Showing papers by "Central Tuber Crops Research Institute published in 1970"

Cytological evidence on the origin of sweet potato.

Abstract: The results of intensive meiotic studies, particularly of the karyology and chromosomal homology at the pachytene stage, in the sweet potato (Ipomoea batatas L.), which is a hexaploid (2 n = 90), have thrown considerable light on its origin and genome relationships. Using suitable criteria, such as relative length of chromosomes, centromere position, chromomere pattern, absence of light staining segments in one of the arms, presence of telochromomere etc., 40 of the 45 haploid chromosome complement at pachytene were identified and assigned to 19 chromosomal types. Among these types, eight were present singly; in six of the types, chromosomes were present in duplicate, and in two types, in triplicate. The occurrence of higher multivalent chromosomal associations such as hexavalents and pentavalents, in addition to the quadrivalents already reported, was recorded for the first time at the pachytene and metaphase I stages. The hexavalents at pachytene were resolved into three distinct types based on the morphology of the participating chromosomes. A maximum number of nine quadrivalents at the metaphase I stage and four in the incompletely analyzed pachytene nuclei were recorded. The constituent chromosomes of three of the quadrivalents at pachytene stage were identified. From these observations, it is suggested that (i) the three parental genomes are partly homologous (ii) two of the genomes show closer homology to one another than to the third and (iii) the three genomes differ with respect to one or more of the eight chromosomal types occurring singly. The available information rules out an autopolyploid origin for sweet potato and suggests that the parental genomes are from closely related taxa. The advantages are emphasized of pursuing similar studies in other American Ipomoea species to unravel their relationship with the sweet potato. Among other meiotic irregularities, a translocated chromosome and a chromosome carrying inversion were detected at the pachytene stage and the possible role they may play in varietal differentiation is discussed.

Cytogenetics of the F1 hybrid between cassava and ceara rubber, and its backcross

Abstract: Cytogenetical studies of the F1 hybrid between the commercially cultivated tuber crop, cassava (Manihot esculenta Crantz.) and the closely related wild speciesManihot glaziovii Muell. (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.

Desynapsis in Colocasia antiquorum Schott.

Abstract: A culture ofColocasia antiquorum Schott. (2n=28) exhibiting desynapsis was detected in the course of cytological screening of a large number of cultivars maintained at the Institute. Intensive studies on the course of meiosis in this material as well as in the normal plants were made commencing from pachytene stage. Meiosis in the normal plants was regular, leading to the production of 95% stainable pollen. On the other hand, meiosis in the desynaptic material was found to be highly irregular resulting in the formation of over 80% sterile pollen. In its meiotic behaviour, the desynapticColocasia conforms to the “complete” type of desynaptics. The available evidence suggests a spontaneous origin of the desynaptic through gene mutation. The utility of this material in the study of unravelling basic cytogenetical problems is discussed.