Principles of Plant Genetics and Breeding

(a)
Hybrids with GP3
Anomalous, lethal, or completely sterile
GP4
All species that
can be crossed with GP2
with at least some fertility
GP4
GP1
Subspecies A:
cultivated races
Biological species
Subspecies B:
spontaneous races
GP1
GP4
Gene transfer possible but
may be difficult
Gene transfer not possible or
requires radical techniques
GP4
PLANT GENETIC RESOURCES FOR PLANT BREEDING 93
early growth stages. The advantages and disadvantages of some commonly used techniques for characterization of PGRs are
summarized in Table 2 (FAO 1996; see also Hammer 2004, p. 127).
Germplasm enhancement
PGRs are fundamental in improving agricultural productivity. These resources, fortunately stored in gene banks around the world,
include an assortment of alleles needed for resistance and tolerance to the diseases, pests, and harsh environments found in their
natural habitats. However, only a small amount of this variability has been introgressed to crop species (Ortiz 2002). Most cereal
breeders do not make much use of the germplasm of landraces and wild and weedy relatives existing in active collections. The
valuable genetic resources are essentially “sitting on the shelf” in what have been dismissively termed “gene morgues”
(Hoisington et al. 1999). Germplasm enhancement may be one of the keys to maximizing the utilization of this germplasm. It has
become an important tool for the genetic improvement of breeding populations by gene introgression or the incorporation of wild
and landrace genetic resources into respective crop breeding pools. The term “germplasm enhancement” or “prebreeding” refers
to the early component of sustainable plant breeding that deals with identifying a useful character, “capturing” its genetic diversity,
and the transfer or introgression of these genes and gene combinations from non-adapted sources into breeding materials
(Peloquin et al. 1989).
The gene pools as defined by Harlan and de Wet (1971) have formed a valid scientific basis for the definition and utilization of
plant genetic resources (Figure 1). More recently, however, plant transformation and genomics have led to a new quality that has
been defined by Gepts and Papa (2003) as a fourth gene pool, whereas Gladis and Hammer (2002) earlier concluded that information
and genes from other species belong to the third gene pool. The fourth gene pool should contain any synthetic strains with
nucleic acid frequencies (DNA or RNA) that do not occur in nature.
The most widespread application of germplasm enhancement has been in resistance breeding with genetic resources of wild
species. Backcross followed by selection has been the most common method for gene introgression from wild germplasm to
breeding materials.
However, some problems still remain for genetic enhancement with wild species: linkage drag, sterility, the small sample size
of interspecific hybrid population, and restricted genetic recombination in the hybrid germplasm (Ortiz 2002). Transgenesis
allows us to bypass sexual incompatibility barriers altogether and introduce new genes into existing cultivars. In recent years,
(b)
GP4 GP3 GP2
GP1
GP2 GP3 GP4
Figure 1 (a) The modified gene pool concept, established by Harlan and de Wet (1971). GP1: the biological
species, including wild, weedy, and cultivated races. GP2: all species that can be crossed with GP1, with some
fertility in individuals of the F 1 generation; gene transfer is possibly but may be difficult. GP3: hybrids with GP1 do
not occur in nature; they are anomalous, lethal, or completely sterile; gene transfer is not possible without applying
radical techniques. Information from other genes refers to comparative genomic information on gene order and
DNA sequence of homologous genes. GP4: any synthetic strains with nucleic acid frequencies (DNA or RNA) that
do not occur in nature. (b) Example of an organismoid or hypothetically designed crop with a genome composed of
different gene pools and synthetic genes (for further explanation, see Gladis and Hammer (2002)).