Molecular characterization of Sclerocarya birrea (Marula) field genebank collections
Gechemba, Machani Fridah
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Scelerocarya birrea is an important fruit tree that is widely used by the rural populations in most regions in which it is found, for its fruits, bark, timber, and even its roots. Studies have shown that the tree has the potential to boost nutrition, health and income security in Africa. However, the local communities harvest the tree products from the wild with minimal attempts to grow it on-farm. The sustainability of such wild harvests is threatened by agriculture, overgrazing, and overexploitation for other purposes. Therefore, the species needs urgent conservation measures in addition to selecting superior germplasm for on-farm tree management that will facilitate ease of species cultivation. This has prompted ICRAF and partners to collect and manage S. birrea germplasm in Tanzania and Malawi field genebanks as the initial step for its domestication and conservation. However, maintenance of field genebanks is very costly because trees take up a large space and they are prone to human and environmental threats. As the data gathering phase ends in the provenance trials, there is need to make an informed decision on what stands to continue being managed as a field genebank. It is important to ensure that diverse material is conserved. In order to effectively manage germplasm, genetic diversity studies are pre-requisite. The provenance trial data have shown great variability in morphological traits across populations. There no prior molecular characterization of these field genebank collections has been conducted to date. The present study was aimed at understanding the genetic diversity of S. birrea ICRAF field genebank collections using inter-simple sequence repeat (ISSR) markers. ISSR markers are informative, reproducible and cost-effective. Presence or absence of ISSR bands were scored manually from the gels (twice) by visual inspection of the gel images. Power Marker version 3.25, GenAlex version 6.5, Tools for Population Genetics Analysis software, and DARwin 6.0 software were used to analyze the data. Six markers yielded a total of 76 polymorphic bands across the 257 accessions. Percentage of polymorphic loci and observed heterozygosity ranged from 75% to 7.89%, and H= 0.362 to H= 0.043, respectively. The partitioning of genetic diversity found a higher (86% P> 0.001) intra-population variation and low inter-population variation, typical of the outcrossing nature of S. birrea. According to Jaccard‘s dissimilarity index, the highest genetic distance between accessions was 1.000 and the least genetic distance was 0.000. Neighbor-joining clustering grouped the accessions into three major clusters and twenty probable duplicates were identified, which should be eliminated to cut down the cost of conservation. The results obtained suggest that S. birrea ICRAF field genebank collections have a comparatively rich gene pool and, hence, valuable for conservation and domestication of the species. The twenty accessions that clustered together would be good for evaluating performance of this long-lived tree species in both locations. Choma-M, Missira-M, Ntcheu-M, Magunde, Ohangwena-M, Magamba-M, Ngundu-M, Muzarabani-M, Matembeleland-N-M, Matembeleland-S-M, and Siavonga-M populations should be the focus of conservation efforts and resources. There is need to cultivate the species in Kenya to improve food security, farmers' incomes and climate resilience.