In Vitro Breeding Techniques

variation that can be expected will vary with the clone, age of the culture, use of mutagenic agents, and selection pressure applied to cell suspensions or callus cultures of clones for stress conditions such as salt level, herbicides, microorganisms or their byproducts, and specific metabolites (Skirvin, 1978). The exploitation of such natural and induced variation seems especially applicable to old woody plant cultivars, such as ‘Bartlett’ pear ( Pyrus communis L.) and ‘Delicious’ apple ( Malus domestica Borkh.) (introduced in 1770 and 1893, respec-tively) (Skirvin, 1977), which could be expected to have accumulated many mutant cells that may have stabilized into chimeras of various complexity.Skirvin and Janick (1976a) were among the first to emphasize the importance of clonal variation in genotype improvement of horticul- tural species. et al. (1980) demonstrated extensive variability among plants regenerated from cultured leaf protoplasts (protoclones) of ‘Russet Burbank’ potato ( Solanum tuberosum L.). In investigating the extent and possible significance of this source of genetic variabil- ity, and Scowcroft (1981), who have reviewed the topic extensively, first suggested that the term “somaclonal variation” be used to describe the phenotypic variation Since the technique of anther culture was first reported (Guha and Maheshwari, 1966), its promise as a means of generating haploid plants that might be instrumental in developing new cultivars of major crop species has been impatiently awaited. By placing immature anthers containing microspores at the late uninucleate/early binucleate stage of development onto a synthetic medium, development can be modified such that some microspores proceed along an embryogenic, or androgenetic, pathway. By conversion of such embryos into plants, or regeneration of microspore-derived callus, haploid plants can be obtained. Subsequently, the chromosome number of such haploids can be doubled to yield homozygous inbred lines. At first, the technique was limited to only a few species; however, with refinements in tissue culture and selection of responsive genotypes, androgenesis has been observed in a wide array of crops representing diverse taxonomic groups (Bajaj, 1990). Given the length of time generally required to develop new cultivars, we can only now begin to expect release of cultivars that owe their origin to anther culture (Table 1). The citations in this table are not necessarily the first anther culture reports for each crop, but the first reports where haploid green plants were produced. Some species used in early anther culture work are not included in the list because of the difficulties that subsequent researchers encountered when applying the techniques to obtain haploid plants (e.g., tomato). Using anther culture for plant improvement depends on the breed- ing system of a crop. For self-pollinated crops, such as wheat ( Triticum aestivum L.), rice ( Oryza sativa L.), or tomato ( Lycopersicon esculentum Mill.), anther culture can be most effectively applied to an F 1 hybrid between two breeding lines with complementary characteristics of agricultural significance to fix homozygosity in a single generation, thus avoiding time required