Oats {Avena sativa) 



Wheat {Triticum aestivum) 



Rice (Oryza sativa) 



Sugarcane (Sacctiarum officinale) 



Corn [Zea mays) 



Potato (Solanum tuberuosum) 



Tobacco (Nicotiana tabacum) 



Alfalfa {l^edicago sativa) 

 Brassicas (Brassica spp.) 



Immature embryo, 



apical meristem 



Immature embryo 



Seed embryo 



Various 



Immature embryo 



Protoplast leaf callus 



Anthers, protoplasts, 

 leaf callus 



Immature ovaries 



Anthers, embryos, 

 meristems 



Plant height, heading date, leaf striping Seed 



Plant height, spike shape, maturity Seed 



tillering, leaf wax giliadins, amylase 

 Tiller number, panicle size, seed fertility. Seed 



flowering date, plant height 

 Disease resistance, auricle length. Vegetable 



isoenzyme alterations, sugar yield 

 Endosperm and seedling mutants. Seed 



pathogen toxin resistance, DNA 



sequence, changes in mitochondria 

 Tuber shape, yield, maturity date, plant Vegetable 



form, stem, leaf, and flower structure, 



disease resistance 

 Plant height, leaf size, yield grade index. Seed 



alkaloids, reducing sugars, leaf 



chlorophyll 

 Leaves, petiole length, plant form and Vegetable 



height, dry matter yield 

 Flowering time, growth form, waxiness. Seed 



glucosinolates, disease tolerance 



^Seed = inherited in seeds of variant plants; vegetable = transmitted to clonally reproduced plants. 



SOURCE: W R. Scowcroft, S.A. Ryan. R.I.S. Brettel. and P.J. Larkin. "Somaclonal Variation; A 'New' Genetic Resource," Crop Genetic Resources: Conservation and 

 Evaluation. J.H.W. Holden and J T. Williams (eds.) (London: George Allen & Unvi/in. 1984). 



and tissues may not regenerate into whole 

 plants or may produce abnormal or sterile 

 plants (75). 



Progress in developing somaclonal variation 

 for plant improvement has been promising for 

 a few plant species (5,90,92). However, its gen- 

 eral application remains unproven. Further, it 

 is not yet possible to select through in vitro cul- 

 ture many valuable traits, such as yield or qual- 

 ity characters. This inability reflects a basic lack 

 of knowledge of the genetic mechanisms con- 

 trolling many such traits [38). 



Somatic Hybridisation 



A report conducted more than a decade ago 

 on the fusion of leaf protoplasts (cells from 

 which the cell walls have been enzymatically 

 removed) from two species of tobacco heralded 

 exciting possibilities (11). The process, termed 

 somatic hybridization, held promise of bridg- 

 ing many barriers to hybridization. Questions 

 of whether "impossible hybrids" could be ob- 

 tained were partially answered with reports of 

 a successful protoplast fusion from a tomato 

 and potato (72). Unfortunately, as with a hy- 

 brid sexually produced 50 years earlier by cross- 

 ing radish and cabbage, the resulting plant ex- 



hibited the least desirable characteristics of 

 each parent and was sterile (75). 



Research on irradiation and protoplast fusion 

 shows promise. By irradiating one set of pro- 

 toplasts, the genetic material is broken into 

 short sequences, some of which will make its 

 way into the fusion partner. The technique, 

 with considerable development, may eventu- 

 ally enable transfer of genes between sexually 

 incompatible species. 



Recent studies show the potential for trans- 

 ferring cellular organelles with their genetic in- 

 formation (chloroplasts and mitochondria) to 

 other species (15,41,75). This technique may be 

 useful in the transfer of genes for the limited 

 number of traits (e.g., photosynthetic efficiency, 

 herbicide tolerance, cytoplasmic male sterility) 

 found in these organelles. 



Application of somatic hybridization has 

 been limited to plants from three families: 

 Solonaceae (e.g., potato, tomato, tobacco); 

 Cruciferaceae (e.g., cabbage, rape); and Umbel- 

 liferaceae (e.g., carrots) (75). Regeneration of 

 whole plants from protoplasts often remains 

 an obstacle because little is known about the 

 culture conditions needed to cause protoplasts 



