Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 344

Clones, as previously stated, are exact replicas of the genotype from which their source tissue had been derived. Commonly, however, clonal propagation, occurring under a tissue culture environment, produces materials that are not exact replicas of the original material used to initiate the culture. Such variation resulting not from meiosis but from the culture of somatic tissue is referred to as somaclonal variation, and the variants somaclones. The variation observed may be transient (epigenetic) or heritable (genetic in origin). It is important to authenticate the presence of a true mutational event before using the somaclone in a breeding program as a valuable source of variation. Somaclonal variants can be recovered in tissue culture with selection pressure (e.g. deliberate inclusion of a toxic agent in the culture medium) or without selection pressure (the basic cultural medium).

A variety of mechanisms have been implicated in this phenomenon. Chromosomal changes, both polyploidy and aneuploidy have been observed in potato, wheat, and ryegrass. Some research suggests mitotic crossovers to be involved whereas cytoplasmic factors (mitochondrial genes) have been implicated by others. Further, point mutation, transposable elements, deoxyribonucleic acid (DNA) methylation, and gene amplification are other postulated mechanisms for causing somaclonal variation. One more trivial source of variation in plants derived from tissue culture is that they derived from mutated section of the explants. Somatic cells may have undergone mutations (leading to chimeras, see Chapter 25). Tissue from chimeric plants may lead to genetically different progeny.

As a breeding tool, breeders may deliberately plan and seek these variants by observing certain factors in tissue culture. Certain genotypes are more prone to genetic changes in tissue culture, polyploids generally being more so than diploids. Also, holding the callus in undifferentiated state for prolonged periods of time enhances the chances of somaclonal variation occurring. Not unexpectedly, the tissue culture environment (medium components) may determine the chance for heritable changes in the callus. The inclusion of auxin 2, 4‐D enhances the chances of somaclonal variation.

The value of somaclonal variation as a breeding tool is evidenced by the successes in various species (Table 7.1). These include disease resistance (e.g. Helminthosporium sacchari in sugarcane, and Fusarium in Apium graveolens) and resistance to various abiotic stresses.

Table 7.1 List of crops where desirable and heritable somaclonal variations have been reported.

Species	Characters, which were modified
A. Monocotyledons
1. Allium sativa	Bulb size and shape; clove no.; aerial bulbit
2. Avena sativa	Plant ht.; heading date; awns
3. Hordeum spp.	Plant ht.; tillering
4. Lolium hybrids	Leaf size; flower, vigor; survival
5. Oryza sativa	Plant ht.; heading date; seed fertility: grain no and wt.
6. Saccharum officinarum	Diseases (eye spot, fiji virus, downy mildew, leaf scald)
7. Triticum aestivum	Plant and ear morphology; awns; gliadins; amylase; grain wt., yield
8. Zea mays	T toxin resistance; male fertility; mtDNA
B. Dicotyledons
9. Lactuca sativa	Leaf wt., length, width, flatness, and color
10. Solanum lycopersicum	Leaf morphology; branching habit; fruit color; pedicel; male fertility; growth
11. Medicago sativa	Multifoliate leaves; elongated petioles; growth; branch no.; plant ht.; dry matter yield
12. Solanum tuberosum	Tuber shape; maturity date; plant morphology; resistance for early and late blight; photoperiod; leaf color; vigor; height.; skin color.