Somaclonal Variation for the Improvement of Tree Species

Genetic variability is an indispensable part of any conventional tree improvement programme. Though breeders are well aware of the traits that need to be improved, the knowledge on the underlying genetic factors (alleles) controlling those traits as well as their distribution in the population is limited. Therefore, it becomes essential to maintain as variable a gene pool as possible in order to reap continuous genetic gains over a long time period. Sexual hybridization and selection are employed to recombine desirable genes from cultivated as well as wild germplasm for breeding for desirable traits. Breeding cultivars for desirable traits through conventional methods is a time-taking process and it gets further lengthened when improvement of tree species encompassing long generation times are sought. Woody perennials tend to possess medicinal properties and are also an important source of food, feed, and fodder throughout the world. With the ever-increasing population and hence rising demand, degradation of soil, paucity of cultivable lands, reducing forest cover, environmental pollution, and changing climatic conditions owing to global warming, the tree breeders are under enormous pressure to sustain production of valuable tree products. Forest tree improvement can be accelerated by integrating new technologies with conventional breeding methods. The occurrence of genetic variations among in vitro raised cell cultures, referred to as the somaclonal variation by Larkin and Scowcroft (1981), is one such method that has been utilized for recovering the desired variant in perennial plant species. Somaclonal variants are mostly recovered but are not restricted to plants regenerated from an intervening callus phase. Numerous factors including wounding, sterilant exposure, culturing of cell wall less protoplasm, media component imbalance, imbalance of growth hormones, light availability, and presence of oxidative stresses might be responsible for causing heritable variation in tissue culture raised plants. The genetic alteration may be attributed to changes in DNA sequence (single base mutations), chromosome structure (deletion, duplication, insertion, translocation), and number (aneuploidy, polyploidy) or may involve epigenetic modifications (gene methylation). Somaclonal variations have been witnessed in many perennial species and this chapter provides further insights into somaclonal variation, its origin, mechanism as well its role in improvement of woody perennials.