Crop Improvement Through Plant Tissue Culture - Crop Improvement: New Approaches and Modern Techniques - page 127

low frequency with which they occur in nature (usually 0.001-0.01 %). Spontane-
ous production of haploids usually occurs through the process of parthenogenesis
(embryo development from an unfertilized egg). However, they reproduce the char-
acters of the male parent alone suggesting their origin through ovule androgenesis
(embryo development inside the ovule by the activity of the male nucleus alone).
Haploid plants have the gametophytic number of chromosomes (Atanassov et al.
1995 ; Zapata Arias et al. 1995 ). The production of haploids in tomato has been tried
and is still at a poor stage of development. The process of early embryogenesis from
isolated microspores and the disruption of normal meiotic development and change
of developmental fate towards callus proliferation, morphogenesis and plant regen-
eration have been shown in tomato by using light and electron microscopy (Segui-
Simaro and Nuez 2007 ). For cell culture studies and breeding in flax, haploid and
double haploid material and homozygous lines need to be produced. Anther culture
has proved to be the most successful method producing doubled haploid lines in
flax (Obert et al. 2009 ).
A fast and cheap method to obtain pure or homozygous lines is a priority for
hybrid seed production in important crop plants. Pure lines can be produced tra-
ditionally by inbreeding and selection techniques, which are time consuming and
costly. Alternatively, it has become possible through a biotechnological approach to
accelerate the production of homozygous lines i.e., the induction of androgenesis to
generate double haploid plants. Androgenesis reduces this process to a single gen-
eration, which implies time and cost saving. Due to these advantages, androgenic
doubled haploids are the choice in a number of important crop plants where the
methodology is well set up. In solanaceae family, crops like eggplant and pepper
anther cultures are used for doubled haploid production and recent advances in the
knowledge of embryo development are opening new ways to achieve the final goal
of an efficient protocol in recalcitrant species (Segui-Simaro et al. 2011 ).
Gynogenesis is the phenomenon of production of whole plants from the unfer-
tilized ovules. In Gentian ( Gentiana triflora , G. scabra and their hybrids) an or-
namental flower, unfertilized ovules were cultured in a medium containing a high
concentration of sucrose (Doi et al. 2011 ) results in production of young plantlets.
Although the embryos showed genotypic variation, all the genotypes gave response.
The ovules collected from flower buds just before anthesis showed higher response.
The dark culture condition also gave more number of haploid embryos as in 16-hour
light condition.
SomaclonalVariation
The phenotypic variation of plants regenerated from cell culture is referred to as so-
maclonal variation. Apart from the mutant cell lines and plants obtained as a result
of mutations many variants have been obtained through the tissue culture process
cycle itself. The somaclonal variants may be genetic or epigenetic and are usually
observed in the regenerated plantlets (Velker et al. 2012 ). These are dependent on
the natural variations in a population of cells. Somaclonal variation may be due to