Cytogenetics of the F1 hybrid between cassava and ceara rubber, and its backcross
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Cytogenetical studies of the F1 hybrid between the commercially cultivated tuber crop, cassava (Manihot esculentaCrantz.) and the closely related wild speciesManihot glazioviiMuell. (Ceara rubber) used as donor specles for “Cassava mosaic” discase and drought-resistant genes and back crosses (to cassava parent) were made. The contrasting parental characters showed partial to total dominance in the F1 hybrid, while the back cross plants were similar to cassava in most of their characters. Eleven of the twelve backeross plants exhibited resistance to “Cassava mosaic” under field conditions. Karyological similarities and differences as resolved on the basis of a comparative study of the karyotypes of the cassava parent and coara rubber were corroborated by the study of chromosomal pairing in the F1 at pachytene. Major chromosomal differentiation in the two species involved three chromosomes of their haploid complement which were represented by three heteromorphic bivalent associations in F1 each consisting of a probably basic chromosomal type and a derived type. Pachytene analyses of three back cross plants provided direct proof for random transmission of marker chromosomes of both the parents through male gametes of the F1 hybrid. An increase in the chiasma frequency in the back cross plants over the F1 hybrid at metaphase I stage was also observed. Pollen fertility of the backeross plants showed considerable variation.
KeywordsChromosomal Pairing Cytogenetical Study Pollen Fertility Marker Chromosome Bivalent Association
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