SOURCE OF VARIATION - Faperta

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12 Δεκ 2012 (πριν από 4 χρόνια και 8 μήνες)

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SOURCE OF VARIATION

Source of variation



Plant genetic resource


Hybridization by sexual means


Induced mutation


Chromosome manipulation (ploidization)


Somatic hybridization


Transformation


Plant genetic resources


Basic for agricultural development


A reservoir of genetic adaptability that acts
as a buffer against environmental change


Its erosion threatens world food security


It is limited and perishable natural resource


It provides raw materials to produce new and
better plant varieties


It is irreplaceable source of important
characters such as resistance to disease and
productivity


It has inestimable value


Distribution of plant genetic
resource


The genetic variability of cultivated plants is
not randomly distributed throughout the
world


Zhukovsky (1965) identified 12 megagene
centers of crop
-
plant diversity and a number
of microgene center of wild growing species
related to crop plants


Zeven & Zukovsky (1975) listed the species
for different megagene center and the range
and extent of distribution of its diversity


Regions of diversity

Chinese
-
Japanese region

Soybean, Citrus, Litchi, Bamboo, Rami, Tea

Indochinese
-
Indonesian
region

Rice, Mango, Banana, Rambutan, Durian, Bread
fruit, Bamboo, Sago palm, Ginger, Coconut

Australian region

Eucalyptus, Acacia, Macademia

Hindustani region

Rice, Eggplant, Okra, cucumber, Banana, Mango

Central Asian region

Onion, Radish, Carrot, Sesame

Near Eastern region

Pear, Apple, Pea, Sesame

Mediterranean region

Durum, olive, Radish

African region

Durum, Cotton, kenaf, coffee

European Siberian
region

Peach, Chicory

South American region

Potato, Tobacco, Tomato, Groundnut, cassava,
cacao, rubber

Central American and
Mexican region

Maize, Chili, Cotton

North American region

Sunflower, plum, strawberry

Classification of plant
genetic resource



Cultivated species



1. Commercial varieties



2. Landraces or traditional local varieties


3. Breeding lines



4. Special genetic stocks



wild species



1. For direct use



2. For Indirect use



3. Potentially utilizable

Commercial varieties


Standardized and commercialized
varieties


Cultivars


They have been obtained by
professional plant breeder


They are characterized by high
productivity and high genetic
vulnerability

Landrace


They are primitive varieties or cultivars which have
evolved over centuries or even millennia and have
been influenced decisively by migration and both
natural and artificial selection


There is a large diversity between and within these
varieties


They are adapted to survive in unfavorable condition,
have low but stable levels of productivity and are
characteristic of subsistence agriculture


Greatest attention, due to:

1. the abundance of potentially useful genetic variation
they contain in already co
-
adapted gene complexes

2. The speed with which they are disappearing when
replaced by commercial varieties

Landrace


These varieties are not adequately represented in
existing collection today, due to:

1.
In many collections, more importance is given to pure
lines and selected materials

2.
Many of the populations collected in the field have
been subject to selection before being store,
thereby decreasing their genetic variability

3.
Most collections have been maintained traditionally
through periodic multiplications in small adjoining
fields with a consequent genetic erosion due to
hybridization, natural selection and the genetic drift
characteristic of small populations

Breeding lines


They are materials obtained by plant breeder as
intermediate product


They have a narrow genetics base because they have
originated from a small number of varieties or
populations

Special genetic stock


Stocks include other genetic combination, such as:


1. Mutant


2. Inter
-
specific hybridization product


3. Somatic hybridization product


4. Transgenic product

Direct use


Genetic erosion doesn’t occur by chance, but
selectively, against the most valuable material


People often select and consume the plant
possessing the most desired characteristics


Such consumption frequently involves the
destruction of either the seed or the plants
before seeds have been produced, setting of
negative selection that ends with the
elimination of those characters in a few
generation

Indirect use


There are wild species related to cultivated species
possessing beneficial characters that can be
transferred to cultivar relatives through crossing,
somatic hybridization even genetic engineering


In vegetative reproduced species, the wild relatives
can sometimes be used as rootstock for grafting.
Through this system, the crop can be extended to
marginal areas and also possible to prevent certain
infectious disease

Potentially utilizable


These species which are not used today have
characteristics or composition which make their use
in the future probably


This includes many wild species for which analysis in
pharmaceutical laboratories has revealed contents of
certain medicinal substances which are higher than in
species traditionally used to obtain these product

Conservation of plant
genetic resource

Objective


To conserve sufficient diversity within each
species to ensure that its genetic potential
will be fully available for breeding work


Conservation system

1.
In Situ

2.
Ex Situ

These two system should be considered complementary,
not antagonistic

In situ conservation


It consists in the legal protection of the area and
habitat in which the species grows


This is the preferred technique for wild plant


The advantage is the evolutionary dynamics of the
species are maintained


The drawback is the cost, and the social and political
difficulties which occasionally arise

Ex situ conservation


It implies the collection of representative samples of
the genetic variability of a population/cultivar, and
their maintenance in germ
-
plasm banks or botanical
gardens as seeds, shoots, in vitro culture, plants


It is mainly used for cultivated plants multiplied by
seeds

Advantage

Drawback

1. The control materials in a small
space under intensive care

1. The germ
-
plasm cease to evolve,
and the natural processes of
selection and continuous
adaptation to local habitat are
halted

2. The materials is easily access to
plant breeder

2. Genetic drift (random loss of
diversity due to the fact that the
samples collected and multiplied
are necessarily very small)

3. Selection pressure (the material
is usually multiplied in
phytoecogeographical area
different from those where it was
collected)

Ex situ conservation of genetic resource



Collection



Maintenance



Multiplication



Evaluation



Exchange

Collection


The team should have adequate knowledge of botany,
ecology, population genetics, plant breeding and plant
pathology


The team must be familiar with species to be
collected and to have a good knowledge of the
country or region where the expedition is conducted
including socio
-
ecological and cultural aspects of the
farming


Team must have a good knowledge of the plant habit
and breeding system

To collect the maximum genetic diversity and, if
possible, to obtain samples that maintain the allelic
frequencies of the collected populations or
varieties

Collection


Base collection


Collections stored under long
-
term conditions


Active collection


Collections stored under medium
-
term
conditions


Working collection


Collections usually stored under short
-
term
conditions (breeder’s collection)

Sample


A most important aspect of the collection of the
material, since a sample must be representative of
the population genetic variability



The main decisions:

1.
The number of samples to be collected from each
plant

2.
The number and distribution of the plant to be
collected in each site

3.
The number and distribution of the sites within a
given area where collecting will be carried out




The answer are not always the same and will depend
on the specific circumstance of each case

Collection’s usefulness



A very relative term


It may vary according to the collectors


Plant breeders will look for useful agronomic
characteristics (selective sampling)


Population geneticist may try to collect randomly
(random sampling)

Field Passport


A very important data



Including:

1.
Climatic characteristic

2.
Soil characteristic

3.
Type of vegetation

4.
Type of integrated pest



The information provided by farmers and field
workers living in collecting area will be of unique
value

Maintenance

In the form of
core collection



Development of a small group accessions


It represents collection with a minimum of
repetitiveness in the genetic diversity of crop
species and its wild relative


It is believed to contain most of known genetic
diversity


It can be used as a point of entry to the available
collections of a crop


It should not replace existing collections


It is a way of making existing collections more
accessible


Maintenance System


Dependent upon propagation system:


seed propagated species


vegetative propagated species

Seed propagated species

1. Its storage longevity can be induced by decreasing its storage
temperature and humidity

2. It is affected by seed type


a. Orthodox


b. Sub orthodox


c. Recalcitrant


Safe long
-
term of orthodox seeds


It requires careful control of the environment in
which the seed are kept


Seed moisture content is the most important factors
affecting seed storage life


Seed stored under moisture proof container at about
-
18
°
C can maintain good viability for a century or
longer


For medium
-
term storage, 5 % MC seed can be stored
below 15
°
C


Recalcitrant seeds


The period of viability varies between 2 weeks and
several months


There are some major economic value i.e. cocoa,
coconut, rubber

Vegetative propagated species


Field growing collections as in arboretum, a field gene
bank, a botanical garden or a nature reserve


Controlled humidity and temperature conditions for
cuttings, bulbs and tubers. It is only practical for
short and medium term or used in conjunction with a
field gene
-
bank


In vitro technique in slow growth conditions under
minimum media, low temperature and low light
intensity

Multiplication


Problem of germ
-
plasm collection


loss of the
germination capacity of stored materials


It varies according to species and variety


Germination test is necessary

The important use of multiplication:

1.
Keeping storage material a life

2.
Meeting the demand

Important aspect of
multiplication


Avoiding genetic contamination by taking into account
the reproductive characteristics of species,
particularly the out
-
crossing rate


The site should have ecological characteristics similar
to those where the materials was collected, in order
to prevent selection that can change the allelic
frequencies


It is extremely important to take advantage of the
process of rejuvenation or multiplication in order to
eliminate viral or other infectious diseases

Evaluation


To able to be used with maximum efficacy, stored
materials must be evaluated


Pre
-
breeding (Other term)


It can deal with one or several possible aspects i.e.
agronomic, pathological, morphological, biochemical,
cytological and other things


All data can help toward detecting duplications and
differences among the conserved samples

Development breeding (Germplasm enhancement
)

Program which aim to facilitate the utilization of plant
germ
-
plasm include the process of pre
-
breeding

Descriptor


Characters considered important or useful in the
description of a population


Differ according to species as to whether they have
been selected by plant breeders, botanists,
geneticists or experts in other disciplines


The degree of usefulness depends on the objectives


There is a tendency toward accepting compromise
solutions through selection of a minimum number of
universally accepted descriptor that can facilitate
the exchange of information and material (protein
and molecular marker as a fingerprint)

A good documentation system is the key to
the effective utilization of the materials
deposited in a germ
-
plasm bank

Exchange


It can be achieved with the consent and/or
agreement of the parties involve and often
require international cooperation and
agreement


The exchange of material also requires
adequate inspection and testing services as
well as quarantine facilities that can reduce
to a minimum the risks of spreading pests and
diseases


In vitro techniques for transfer of germ
-
plasm are widely used for some crops