Principles of Plant Genetics and Breeding

into wild habitats by livestock farmers, destroys wild
species and wild germplasm resources.
Action of breeders
Farmers plant what breeders develop. Some methods
used for breeding (e.g., pure lines, single cross, multilines)
promote uniformity and a narrower genetic base.
When breeders find superior germplasm, the tendency is
to use it as much as possible in cultivar development. In
soybean, as previously indicated, most of the modern
cultivars in the USA can be traced back to about half a
dozen parents. This practice causes severe reduction in
genetic diversity.
Problems with germplasm conservation
In spite of good efforts by curators of germplasm repositories
to collect and conserve diversity, there are several
ways in which diversity in their custody may be lost. The
most obvious loss of diversity is attributed to human
errors in the maintenance process (e.g., improper storage
of materials leading to loss of variability). Also, when
germplasm is planted in the field, natural selection pressure
may eliminate some unadapted genotypes. Also,
there could be spontaneous mutations that can alter
the variability in natural populations. Hybridization as
well as genetic drift incidences in small populations are
also consequences of periodic multiplication of the
germplasm holdings by curators.
General public action
As previously indicated, there is an increasing demand
on land with increasing populations. Such demands
include settlement of new lands, and the demand for
alternative use of the land (e.g., for recreation, industry,
roads) to meet the general needs of modern society.
These actions tend to place wild germplasm in jeopardy.
Such undertakings often entail clearing of virgin land
where wild species occur.
Selected impact of germplasm acquisition
Impact on North American agriculture
Very few crops have their origin in North America. It
goes without saying that North American agriculture
owes its tremendous success to plant introductions,
which brought major crops such as wheat, barley, soybean,
rice, sugar cane, alfalfa, corn, potato, tobacco,
PLANT GENETIC RESOURCES FOR PLANT BREEDING 97
and cotton to this part of the world. North America
currently is the world’s leading producer of many of
these crops. Spectacular contributions by crop introductions
to US agriculture include the following (see also
Plant introductions, p. 103):
1 Avocado: introduced in 1898 from Mexico, this crop
has created a viable industry in California.
2 Rice: varieties introduced from Japan in 1900 laid the
foundation for the present rice industry in Louisiana
and Texas.
3 Spinach: a variety introduced from Manchuria in
1900 is credited with saving the Virginia spinach
industry from blight disaster in 1920.
4 Peach: many US peach orchards are established by
plants growing on root stalks obtained from collections
in 1920.
5 Oats: one of the world’s most disease-resistant oat
varieties was developed from germplasm imported
from Israel in the 1960s.
Other parts of the world
A few examples include dwarf wheat introduced into
India, Pakistan, and the Philippines as part of the Green
Revolution, and soybean and sunflower into India;
these have benefited the agriculture of these countries.
Nature of cultivated plant genetic resources
Currently five kinds of cultivated plant materials are
conserved by concerted worldwide efforts – landraces
(folk or primitive varieties), obsolete varieties, commercial
varieties (cultivars), plant breeders’ lines,
and genetic stocks. Landraces are developed by indigenous
farmers in various traditional agricultural systems
or are products of nature. They are usually very variable
in composition. Obsolete cultivars may be described
as “ex-service” cultivars because they are no longer
used for cultivation. Commercial cultivars are elite
germplasm currently in use for crop production. These
cultivars remain in production usually from 5 to 10 years
before becoming obsolete and replaced. Breeders’ lines
may include parents that are inbred for hybrid breeding,
genotypes from advanced yield tests that were not
released as commercial cultivars, and unique mutants.
Genetic stocks are genetically characterized lines of
various species. These are advanced genetic materials
developed by breeders, and are very useful and readily
accessible to other breeders.