A field study was carried out on a sixyear-old on-farm field trial during long-rains season (April-August) 2003 to investigate the effect of improved fallow systems and phosphorus application on arbuscular mycorrhiza fungi (AMF) symbiosis in maize. The trial comprised of maize rotated with a fast growing leguminous Crotalaria grahamiana fallow and a non-leguminous Tithonia diversifolia fallow for 3 years followed by continuous maize. The experiment was randomized complete block design with three cropping (continuous maize, Crotalaria fallow and Tithonia fallow) systems and two phosphorus levels (0 and 50 kg P/ha). AMF colonization in maize roots, maize yield and macro-nutrients uptake were recorded. Phosphorus applications improved (P \ 0.05) early (\8 weeks old maize) AMF colonization, nutrient uptake and maize yield in improved fallow systems. Greater differences due to phosphorus application were noted in maize in Tithonia fallow than in Crotalaria fallow. Following phosphorus application, a positive relationship existed between early AMF colonization and maize yield (r = 0.38), and phosphorus and nitrogen uptake (r = 0.40 and r = 0.43, respectively), demonstrating the importance of phosphorus fertilization in enhancing low-input technologies (improved fallows systems) in phosphorus deficient and acidic soils of western Kenya.
Jatropha curcas L is an economically potential tree species gaining interest globally because of its feasible contribution towards production of commercial biofuel. Little is known however, of its genetic variation patterns within Kenyan accessions for maximum exploitation. Eight populations covering most of its distribution range in Kenya were sampled and leaves of 160 individual trees collected. Analysis of molecular diversity was carried out using random amplified polymorphic DNA (RAPD) markers. Ten random primers generated 251 loci that were scored for diversity. Shannon's diversity index varied from 0.116 (Likoni) to 0.360 (Namanga). Analyses of molecular variance (AMOVA) showed that, more variation (53%; P = 0.01) was partitioned among populations while 47% (P = 0.01) variation was partitioned within populations. Nested analysis of variance showed no variation across regions (0%; P > 0.01). The level of genetic structure and diversity may be explained by the modes of germplasm introductions and the biological traits of J. curcas. Based on these results, Jatropha accessions are quite variable and seeds collected from various populations are bound to exhibit wide variation. Selection of a J. curcas breeding population should encompass a large number of individuals across all accessions in order to capture the wide genetic variation.
The development of genetic tools for non-model organisms has been hampered by cost, but advances in next-generation sequencing (NGS) have created new opportunities. In ecological research, this raises the prospect for developing molecular markers to simultaneously study important genetic processes such as gene flow in multiple non-model plant species within complex natural and anthropogenic landscapes. Here, we report the use of bar-coded multiplexed paired-end Illumina NGS for the de novo development of expressed sequence tag-derived simple sequence repeat (EST-SSR) markers at low cost for a range of 24 tree species. Each chosen tree species is important in complex tropical agroforestry systems where little is currently known about many genetic processes. An average of more than 5,000 EST-SSRs was identified for each of the 24 sequenced species, whereas prior to analysis 20 of the species had fewer than 100 nucleotide sequence citations. To make results available to potential users in a suitable format, we have developed an open-access, interactive online database, tropiTree (http://bioinf.hutton.ac.uk/tropiTree), which has a range of visualisation and search facilities, and which is a model for the efficient presentation and application of NGS data.
& Key message High genetic diversity and significant population structure of Vachellia tortilis population in Kenya has been determined using nuclear microsatellite markers. Despite wide distribution and long-distance gene dispersal of the species, geographic barriers affect its gene flow patterns. The Gregorian Rift Valley that cuts across the country is suggested to have differentiated the populations into eastern and western clusters. & Context Vachellia tortilis is an important dryland tree species valued for fuelwood and fodder production; however, no strategy has been put in place for sustainable management of the species genetic resources. Furthermore, there is inadequate information on the species population genetics to aid the development of such strategies. & Aims We evaluated the amount and structure of neutral genetic diversity of V. tortilis population in Kenya and provided recommendations necessary for improvement and conservation of the species genetic resources. We hypothesized that the current genetic diversity of V. tortilis is high because of its demographic history and that no population structuring was expected to occur due to the presumed long-distance and effective gene flow patterns within the species. & Methods Leaf tissues were collected from 15 putative natural populations of V. tortilis covering the whole distribution range in Kenya. DNA was isolated from the leaf tissues and analyzed using microsatellite markers. In total, 450 trees were genotyped using ten polymorphic nuclear microsatellite loci, and genetic diversity and population structure parameters were determined. & Results We found high levels of genetic diversity within the populations with a mean gene diversity at 0.85. However, significant population differentiation was evident (F ST = 0.026, P = 0.007; R ST = 0.032, P = 0.004) despite a large number of migrants per generation (N m = 5.3). Population structure detected suggests the presence of two clusters, although many individuals showed mixed ancestry. The groups reflect the influence of geographic patterns and historical population gene flow. & Conclusion There exists high genetic diversity in V. tortilis in Kenya with significant population structuring into two clusters. We recommend the consideration of the two distinct groups in the development of the species improvement, breeding, and conservation programs. Such programs should ensure maintenance of the majority of the extant genetic diversity.
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