Agriculture Reference
In-Depth Information
Individual selection is practised by sepa-
rating the descendants of each plant selected
from a given population. The value of a
selected plant is evaluated by testing its
descendants. Individual selection is basically a
selection of genotypes and is capable of deliv-
ering significant genetic improvement com-
pared to mass selection, which is essentially a
form of phenotypic selection. The efficiency
of the selection process is, however, rendered
more difficult by the fact that many of the
important traits for selection (e.g. hurd yield,
fibre percentage, quality, etc.) can only be
measured and evaluated after flowering. This
signifies that a mixture of pollen from com-
patible flowers and non-desirable flowers
contributes to the pollination of selected
plants. The control of cross-pollination has
been undertaken using the Bredemann tech-
nique (Bredemann, 1942), named after
German creator who used it for the selection
of hemp. However, undesirable plants will
not have been identified and removed and
could therefore pollinate the selected plants.
The Bredemann method is based on the
determination of the fibre content of the male
plant stems, generally measured on a cut
branch before the opening of the flowers.
Only those male plants with a high fibre con-
tent will be allowed to release their pollen and
pollinate female plants. The seeds obtained
from those females, themselves with a high
fibre content, will then be used to prepare the
next generation.
Genetic markers identifying specific sec-
tions of DNA closely associated with certain
traits can help give an idea of the plant's geno-
type and aid in selection (marker-assisted
selection) (Mandolino and Ranalli, 2002).
Genetic markers have been identified for
female characteristics by Shao and Shong
(2003), for male characteristics by Törjék
et al
(2002) and for cannabinoid profiling, as well
as for other interesting traits.
provide a base for further desirable genotype
selection. As the precise performance of each
individual and all of its descendants is preserved
throughout the whole selection process, com-
bination selection is often called genealogical
selection.
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SELECTION BY HYBRIDIZATION
.
the selection of
hybrids is founded on the use of the phenom-
enon known as heterosis, or hybrid vigour.
Hybrid vigour describes the increase in size,
vigour, productivity and general performance
of hybrids relative to the parent lines or race.
Bócsa (1954) was one of the first to create
populations of hybridized hemp.
Cultivars do not completely transmit all
their characteristics to their descendants.
Instead, they establish divisions within the next
generations. As a result, farmers find them-
selves having to buy certified hybrid seed stock
from breeders every year if they wish to benefit
from the advantages of hybrid cultures. Until
production of hybridized hemp was unknown.
Dr Ivan Bócsa from the Fleischmann Rudolf
Agricultural Research Institute in Kompolt,
Hungary, was the pioneer in the development
and practice of this phenomenon.
Unisex plants have their origin in the
crossing of dioecious female plants with mon-
oecious parent plants. The first generation,
after this initial crossing, is called unisex and
consists of 70-85% female plants, 10-15%
monoecious plants and only 1-2% male plants.
Manual elimination of all the male plants, origi-
nating from dioecious female plants, before
pollination allows the chance of crossings to be
controlled. The crossing is then undertaken
again in order to improve the yield in certified
seeds, which can then be used as seed stock for
cultivations destined for commercial use. The
majority of French cultivars belong to this
category.
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SELECTION BY MUTATION
.
Mutations arise sud-
denly in the genes and chromosomes and
heredity changes unpredictably. Depending
on the cause, mutations may be spontaneous
or may be provoked. The unpredictable
nature of engineered mutations makes their
use on a large scale difficult as a means of
selection.
RECOMBINANT SELECTION
.
As the variability of
indigenous species has been exhausted by
selection, new genetic variations for more elab-
orate selections have been obtained by selected
crossings of parent plants carefully. In combi-
nation selection, a recombination of parental
genes with a unique genotype is undertaken to
