Agriculture Reference
In-Depth Information
7 Application of Haploid Production
7.1 Development of Pure Homozygous Lines
Homozygous true breeding cultivars are highly important for screening of high
yielding lines and to produce hybrid vigour as a method of crop improvement. Ob-
taining homozygous diploid plants by conventional methods is difficult in perenni-
als. From several decades to over a hundred years are required to obtain a pure line
by means of successive inbreeding throughout many generations. The seed set by
inbreeding in many trees is very low, usually only a few of ten thousandth or some-
times no seed can be obtained at all; therefore, it is impractical to obtain pure lines
by inbreeding (Chen
1986
). Moreover, conventional method of haploid production
by inbreeding is impossible if the plant is strictly cross-pollinating in nature. On the
other hand, homozygous diploid plants can be achieved in a single generation by
diploidization of in vitro raised haploids through colchicine treatment.
7.2 Genetic Studies
Because of the lack of accurate material in research work, the progress in the study
of genetics in trees is much slower than that in annual herbaceous plants. The ge-
netics of a lot of important traits in economically important plant species has not
been clearly demonstrated as yet. As a result, it is still unknown whether the desir-
able characters of the parents will appear in their progenies. Only when crossing
between different homozygous diploid plants is carried out, we can gain a clear idea
of dominance of genes controlling various characteristics and that these character-
istics are either monogenic or polygenic (Chen
1986
). Furthermore, if we can use
the haploid plants as samples of gametes, then we can obtain directly the recombi-
nation value between genes. Moreover, we can also use the haploid plants to study
chromosome homology within genome or between genomes.
7.3 Gametoclonal Variation
The “gametoclonal variation” arises among plants regenerated from cultured ga-
metic cells consisting of differences in morphological and biochemical character-
istics as well as in chromosome number and structures that are observed. Besides
yielding haploids, in vitro androgenesis helps in the screening of gametophytic
variation at plant level. Pollen grains within an anther form a highly heterogeneous
population because they are the product of meiosis which involves recombination
and segregation. Therefore, each pollen plant is genetically different from the other.
The gametoclonal variants being hemizygous in nature, express also the recessive
characteristics in the R
0
plants (Bhojwani and Razdan
1996
). Different sources of
