Principles of Plant Genetics and Breeding

330 CHAPTER 17
Genetic issues
The highest yield performance is obtained in the syn-1
generation, hybrid vigor declining with subsequent
generations. It is generally estimated that a synthetic
forage cultivar of cross-fertilized diploid or polyploid
species will experience a maximum yield decline of
10–12% from the syn-1 to syn-2 generation, as previously
stated. The yield decline is less in subsequent generations.
Sewall Wright proposed a formula to predict
the F2 yield of a group of inbred lines:
F 2 = F 1 − [(F 1 − P)/n]
where F 2 = expected performance of the F 2 , F 1 = mean
F 1 hybrid performance from combinations of inbred
lines, P = average performance of inbred lines, and n =
number of inbred lines. That is, one can increase the F 2
yield by increasing the average F 1 yield, increasing the
yield of parental lines, or increasing the number of lines
used to create the synthetic. This formula assumes that
the species has diploid reproduction and that the parents
are inbred. Hence, even though shown to be accurate
for maize, it is not applicable to polyploid species and
those that are obligate outcrossers.
The formula may also be written as:
syn-2 = syn-1 − [(syn-1 − syn-0)/n]
Studies involving inbred lines and diploid species have
indicated that as the number of parental lines increase,
the F 1 performance increases. Parental lines with high
combining ability will have a high F 1 performance. In
practice, it is a difficult task to find a large number of
parents with very high combining ability. Furthermore,
predicting yield performance of synthetic cultivars of
cross-fertilized diploid and polyploid forage species is
more complicated than is described by their relationship
in the equation. Given a set of n inbred lines, the total
number of synthetics, N, of size ranging from 2 to n is
given by:
N = 2n − n − 1
As inbred lines increase, the number of possible synthetics
increases rapidly, making it impractical to synthesize and
evaluate all the possible synthetic cultivars.
The theoretical optimum number of parents to
include in a synthetic is believed to be about 4–6.
However, many breeders favoring yield stability over
yield ability tend to use large numbers of parents
ranging from about 10 to 100 or more. Large numbers
are especially advantageous when selecting for traits
with low heritability.
Synthetics of autotetraploid species (e.g., alfalfa) are
known to experience severe and widespread decline in
vigor between syn-1 and syn-2, which has been partly
attributed to a reduction in triallelic and tetrallelic loci.
Higher numbers of tetrallelic loci have been shown to
be associated with higher agronomic performance (e.g.,
forage yield, seed yield, height) of alfalfa. The number
of selfed generations is limited to one. Selfed seed from
selected S 0 plants are intermated to produce the synthetic
population. The rationale is that S 0 plants with
high combining ability should contain many favorable
genes and gene combinations. Selecting specific individuals
from the segregating population to self could
jeopardize these desirable combinations.
Additive gene action is considered more important
than dominance genotypic variance for optimum
performance of synthetic cultivars. In autotetraploids
where intralocus and allelic interactions occur, high
performing synthetic cultivars should include parents
that have a high capacity to transfer their desirable performance
to their offspring. Such high additive gene
action coupled with a high capacity for intralocus or
allelic interactions will likely result in higher performing
synthetics.
Synthetic cultivars exploit the benefits of both heterozygosity
and heterosis. J. W. Dudley demonstrated
that yield was a function of heterozygosis by observing
that, in alfalfa crosses, the F 1 yields reduced as generations
advanced. Further, he observed that allele
distribution among parents used in a cross impacted
heterozygosity. For example, a cross of duplex × nulliplex
(A 2 × A 0 ) always had a higher degree of heterozygosity
than, say, a cross of simplex × simplex (A 1 × A 1 ),
regardless of the clonal generation used to make the
cross.
Natural selection changes the genotypic composition
of synthetics. The effect can have significant consequences
when the cultivar is developed in one environment
and used for production in a distinctly different
environment. There can be noticeable shifts in physiological
adaptation (e.g., winter hardiness) as well as
morphological traits (e.g., plant height). For example,
when growing alfalfa seed in the western US states (e.g.,
California) for use in the midwest, the cultivars may
loose some degree of winter hardiness, a trait that is
desired in the production region of the midwest. A way
to reduce this adverse impact is to grow seed crop in the
west using foundation seed from the midwest.