Genetic diversity and differentiation of Cassava (Manihot esculenta Crantz) accessions in Puerto Rico
Montero-Rojas, María del Milagro
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Cassava is consumed daily by 600 million people living in the poorest places in Africa, Asia and South America. Natural genetic variation that exists among different cultivars of cassava has been studied in order to assist in the reduction of constraints present in cassava. In Puerto Rico little is known about the genetic diversity of cassava. In order to access genetic diversity, 162 samples were collected from different townships of the island and were analyzed by 33 SSR markers on polyacrylamide gels (detected using a fluorescence-based technique using an automated DNA sequencer), and compared with the Puerto Rican cassava collection using NTSYS, GENSURVEY, FSTAT and GEAIE software programs. Our results showed that genetic diversity (He) of the unknown cassava samples (0.7242) is higher than in the Puerto Rican cassava collection (0.7026). Furthermore the observed proportion of heterozygotes (Ho) was higher in Puerto Rican cassava collection compared to the unknown samples. The proportion of inbreeding coefficient (Fis) showed a larger proportion of heterozygotes in the unknown samples than in the Puerto Rican cassava collection. The genetic diversity evaluated across all loci and all populations (Ht) was high (0.7416). In general the highest genetic diversity was present within populations (0.7134). There was no genetic differentiation between populations (Dst= 0.0235) and there is no gene differentiation of populations (Gst= 0.0375 ± 0.0466). The UPGMA analysis showed that most of the samples clustered with the varieties of the Puerto Rican cassava collection. However there was one cluster with only unknown cassava. From this cluster the unknown sample SpL-27 presented the highest genetic dissimilarity (71%). At least 4 pairs of unknown cassava gathered from different regions showed the same identity (Fal071-129 and Fal017-130; Fal041-121 and Fal067-124; Fal017-131 and Fal017-132; Fal0-19 and Fal0-20). The high levels of heterozygosity found in Puerto Rican cassava can be due to three reasons. First, the high levels of heterozygosity in the local accessions could be due to the high heterozygosity inherited from cassava ancestors. Second, sexual reproduction occurring in vegetative propagated crops have been shown to increase heterozygosity due to farmer practices which allow sexually reproduced seedlings to germinate and to be subsequently incorporated in to the next planting. This new genotype might then be named based on an existing morphologically similar variety. Third, the accumulation of fixed somatic mutations, the frequency of mutations depends on both the velocity of those changes occurring and the selection of the individuals which have them. The levels of genetic diversity found in this study, revealed a high potential for local Puerto Rican cassava to be part of a genetic improvement program in the future.