Genetic Stability Studies in Regenerated Plants of Allium tuberosum Rottl. ex Spreng.-A Cytological Approach

Nuclear instability of plant cells grown in vitro is very common and has been demonstrated by a number of researchers in wide array of plant materials (D'Amato 1975, Sunderland 1977, Bayliss 1980, Orton 1980). Quite often this instability is manifested in the form of numerical and structural alteration in chromosomes of cultures as well as plants regenerated from them (Kao et al. 1970, McCoy and Bingham 1977, Browers and Orton 1982, Sree Ramulu et al. 1983). The frequency of such aberrations, both structural and numerical, is reported to be very high, especially when the regeneration is through callus (Sacristan 1971, Yamere 1975, Sunderland 1977, Yeoman and Street 1977, Roy 1980) as compared to direct regeneration (Sheridan 1974, Mathur et al. 1987, Sen and Sharma 1991). In both the cases, however, any attempts for clonal multiplication especially when they are made with a prime objective of ex-situ conserva tion should be made with a frequent monitoring and verification of genomic integrity of the cultured material. Cytogenetical analysis, besides biochemical, histological and histochemi cal, is one of the most informative and reliable techniques to ascertain whether any changes occured in the nuclear material during the process of regeneration and organogenesis. Among cytogenetical aspects, karyological studies are of paramount importance as they often pro vide authentic information pertaining to chromosome structure, number and in general their gross morphology (Darlington and La Cour 1976). Meiotic studies on the other hand focus on details of pairing behaviour of chromosomes, recombination frequencies and more impor tantly their disjunctional pattern at anaphase I and II which are not deducible from the mitotic studies (John and Lewis 1965). Allium tuberosum Rottl. ex Spreng. (Familly Alliaceae) is an important wild relative of garden onion (Jones and Mann 1963) and both diploid (2n=16) and tetraploid (2n=32) cytotypes are known to occur (Mathur and Tandon 1965, Gohil and Koul 1973). The wild genetic resources of this economically important species have been declining in the natural habitat in recent years due to over exploitation by local inhabitants mainly for inflorescences and leaves with garlic-like flavour, and added to this is complete lack of organised cultivation. Necessiated by this fact, efforts were made at National Plant Tissue Culture Repository for in vitro conservation of this important species along with other related types. Subsequently clonal propagation through in vitro shoot proliferation was successfully accomplished (Pandey et al. 1992). In the present investigation, attempts were made to study the genetic stability of regenerated plants in various generations using cytological techniques and this paper deals with a comprehensive account of the results obtained in four generations of A. tuberosum plants raised in vitro.