Agriculture Reference
In-Depth Information
that new characters are introduced into already existing
cultivated genotypic background.
Why is genetic variability so important? Well it has
been continually stated that without genetic variability,
there can be no gain from selection. A further need is
related to the appearance of new forms of pest or disease
or new husbandry techniques, or new environmental
challenges. If a new disease became important in an
agricultural area to which all known cultivars were sus-
ceptible then it may be possible to identify new sources
of disease resistance from closely related wild or weedy
species.
There is a growing awareness of reduced germplasm
resource throughout modern agriculture. The greater
use of mono-culture crops and homozygous cultivars
has greatly reduced the genetic variability within our
agricultural crop species. For example, at the turn of the
century, farmers growing cereal crops were propagating
land races which were a collection of genetically differ-
ent types grown in mixture. Land races have been almost
completely replaced, in most countries, by homozy-
gous lines or hybrids and much of the variability that
existed has already been lost. Disease epidemics can also
greatly reduce genetic variability within a crop species.
The potato blight which affected western Europe (not
just Ireland) had the effect of greatly reducing the
genetic variability within European potato lines. Mod-
ern agricultural has become heavily reliant on chemical
weed control. In our agricultural systems weeds can
be almost completely eliminated leaving only the sin-
gle homozygous genotype that was planted by the
farmer.
Worldwide organizations have been formed with the
specific aim of conserving germplasm which is accessible
to breeders to search for new traits that are not avail-
able within the cultivated crops. The International Plant
Genetic Resources Institute (IPGRI) is one organiza-
tion which coordinates germplasm collection activities
on an international level. IPGRI is part of the Con-
sultative Group on International Agricultural Research
(CGIAR).
In addition to the national germplasm collections
and IPGRI, other organizations in the Consultative
Group on International Agricultural Research (CGIAR)
research centres, such as the International Potato
Research Center (CIP, Peru), the International Cen-
ter for Maize and Wheat Improvement (CIMMYT,
Mexico), the International Rice Research Institute
(IRRI) and the International Crops Research Institute
for the Semi-Arid Tropics (ICRISTAT, India) have
remits to maintain germplasm collections on specific
crop species.
Germplasm is available within the United States from
the Plant Introduction System. Genotypes are made
available from the location which maintains plant intro-
duction material or from one of the regional stations.
Some of the major crop responsibilities of each station
are as follows:
•
Northeastern Regional Plant Introduction Station,
Geneva, New York
: Perennial clover, onion, pea,
broccoli and timothy
•
Southern
Regional
Plant
Introduction
Station,
Georgia:
Cantaloupe,
cowpea,
millet,
peanut,
sorghum and pepper
North Central Regional Plant Introduction Station,
Ames, Iowa:
Corn, sweet clover, beets, tomato and
cucumber
•
Western Regional Plant Introduction Station, Pull-
man, Washington:
Alfalfa, bean, cabbage, fescue,
wheat, grasses, lentils, lettuce, safflower and chickpea
•
•
State and Federal Inter-regional Potato Introduction
Station (IR-1), Sturgeon Bay, Wisconsin:
Potato
Germplasm in itself is of little use to a plant breeder
unless there is information regarding the attributes
or defects of different genotypes. Most germplasm
collections have associated data banks detailing and clas-
sifying material within the collection. For example, the
Germplasm Resources Information Network (GRIN)
is a computerized data base containing information on
the location, characteristics and availability of accessions
within the plant introduction scheme. This informa-
tion is available to any breeder through the Database
Management Unit of the Agricultural Research Service,
Plant Genetics and Germplasm Institute, Beltsville,
Maryland.
THINK QUESTIONS
(1) Selection in a plant breeding programme can be
divided into three different stages: early generation
selection; intermediate generation selection; and
advanced generation selection. Briefly, state the
major differences between the three above stages.
(2) A 10
×
10 half diallel crossing design was used
to examine the potential of each of ten parents in
