Source Material and Culture Conditions, Genotypes, Critical variables, Polyploid, Chromosome, Polyploidization

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	Source Material and Culture Conditions - Attempts directed towards the evolution of new crop varieties through somaclonal variation are by and large influenced by genotype, explant source, duration of culture, and growth hormone effects. These factors can be readily varied with respect to some crops, while in others the constraints of regeneration may limit flexibility in manipulating one or more of these critical variables. The genotype influences both the frequency of regeneration and the frequency of somaclonal variation. Sun et al. (1983) compared the frequency of polyploid regenerates in 18 varieties of rice under identical culture conditions and recovered multiploids only in the indica variety but not in the japonica.
Explants are generally taken from any tissue, namely leaves, internodes, ovaries, roots; and inflorescences. The source of the explant has been considered a critical variable for somaclonal variation. It is likely that different selective pressures would be exerted against different explant resources in culture, resulting in the spectrum of soma clonal variation among regenerated plants. Many established cell cultures show chromosomal mosaicism. Several factors contribute to variation in the chromosome number such as 1) preexisting chromosome mosaicism (polysomaty the phenomenon of polyploidization of body cells) in explants used for culture initiation, 2) nuclear fragmentation associated with first cell division of callus initiation,
3) endoreduplication or endomitosis occurring during cell culture initiation, and 4) abnormalities of the mitotic process leading to aneuploidy. Barbier and Dulieu (1980), using a genetically marked explant source, demonstrated that genetic changes occurring in the first few mitoses in culture increase with the age of cultures. Since longer periods (six months) are required to recover somatic hybrid plants in culture, it has been observed that such protoplast derived hybrids are a rich source of somaclonal variability.
High proportions of growth regulators effect karyotypic alterations in cultured cells. 2, 4-0, BAP, and others have been shown to induce chromosomal variability in cultured cells, leading to the formation of somaclones. Growth hormones are essential for induction of organogenesis and shoot differentiation; however, high concentrations of these substances may not permit recovery of whole plants in tissue cultures and the proportion of hormones in the medium needs to be carefully monitored in establishing culture systems for in vitro propagation of somaclonal variants.