Principles of Plant Genetics and Breeding

patterns derived from the same open-pollinated cultivars
were reported. Other patterns include “ETO-composite”
× “Tuxpeno” and “Suwan 1” × “Tuxpeno” in tropical
regions. Alternate heterotic patterns continue to be
sought.
In rice, some research suggests two heterotic groups
within Oryza indica, one including strains from southeastern
China, and another containing strains from South
East Asia. In rye, the two most widely used germplasm
groups are the “Petkus” and “Carsten”, while in faba
bean three major germplasm pools are available, namely,
“Minor”, “Major”, and “Mediterranean”.
Even though various approaches are used for the
identification of heterotic patterns, they generally follow
certain principles. The first step is to assemble a large
number of germplasm sources and then make parent
populations of crosses from among which the highest
performing hybrids are selected as potential heterotic
groups and patterns. If established heterotic patterns
already exist, the performances of the putative patterns
with the established ones are compared. Where the
germplasm accession is too large to permit the practical
use of a diallel cross, the germplasm may first be grouped
based on genetic similarity. For these groups, representatives
are selected for evaluation in a diallel cross.
According to Melchinger, the choice of a heterotic
group or pattern in a breeding program should be based
on the following criteria:
1 High mean performance and genetic variance in the
hybrid population.
2 High per se performance and good adaptation of the
parent population to the target region.
3 Low inbreeding of inbreds.
Estimation of heterotic effects
Consider a cross between two inbred lines, A and B,
with population means of X¯ P1 and X¯ P2 , respectively.
The phenotypic variability of the F1 is generally less
than the variability of either parent. This indicates that
the heterozygotes are less subject to environmental
influences than the homozygotes. The heterotic effect
resulting from the crossing is roughly estimated as:
H F1 = X ¯ F1 − 1 / 2 (X ¯ P1 + X¯ P2 )
This equation indicates the average excess in vigor
exhibited by F 1 hybrids over the midpoint (midparent)
between the means of the inbred parents. K. R. Lamkey,
and J. W. Edward coined the term panmitic midparent
BREEDING HYBRID CULTIVARS 343
heterosis to describe the deviation in performance
between a population cross and its two parent populations
in Hardy–Weinberg equilibrium. Heterosis in the
F 2 is 50% less than what is manifested in the F 1 .
Types of hybrids
As previously discussed, the commercial applications of
hybrid breeding started with a cross of two inbred lines
(a single cross: A × B) and later shifted to the more economic
double cross [(A × B) × (C × D)] and then back
to a single cross. Other parent combinations in hybrid
development have been proposed, including the threeway
cross [(A × B) × C] and modified versions of the
single cross, in which closely related crosses showed
that the single cross was superior in performance to the
other two in terms of average yield. However, it was
noted also that the genotype × environment interaction
(hybrid × environment) mean sum of squares (from the
ANOVA table; see Chapter 23) for the single cross was
more than twice that for the double crosses, the mean
sum of squares for the three-way cross being intermediate.
This indicated that the single crosses were more
sensitive or responsive to environmental conditions than
the other crosses. Whereas high average yield is important
to the producer, consistency in performance across
years and locations (i.e., yield stability) is also important.
As R. W. Allard and A. D. Bradshaw explained, there are
two basic ways in which stability may be achieved in
the field. Double and three-way crosses have a more
genetically divergent population for achieving buffering.
However, a population of single-cross genotypes
that are less divergent can also achieve stability on the
basis of individual buffering whereby individuals in the
population are adapted to a wide range of environments.
Today, commercial hybrids are predominantly single
crosses. Breeders continue to develop superior inbred
lines. The key to using these materials in hybrid breeding
is identifying pairs of inbreds with outstanding combining
ability.
Germplasm procurement and
development for hybrid production
As previously indicated, the breeder needs to obtain
germplasm from the appropriate heterotic groups,
where available. It is critical that the source population
has the genes needed in the hybrid. Plant breeders in
ongoing breeding programs often have breeding lines in