Variability of chloroplast DNA and nuclear ribosomal DNA in cassava (Manihot esculenta Crantz) and its wild relatives

Abstract

Chloroplast DNA (cp) and nuclear ribosomal DNA (rDNA) variation was investigated in 45 accessions of cultivated and wild Manihot species. Ten independent mutations, 8 point mutations and 2 length mutations were identified, using eight restriction enzymes and 12 heterologous cpDNA probes from mungbean. Restriction fragment length polymorphism analysis defined nine distinct chloroplast types, three of which were found among the cultivated accessions and six among the wild species. Cladistic analysis of the cpDNA data using parsimony yielded a hypothetical phylogeny of lineages among the cpDNAs of cassava and its wild relatives that is congruent with morphological evolutionary differentiation in the genus. The results of our survey of cpDNA, together with rDNA restriction site change at the intergenic spacer region and rDNA repeat unit length variation (using rDNA cloned fragments from taro as probe), suggest that cassava might have arisen from the domestication of wild tuberous accessions of some Manihot species, followed by intensive selection. M. esculenta subspp flabellifolia is probably a wild progenitor. Introgressive hybridization with wild forms and pressures to adapt to the widely varying climates and topography in which cassava is found might have enhanced the crop's present day variability.

Genetic relatedness between cassava (Manihot esculenta Crantz) and M. flabellifolia and M. Peruviana based on both RAPD and AFLP markers

“…Similarly, spontaneous hybrids of M. esculenta and M. glaziovii were described in Africa by Nichols (1947), Cours (1951), INEAC (1952 and Lefèvre (1989). Contrary to our results, Fregene et al (1994) found no difference between M. glaziovii and M. michaelis, based on an analysis of DNA extracted from chloroplasts. Likewise, Haysom et al (1994) used RFLP markers to classify the Mexican species M. chlorostica as the species most closely related to M. flabellifolia, followed by M. esculenta and M. glaziovii.…”

“…Schaal et al (1997) reached the same conclusion using RAPD markers and two ITS regions of nuclear ribosomal DNA. Based on the analysis of DNA extracted from the chloroplasts of the two species, Fregene et al (1994) classified M. flabellifolia as being very closely related to cassava. These results suggest that M. flabellifolia/M.…”

Origin and Domestication of Native Amazonian Crops

“…Later, another Mesoamerican species, M. carthaginensis, was proposed as a wild relative [15,16]. Early studies with molecular markers did not clearly resolve the phylogeny of the genus [17,18]. However, they did identify a clear separation between the Mesoamerican and South American lineages, and found that cultivated manioc always grouped with South American species, suggesting that the crop was domesticated in South America.…”

Evidence on the origin of cassava: Phylogeography of Manihot esculenta

“…The genus Manihot is believed to have arisen and diversified recently (18), an argument supported by a lack of variability in chromosome number, by low levels of divergence in floral morphology (18) and DNA sequence data (40), and by interfertility between morphologically divergent species in artificial crosses (26,27). Thus, M. esculenta and M. pruinosa are likely to share very recent common ancestry, indicating a high probability of lineage sorting (51,52).…”

“…More recently, wild populations of M. esculenta that are likely to be the crop's direct progenitors have been identified in South America (19,25). However, the evolutionary relationship between cassava and its conspecific wild relatives is only beginning to be examined (19,26,27), and there is continued speculation that the crop's origins may extend beyond M. esculenta to hybridizing Manihot species (26,27).…”