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Final Activity and Management Report Summary - ASGEN (Identification of genes linked to gum quality in the gum producing gum arabic tree - Acacia senegal (L.) Willd. ...)

The 'gum arabic' tree, scientifically known as Acacia senegal, is native to the sub-Saharan region of Africa, and also occurs in the Arabian peninsula and Indian sub-continents. Gum arabic is a valuable product of the tree and is traded internationally. Given its economic potential and the prospect of providing a valuable new income source to people in arid and semi-arid sub-Saharan Africa, there is considerable interest in developing improved planting material to produce more yield and better quality gum. Therefore, the scientific objective of this project was to investigate the potential of association genetics to identify markers or genes that are linked to gum quality and productivity and to create a framework for applying these tools, including creation of a sample archive, and generation of new non-coding and gene-based datasets.

More than 1600 DNA samples were extracted from A. senegal tissues collected from Kenyan populations and stored at -20 degrees Celsius, supplementing the initial 1 500 DNA rangewide samples that were extracted and stored during period 1. The DNA samples are now stored and archived at the Kenya Forestry Research Institute (KEFRI) and the Centre for Hydrology and Ecology (CEH, Edinburgh, United Kingdom).

Biological N2-fixation (BNF) and water use efficiency (WUE) are known to be associated with gum yields and quality in A. senegal. As part of the phenotyping, these two parameters were determined in natural populations and provenance trials using the stable isotopes 15N and 13C techniques, represented as delta values in parts per thousand (‰). There was high variability in delta 15N values ranging from 2.33 to 11.22 ‰. The key factors responsible for the variation were site (experimental location), season (dry versus wet) and provenance. BNF potential was higher during wet than dry seasons. The delta 13C values ranged from -31.82 to - 27.76 ‰.

The key finding was that delta 13C values were less negative during the dry season than wet season which suggested a shift in water use strategy from low to high WUE during the wet and dry seasons, respectively. When repeated (paired analysis) assessments were done, the differences between dry and wet delta 13C values were significantly lower than < 1.0 ‰, except for Kordofan (Sudan) provenance, which had a difference of 1.19 ‰. This result differentiated the Kordofan provenance from the West African populations, and suggested that the Kordofan provenance was less drought-tolerant.

Genotyping work was continued on the expanded DNA sample databank using nuclear and chloroplast markers (cpSSR, trnH-psbA, ITS, SSR). They showed a high diversity index (HT = 0.593) and a geographic genetic structuring pattern that separated the East and South African populations from those in West and Central Africa. The nuclear ribosomal DNA (nrDNA) internal transcribed spacer (ITS) molecular marker distinguished variety leiorhachis from the other three (varieties senegal, kerensis and rostrata). This result represents the first molecular marker evidence that distinguishes variety leiorhachis from the others, hitherto based on morphological traits. The nrDNA and genome size analyses also revealed prevalence of hybridisation, and that polyploidy was more common in West African than East and Southern African A. senegal populations.

These results reaffirm the complex interaction of genetic and environmental factors in producing the high phenotypic variation in A. senegal, also observed for gum yield and quality. A field trial has been established to initiate molecular marker heritability and breeding programme. Specifically, this field trial will provide a medium- to long-term platform to monitor environmental parameters, assess growth, gum yield and quality, and generate crucial phenotypic data for association analyses with genetic data. The fellowship has ensured the capability of the home institution (KEFRI) to pursue these objectives to fruition through international networking, laboratory equipment upgrades and hands-on training of key technical staff, junior scientists and students.

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