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.
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