between geographic distribution and genetic diversity.

Ecogeographical distribution of wild, weedy and cultivated Sorghum bicolor (L.) Moench in Kenya: implications for conservation and crop-to-wild gene flow

The potential gene flow between a crop and its wild relatives is largely determined by the overlaps in their ecological and geographical distributions. Ecogeographical databases are therefore indispensable tools for the sustainable management of genetic resources. In order to expand our knowledge of Sorghum bicolor distribution in Kenya, we conducted in situ collections of wild, weedy and cultivated sorghum. Qualitative and quantitative morphological traits were measured for each sampled wild sorghum plant. Farmers’ knowledge relating to the management of sorghum varieties and autecology of wild sorghum was also obtained. Cluster analysis supports the existence of several wild sorghum morphotypes that might correspond to at least three of the five ecotypes recognized in Africa. Intermediate forms between wild and cultivated sorghum belonging to the S. bicolor ssp. drummondii are frequently found in predominantly sorghum growing areas. Crop-wild gene flow in sorghum is likely to occur in many agroecosystems of Kenya.

Genetic variability in pepper (Capsicum annuum L.) estimated by morphological data and amplified fragment length polymorphism markers

Abstract.Data on genetic similarity among crop cultivars is of vital importance for the plant breeder. The objectives of this study were to group pepper (Capsicum annuum L.) genotypes into clusters according to their distances as estimated by morphological traits and amplified fragment length polymorphism (AFLP) markers and to assess the relationships between the two. Thirty-nine pepper genotypes obtained from different countries were grown in the greenhouse at University of the Free State, South Africa, during 2001 and 2002 in a randomized complete block design with three replications. A total of 20 different morphological traits were measured and six AFLP primer pairs were used to estimate pairwise genetic distances. Both datasets showed high genetic distances among the different genotypes, indicating high genetic diversity among them. The mean genetic distance among Ethiopian pungent elongated-fruit genotypes, was lower than that between them and the introduced ones. Morphological and AFLP distance estimations generally clustered together genotypes with similar fruit sizes. Significant, positive correlation was observed between morphological and AFLP diversity estimations. The narrow genetic basis among the Ethiopian pungent elongated-fruit cultivars suggests that the pepper breeding program of Ethiopia should focus on enriching its germplasm through local collection and introductions from other parts of the world.

Genetic structure and relationships within and between cultivated and wild sorghum (Sorghum bicolor (L.) Moench) in Kenya as revealed by microsatellite markers

Understanding the extent and partitioning of diversity within and among crop landraces and their wild/weedy relatives constitutes the first step in conserving and unlocking their genetic potential. This study aimed to characterize the genetic structure and relationships within and between cultivated and wild sorghum at country scale in Kenya, and to elucidate some of the underlying evolutionary mechanisms. We analyzed at total of 439 individuals comprising 329 cultivated and 110 wild sorghums using 24 microsatellite markers. We observed a total of 295 alleles across all loci and individuals, with 257 different alleles being detected in the cultivated sorghum gene pool and 238 alleles in the wild sorghum gene pool. We found that the wild sorghum gene pool harbored significantly more genetic diversity than its domesticated counterpart, a reflection that domestication of sorghum was accompanied by a genetic bottleneck. Overall, our study found close genetic proximity between cultivated sorghum and its wild progenitor, with the extent of crop-wild divergence varying among cultivation regions. The observed genetic proximity may have arisen primarily due to historical and/or contemporary gene flow between the two congeners, with differences in farmers’ practices explaining inter-regional gene flow differences. This suggests that deployment of transgenic sorghum in Kenya may lead to escape of transgenes into wild-weedy sorghum relatives. In both cultivated and wild sorghum, genetic diversity was found to be structured more along geographical level than agro-climatic level. This indicated that gene flow and genetic drift contributed to shaping the contemporary genetic structure in the two congeners. Spatial autocorrelation analysis revealed a strong spatial genetic structure in both cultivated and wild sorghums at the country scale, which could be explained by medium- to long-distance seed movement.

Levels of host plant resistance to banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae), in Ugandan Musa germplasm

Forty-five Musa clones, including endemic and introduced cultivars plus hybrids, were evaluated for resistance against the banana weevil, Cosmopolites sordidus, in a field trial in Uganda. The predominant groups of staple crops, East African highland bananas (Musa spp. AAA) and plantains (Musaspp. AAB), as well as plantain-derived hybrids (AAB × AA), showed the highest levels of susceptibility to this pest. These were followed by dessert bananas (Musa spp. AAA), exotic bananas (Musa spp. ABB) and finally diploids of M. acuminata (AA). Hybrids of banana origin were highly resistant. Some East African highland cultivars, especially brewing types (e.g., Kabula, Bagandeseza, Ediirira), showed intermediate levels of resistance. Among the non-highland bananas, high levels of resistance were observed in Yangambi-Km5 (AAA), Cavendish (AAA), Gros Michel (AAA), Kayinja (ABB, Pisang Awak subgroup), Ndiizi (AB, Ney Poovan subgroup)and Kisubi (Ney Poovan subgroup). The highest resistance was observed in banana hybrids TMB2×7197-2, TMB2×8075-7 and the wild banana Calcutta-4 (AA). These were considered the best sources of resistance for a weevil resistance-breeding programme with the two hybrids commonly used as improved male parents.

The Effect of Variety and Location on Cactus Pear (Opuntia ficus-indica) Fruit Quality

Little is known about the performance of South African cactus pear varieties in different agro-ecological regions. Effects of locality on internal quality parameters of available cactus pear varieties were examined. With only one exception, no significant differences among the mean replication values for the different parameters between the different locations were observed. The differences between mean values for most individual parameters at the three localities were highly significant. Highly significant differences between the mean values for the measured characteristics were observed, not only among the locations (except for the pulp glucose values), but also for the influences of genotype and interaction between locality and genotype. Significant variations existed between mean values of the different characteristics between localities. Genotype × environmental interactions were noted. It was concluded that Meyers is the most appropriate cultivar for economical purposes in South Africa.

Genetic relationships between Southern African Solanum retroflexum Dun. and other related species measured by morphological and DNA markers

In this study, the genetic relationship between 14 genotypes of black nightshade, most which were part of the Solanum nigrum complex, was investigated. Fifteen morphological characters were measured and used to compile a dendrogram. Amplified fragment length polymorphism (AFLP) markers were also used to assess the level of polymorphism between the 14 Solanum genotypes. Three EcoR I/Mse I primer combinations with three selective nucleotides per primer were used for screening the respective genotypes. Multiple polymorphisms could be detected to the extent that all the genotypes studied could be distinguished, using any single primer combination, thus showing the usefulness of AFLPs for this purpose. Up to 43 polymorphic bands were detected with a single primer combination among the 14 different genotypes. The three primer combinations generated a total of 359 bands, of which 222 (62%) were clearly polymorphic. This data was used to compile a dendrogram. Both the morphological and AFLP marker analysis clearly separated the different genotypes into similar groups.

The use of sensory attributes, sugar content, instrumental data and consumer acceptability in selection of sweet potato varieties

BACKGROUND As eating quality is important for adoption of new varieties, nine orange-fleshed and three cream-fleshed sweet potato varieties were assessed for sensory characteristics, dry mass and free sugar content, instrumental texture and colour and consumer acceptability (n =  216) in a peri-urban South African setting. RESULTS Cream-fleshed varieties were higher in yellow-green colour and sweet potato-like flavour and lower in graininess. Orange-fleshed varieties were higher in pumpkin-like flavour, orange colour, discolouration and sucrose content. Partial least squares regression analysis showed that the most accepted varieties (Impilo, Excel, Resisto, 2001_5_2, Serolane, W-119 and Monate) were associated with sweet flavour, dry mass and maltose content, while the least accepted varieties (Beauregard, Khano and 1999_1_7) were associated with wateriness. Pearson correlation analysis highlighted correlations of sensory attributes yellow and orange with instrumental colour measurements (colour a* and colour b*), instrumental firmness with sensory firmness, dry mass with sensory wateriness, and maltose content with sensory sweet and sweet potato-like flavour. The varieties were clustered into three groups. Consumer acceptability for eating quality correlated with maltose content, dry mass and sweet flavour. CONCLUSION Chemical and instrumental measurements were identified to evaluate key attributes and will be useful in the intermediate phases of sweet potato varietal development.

Isolation and physicochemical characterisation of starch from cocoyam (Colocasia esculenta) grown in Malawi

BACKGROUND The aim of this study was to determine the physicochemical properties of starches isolated from Malawian cocoyams and compare them with those of cassava and corn starches. RESULTS The purity of the isolated starches varied from 851 to 947 g kg(-1) and pH from 4.93 to 6.95. Moisture, ash, protein, fat and amylose contents ranged from 104 to 132, 0.3 to 1.5, 3.5 to 8.4, 0.9 to 1.6, and 111 to 237 g kg(-1), respectively. Cocoyam starches gave higher potassium and phosphorus but lower calcium levels than the other starches. The shape of starch granules varied from spherical to polygonal with cocoyam starches displaying smaller-sized granules than cassava and corn starches. Cocoyam starches gave a higher wavelength of maximum iodine absorption and blue value but lower reducing capacity values than cassava and corn starches. The extent of acid hydrolysis of the starches also differed. Cocoyam starches exhibited amylopectin molecules of higher molecular weights but amylose molecules of lower molecular weights than cassava and corn starches. Cocoyam starches exhibited lower water absorption capacity and swelling power, paste clarity and viscosity but higher solubility, gelatinisation temperatures and retrogradation tendencies than cassava and corn starches. CONCLUSIONS The physicochemical properties of native Malawian cocoyam starches vary among the different accessions and differ from those of cassava and corn starches.

Estimates of heterosis and association of genetic distance with heterosis in durum wheat under different moisture regimes

The objectives of the present study were to evaluate heterosis for grain yield and yield components in durum wheat, and to assess the prediction potential of amplified fragment length polymorphism (AFLP) based and agronomic trait based genetic distances (GD and MD, respectively) to F 1 performance, mid parent heterosis (MPH), and specific combining ability effects (SCA) under well-watered and moisture stress conditions. Six parental genotypes with different responses to moisture stress and their 15 F 1 crosses were evaluated for their responses to moisture stress conditions in a glasshouse. Some cross combinations showed significant MPH for grain yield and yield components. The expression of heterosis for grain yield was greater under moisture stress conditions than under well-watered conditions. Cluster analysis of the parental lines based on agronomic performance under stress conditions was similar to cluster analysis result based on AFLP marker profiles. F 1 performance was strongly correlated to both SCA effects and MPH under both stress and well water conditions. The correlation between SCA and MPH was very high under both treatment conditions for all traits. Correlation between GD and MD was significant only under stress conditions. Positive correlation was found only for the association between GD v. F 1 performance and GD v. SCA effects for harvest index (HI) under well-watered conditions. None of the correlations between MD and SCA effects were significant. The absence of association between GD and heterosis for yield and most agronomic traits implied that heterozygosity per se diversity is not a good predictor of heterosis or F 1 performance under both well-watered and stressed conditions.