The Eleventh Regional Wheat Workshop For Eastern ... - Cimmyt

Increasing yield potential for marginal areas using genetic resources collections - Skovmand and Reynolds
Genetic resources
The genetic resources available for plant physiologists and breeders are found in the several
Triticeae gene pools. The concept of the gene pool was first proposed in 1971 by Harlan and
deWet (Harlan, 1992), who suggested the circular way of demonstrating the relationships
among gene pools. The primary gene pool consists of the biological species, including
cultivated, wild and weedy forms of a crop species; gene transfer in the primary gene pool is
considered easy. The secondary gene pool consists of the coenospecies from which gene
transfer is possible but difficult. The tertiary gene pool is composed of related genera of
annual and perennial grasses from which gene transfer is very difficult and can only be
exploited through use of special techniques.
Genetic resources of a cultivated plant species have been categorized by Frankel (1977) and
the FAO Commission on Plant Genetic Resources (FAO, 1983), though this categorization is
not followed by all centers involved in genetic resource conservation and utilization. These
categories are:
• Modem cultivars in current use;
• Obsolete cultivars, often the elite cultivars of the past, many found in the pedigrees of
modem cultivars;
• Landraces;
• Wild relatives of crop species in theTriticeae tribe;
• Genetic and cytogenetic stocks; and
• Breeding lines.
In the CIMMYT Wheat Collection, the classification has basically followed the categories as
outlined by Frankel and the FAO Commission on PGR (Skovmand et at., 1992). Recently,
however, a list with 21 categories was defined in the GRIP project (Skovmand et at., 2000b)
to describe the biological status of materials in the collection and other genetic resources.
When such specific categories are applied to collections, the efficiency of utilization IS
enhanced, which makes it easier for users to know exactly what they are working with.
The circles proposed by Harlan and deWet (Harlan, 1992) to describe the gene pools have
been very useful, and the concept has provided a rational basis for comparative taxonomies.
However, it gives the appearance that separations among the pools are clear-cut, with distinct
divisions between one pool and another, though Harlan (1992) states that the line of
demarcation may be fuzzy. Further, the circles do not reflect the relative difficulty of utilizing
the different gene pools, nor the relative cost of utilizing genetic resources within a gene pool
or within a species. Figure 1 presents a schematic diagram of the effort to transfer traits from
genetic resources to farmers' fields (Skovmand et at., 2000c). Within the primary gene pool,
when moving to closely related species, the cost of utilization increases. And again within a
species there are levels of genetic resources, from current high yielding cultivars to landraces,
that determine the cost of using these different genetic resources. Moving away from the
primary gene pool increases geometrically the effort required to utilize genetic resources such
as those in the secondary and teliiary gene pools. This is the reason that it is difficult to
release a commercially acceptable cuItivar if it does not have prior released cultivars in its
pedigree (Rajaram, pers. comm.), i.e., crosses with the secondary and tertiary gene pools tend
to disunite favorable gene complexes and thus affect performance. Technology extends the
gene pools and decreases the cost, as, for example, embryo rescue has done in the recent past.
Also, species found in the secondary gene pool, such as Aegi/ops tauschii, can be used as
readily as species in the primary gene pool through the production of hexaploid synthetic
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