Agriculture Reference
In-Depth Information
chemical methods often fail to provide accurate, fast qualitative or quantitative varietal identiication. This
is particularly a concern when some countries prohibit the use of genetically modiied seeds, others have
some levels of tolerance, and some prefer to take advantage of the value added products developed using
this technology. In all cases, testing for the adventitious presence of transgenic materials in conventional
seed or grain lots has become important for the global seed trade market.
Two principal concerns exist in maintaining genetic purity. First, the genetic composition of the vari-
ety initially developed by the breeder must be the same as that marketed to the grower after several gen-
erations of seed increase. Second, for hybrid seed crops, the success of hybridization must be ensured by
minimizing the percentage of seling and outcrossing.
Within varieties, genetic purity testing is important so that: (1) intellectual property protection through
Plant Variety Protection Act (PVPA) or utility patents can be obtained and then subsequently maintained,
(2) varieties can be created with uniform appearance and agronomic performance that meet the demands
of farmers, conditioners, and consumers, (3) varieties with stable genetic identities can be created so that
plant performance can be as predictable as possible given unpredictable environmental luctuations, and
(4) breeders can more completely and precisely characterize and measure genetic diversity so that genetic
resources can be more thoroughly evaluated in terms of plant performance and be more effectively utilized
for the creation of improved varieties (Smith and Register, 1998).
During seed multiplication, production and conditioning, several factors can affect cultivar purity,
among which are cross pollination, genetic shifts, mechanical mixture, and incorrect labeling. These fac-
tors can modify the genetic integrity of the original cultivar and make varietal identiication imperative to
determine the level of genetic purity of a seed lot.
Traditionally, plant breeders, seed companies and certiication agencies have determined genetic purity
using physical traits expressed by the seed, seedling, or mature plant. However, the success of laboratory
and ield tests is limited because environmental conditions (particularly stress) or post-harvest handling
operations mask or alter speciic seed and seedling morphological features. As a result, genetic purity
determinations have shifted to biochemical characterizations of seed/seedling enzymes, often separated
on electrophoretic gels. The corn seed industry, for example, routinely screens for hybrid genetic purity
using isozyme analysis (McDonald, 1995). Even these techniques are restricted to a few enzymatic assays
that produce differing polymorphic banding patterns for the inbred parents. The future release of superior,
genetically engineered varieties and hybrids places even greater burdens on developing genetic purity tests
with greater sensitivity.
History of Variety Testing
The importance of variety identiication was recognized early in the history of the seed industry. In the late
19th century and early 20th century, new ield crop varieties often lost their identity and became mixed with
other varieties of common, unnamed types. Others became known by different names. For example, 'Fultz'
wheat, distributed irst in 1871, was reported under 24 names and 'Silvermine' oats, introduced in 1895,
was grown under 18 different names (Parsons, 1985). Obviously, such confusion and misrepresentation in
the marketplace could not be tolerated by growers and consumers of seeds. Consequently, the International
Crop Improvement Association (now the Association of Oficial Seed Certiication Agencies) was estab-
lished in 1919 to help unify and standardize a new seed quality control program called seed certiication.
Seed certiication was established to maintain and make available to the public high quality seeds and
propagating materials of genetically distinct varieties. In its early years, it became an established and vital
institution for maintaining genetic integrity and increasing seed of improved varieties that were almost
without exception products of universities or other public agency breeding programs. Today, it remains the
primary method for maintaining identity of varieties on the open market. A four generation scheme has been
devised, including breeder, foundation, registered, and certiied seed. Throughout this process, maintenance
of varietal integrity is paramount. This is accomplished by both ield and laboratory testing as well as a
