SOMACLONAL VARIATION REVIEW

 

SOMACLONAL VARIATION

The accumulation of genetic variability is an important aspect in plant breeding. The increasing use of plant tissue (in vitro techniques) as an unconventional means of crop improvement has resulted in the introduction of genetic changes into such plants. These genetic alterations have been recovered in the plants regenerated from cell cultures, and could be used to develop new breeding lines. The occurrence of genetic variation among plants regenerated from in vitro culture has been referred to as somaclonal variation. Variations have been observed for morphological traits like pigment production, biochemical characters like nicotine synthesis and chromosome number and structure

TYPES OF SOMACLONAL VARIATION

Somaclonal variation results from both pre-existing genetic variation within the explants and the variation induced during the tissue culture phase .There are two types of somaclonal variation: heritable (genetic) and epigenetic. Heritable variation is stable through the sexual cycle or repeated asexual propagation; epigenetic variation may be unstable even when asexually propagated. Epigenetic variation is also known as developmental variation, and includes persistent changes in phenotype that involve the expression of particular genes (Hartmann and Kester, 1983). The best known example of epigenetic variation is the loss of auxin, cytokinin, or vitamin requirements by callus . . Other epigenetic changes include, extreme vigour ex vitro associated with either the reversion to juvenility (Swartz et al, 1981) or virus elimination (Abo El-Nil and Hilderbradt, 1971). Transient dwarfism is probably epigenetic also, and may be due to a carryover effect of growth regulators from the tissue culture medium.

GENETIC BASIS OF SOMACLONAL VARIATION

Although somaclonal variation has been studied extensively, the mechanisms by which it occurs remain largely either unknown or at the level of theoretical speculation in perennial fruit crops (Leva et al. 2012). Few studies have addressed the molecular basis or nature of somaclonal variation (Al-Zahim et al. 1999; Yang et al. 1999), though it was discovered that alteration in DNA methylation probably plays a role (Muller et al. 1990). Prior to the work of Evans and Sharp (1983) to describe somaclonal variation in tomato, the genetic basis of somaclonal variation in asexually propagated crops was not ascertained. It is now understood that for somaclonal variation to be applicable to a wide range of crops, detailed genetic information from the donor crops is necessary. Most of the early works to elucidate the genetic basis of somaclonal variation were done on sugarcane and potato. Both crops are asexually propagated; polyploids and can tolerate variation in chromosome number without concomitant disruption in agronomic characteristics. In particular, sugarcane is plastic, and even the intact sugarcane is a chromosomal mosaics (Krishnamurthi, 1981). However, according to Leva et al. (2012) the following variations have been observed: changes in chromosome number and structure, in which polyploidy is the most frequent. In addition, single nuclear gene mutations and less-defined genetic changes have been observed. Variation in chromosome numbers and structures, and chromosome irregularities (such as breaks, acentric and centric fragments, ring chromosomes, deletions and inversions) are observed during in vitro differentiation and among regenerated somaclones (Hao and Deng 2002; Mujib et al. 2007). Such rearrangements in chromosomes may result in the loss of genes or their function, the activation of genes previously silent, and the expression of recessive genes, when they become haploid. The irregularities in the chromosomes may be lost during plant regeneration and result in the production of ‘normal’ plants, or appear in the regenerated somaclones. Changes in chromosome number are commonly associated with reduced fertility and altered genetic ratios. Chromosome rearrangements have been implicated in the variations in regenerated plants, by analyzing meiosis in regenerated plants. Somaclonal variation may also be due to genetic mitotic recombination, mutations in chloroplast DNA (detected by both maternal inheritance and restriction enzyme analysis). It can also be as a result of the expression of existing genetic differences between the mother plant and the explants, or through the effects induced by the culture media (Tabares et al. 1993). In addition, transpositional events, such as the activation of transposable elements, which are pieces of DNA that move within and between chromosomes, putative silencing of genes and a high frequency of methylation pattern variation among singlecopy sequences, play a role in somaclonal variation

SOMACLONAL VARIATION IN SOME CROPS

Sugarcane (Saccharum officinarum L.): Sugarcane (Saccharum officinarum L.) is an economically important crop widely cultivated in the tropics to subtropics with an annual production of about 60 to 70 % world sugar (Shah et al. 2009).The potential of somaclonal variation for the genetic improvement of characters of agricultural importance was first demonstrated in Saccharum officinarum with the in vitro selection of a commercial variety resistant to Fiji disease (Heinz, 1973). There were variations in the morphology, cytogenetics and isoenzyme traits. Liu et al. (1972) reported on the morphological variation in stooling and erectness amongst somaclones. In addition, some somaclones were reported to be resistant to Fiji disease virus, downy mildew (Krishnamurthi, 1974, Krishnamurthi and Tlaskal, 1974), eyespot disease (Ramos Leal et al. 1996) and sugarcane mosaic virus (Nickel and Heinz, 1973). Salt tolerance somaclones have also been generated by a tissue culture cycle (Khan et al. 2004). Siddiqui et al. (1994) compared the brix % of canes of somaclones with those of their parents and found the somaclones were better than their parents in this character. Conversely, Khan et al. (2004) reported that the brix % of canes of somaclones was found to be less compared to their parents. The somaclones were better in tillers/plant, stalk height, number of nodes/stem and root band width. Evidently, somaclonal variation is very common in sugarcane, and it affects many important traits that can be used in the improvement of some varieties. Potato (Solanum tuberosum): Potato (Solanum tuberosum L.) is one of the most important vegetable crops in the world (Solomon-Blackburn and Baker, 2001). Potato is a native of South America (Peru) where considerable breeding programmes have taken place. With over 70% of the world’s potato grown in Europe, potato is considered a good source of antioxidants (Chen et al. 2007). In North America, the more than 70 year old variety called ‘Russet Burbank’ constitutes about 39% of the potato crop. As a vegetatively propagated, heterozygous and tetraploid crop, traditional breeding of potato is very difficult (Solmon-Blackburn and Baker, 2001). The same applies using botanical seeds on commercial cultivation, which is fraught by low germinability and large variability in the segment generations (Bordallo et al. 2004). Somaclonal variation has been reported in potato plants regenerated from protoplasts of the widely grown variety ‘Russet Burbank’

Maize (Zea mays); Maize is a very important used as food for man and livestock. It is also used in many industrial products such as textiles, ceramics and pharmaceuticals (Earle and Kuehnle, 1990). Moreover, maize has many other features that make it attractive material for studies of somaclonal variation. Matheka et al (2008) used somaclonal variation to select maize varieties resistant to drought in Kenya. Somaclonal variation in maize has also been shown to affect the mitochondrial genome. Selection for resistance in cultures of T-cytoplasm maize (sensitive to southern corn leaf blight T-toxin of Drechslera maydis Race T) by recurrent sub-lethal exposure T toxin resulted in the recovery of toxin-resistant plants. These same plants were also fertile in contrast to the male-sterility of the original parent (Gengenbach et al. 1977). These results have been confirmed by Brettell and Ingram (1979) and Brettell et al. (1980). They further indicated that the frequency of occurrence of these resistant variants was very high even when the toxin was not added to the cultures prior to regeneration. The restored male-sterility and toxin resistance were shown to be cytoplasmically inherited.

Rice (Oryza sativum) : Somaclonal variation on rice is particularly interesting because apart from being a model plant for the grass family (Poaceae) that includes all cereal crops, it is also a major crop that provides food for more than half of the world’s population (Ngezahayo1 et al. 2007). Several studies on somaclonal variation have been carried out in rice by using cultivars of both subspecies: indica and japonica (Yang et al. 1999; Kim et al. 2003; Roy and Mandal 2005). Ngezahayo1 et al. (2007) studied the nature of somaclonal variation at the nucleotide sequence level in the cultivar rice Nipponbare using RAPD and ISSR markers and by pairwise sequence analysis. Earlier reports on somaclonal variation in rice include those of Nishi et al. (1968) and Henke et al. (1978), all from rice callus. Variations were observed in number of tillers per plant, number of fertile tillers per plant, length of panicle, frequency of fertile seeds, plant stature and length of flag leaf. Oono (1978a and b) extensively and carefully analyzed homozygous maternal seeds from selfed double haploids of about 800 somaclones derived from callus. After two selfing generations, all the lines were examined for chloroplast content, flowering date, plant height, fertility and morphology with about 28.1% as true-to-types (normal parents). Variations were however observed in seed fertility, plant height and heading date. There were variations in chlorophyll deficiencies in the second generation. Sectorial analysis of plants derived from a single seed callus showed that at least most the variations were induced during culture.