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Genetic diversity and differentiation of Cassava (Manihot esculenta Crantz) accessions in Puerto Rico
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Abstract
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.
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Date
2010
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Keywords
Cassava, Puerto Rican cassava collection