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Browsing Theses & Dissertations by Subject "(SSR) markers"
Correa-Morales, Ana M.; Siritunga, Dimuth; College of Arts and Sciences - Sciences; Kolterman, Duane A.; Martínez Cruzado, Juan Carlos; Department of Biology; Irish, Brian M.
The genetic diversity and differentiation of 24 cassava (Manihot esculenta Crantz) accessions from the Puerto Rican collection are assessed in this study. In addition, 63 cassava DNA samples from the CIAT (Centro Internacional de Agricultura Tropical) germplasm collection, representing Latin America (Colombia, Costa Rica, Guatemala, Mexico, and Brazil) and Africa (Benin, Cameroon, Ghana, Nigeria, and Sierra Leone) were included from previous cassava diversity studies. Using simple sequence repeat (SSR) markers, variation in allele frequency at 37 unlinked loci was used to estimate genetic diversity and differentiation and to find the relationships between these 24 accessions and their cultivated relatives from primary and secondary diversity centers. The SSR markers were chosen because they represent a broad coverage of the cassava genome with moderate to high polymorphism information content (PIC) and robust amplification. For all the tested SSR loci studied there were on average 96.85 % ± 6.86 polymorphic loci across all country samples with an average of 4.58 ± 1.83 alleles per locus, and a mean observed heterozygosity of 0.7 ± 0.055. Despite the low level of differentiation [FST (theta) = 0.047± 0.005] found between country samples, sufficient genetic distance (Pairwise Nei’s and FST distances) existed between individual accessions to separate them according to their country of origin. UPGMA analysis of country samples revealed that Puerto Rican accessions clustered together with South American landraces. The SSR markers detected two genetically identical accessions in the Puerto Rican collection and led us identify the possibly closest cultivated relatives from Brazil and Colombia for some of the collection’s accessions. Finally, we evaluate the embryogenic capacity of the accessions present in the in vitro collection using the conventional solid induction medium system with 8 mg/l of 2,4-D. The explants used were young leaf lobes isolated from in vitro grown plants. Somatic embryogenesis was achieved in 58 % of the accessions, but only SG804, CM3064, Tremesiana, PI12900, PI12903 and Seda produced germinated somatic embryos. Differences found among the explants for their capacity to form embryogenic structures and germinate could be due to their intrinsic genetic characteristics. Our results support the thought that the capacity to produce embryogenic tissue in cassava varies among different accessions. Furthermore, the solid system used could be forming gradients in the uptake of nutrients and hormones promoting variation and reducing the growth of the embryogenic structures.