Principles of Plant Genetics and Breeding

492 CHAPTER 28
Exotic
germplasm
Recycling
Figure 3 General scheme for the development of maize inbreds
and hybrids.
validated performance in hybrids are considered carefully
for choosing parents to start breeding populations.
The hybrid testing program provides information
about the best inbreds to initiate breeding projects.
Development of parental inbreds
Inbreeding and inbreeding depression
Hybrid development requires the development of
parental inbred lines (Figure 3). The inbred parents
used to produce the hybrids are developed through a
process of inbreeding and selection. The consequence
of inbreeding is the increase in homozygosity that
leads to homogeneous expression of traits and to
inbreeding depression (i.e., loss of vigor and productivity).
Self-pollination is the most common and
fastest system of inbreeding. As inbreeding reduces
the genetic variation within families and increases
the genetic variation among families, the efficiency
of selection among lines increases while it decreases
within lines. The level of inbreeding depression
depends on the trait. Traits that show high inbreeding
depression also show high heterosis (e.g., grain yield). Vigor, plant size, grain yield components, and grain yield are reduced
while time to flowering and incidence of barrenness increase with inbreeding (Hallauer & Miranda 1988). The improvement of
inbred lines over the last decades has reduced the inbreeding depression considerably in temperate maize. Therefore, source
germplasm oriented to develop hybrids has been selected for low inbreeding depression (particularly in the female parent
because high yielding lines reduce the cost of the hybrid seed) in addition to high combining ability.
Breeding methods to develop inbreds
Development of inbred parents can follow different breeding methods such as pedigree breeding, backcrossing, bulking, singleseed
descent, double haploids, etc.
Pedigree breeding is the most widely used breeding system to develop maize inbreds. Typically, specific crosses are made
between inbred lines, and then self-pollination is applied to the F 1 and subsequent generations to develop inbred lines that are
superior to either parent (transgressive segregants) through genetic segregation and recombination. Selection is applied among
progeny rows and among plants within S 1 families. It is common to have replicated nurseries for the S 1 families exposed to different
disease, insect, or abiotic stresses. This process of selfing and selection is repeated in successive generations (S 2 , S 3 , S 4 , S 5 ,...
S n ) until homozygous elite inbreds are developed. Effective phenotypic selection and greater selection intensity can be applied in
initial inbreeding stages for traits with high heritability such as pest resistance, maturity, morphological traits, etc.
The backcross breeding method is used widely in maize breeding to transfer one or a few traits/genes from the donor parent to
the recurrent and most desirable parent. With the advent of genetically modified organisms, major emphasis is devoted to accelerate
backcrosses to transfer the transgenes to elite inbreds. The use of DNA molecular markers has facilitated both the speed and
accurate recovery of the recurrent parent, and the reduction of linkage drag.
The bulk method, where the seeds for each selfing generation are harvested in bulk, and single-seed descent, where one or
a few seeds from each genotype are advanced each generation until approximate fixation is reached, are also used because of
their simplicity and low space requirements. Double haploids derived from maternal (e.g., stock6) or paternal (e.g., indeterminate
gametophyte, ig) gametes have been used to derive homozygous inbred lines instantaneously (Birchler 1994). However,
this method of developing inbred lines has not been used extensively because of the absence of any possibility for phenotypic
evaluation and selection, which generates an unselected, large sample of inbreds that needs to be evaluated for combining
ability.
Breeding methodologies
Segregating population
Inbreeding
and
selection
S 1
S 2
S 3
.
.
Si Superior inbreds
Commercial hybrids
Testcross evaluation
Hybrid evaluation
Improvements of technologies such as off-season nurseries, managed environments for screening against biotic and abiotic
stresses, adoption of experimental equipment (combines, planters, computers, etc.), and applications of molecular tools and
biological research have increased the accuracy and efficiency of inbred development. Experimental screening techniques have
been developed to increase heritability, such as in artificial insect infestation, disease inoculation, and environments managed for
higher plant densities or specific abiotic stress factors.