Developed by Hallauer and Eberhart as modifications
of the method by Comstock and colleagues, the fullsib method requires at least
one of the populations to be prolific. The recombination units are half-sibs.
Developed for maize, full-sib families are produced by pairing plants from two
populations, A and B. The top ear of a plant from population A is crossed with a
plant from populations B. The lower ear is selfed to be saved as remnant seed.
The same is done for the reciprocal plant from population B, if they have two
ears, otherwise, they are selfed.
_ Season 1. Plant population A as
females in an isolated block and population B as males in field 1. Plant
population B as females and population A as males in field 2. The upper ears in
each field are open pollinated, while the lower ears are protected and pollinated
manually. The result is that the upper ear is an interpopulation half-sib
family while the lower ear is an intrapopulation half-sib family.
_ Season 2. Evaluate 100–200 A _
B and B _ A halfsibs in replicated trials. Select best half-sibs from both sets
of crosses.
_ Season 3. Plant the remnant
seed of lower ears selfed by hand pollination that corresponds to the best A _
B half-sibs in ear-to-row as females.The males are the bulk remnant half-sib
seed from population B corresponding to the best B _ A
crosses. They are randomly mated. The open
pollinated seed in populations A and B are harvested to initiate the next
cycle.
Advantages
_ As compared to the half-sib method,
one half of the families are evaluated in each cycle because the evaluation of
each full-sib reflects the worth of two parental plants, one from each
population.
_ Superior S0
_ S0 crosses may be advanced in
further generations and evaluated as S1 _ S1, S2_ S2,. . . . . . ., Sn_ Sn to
allow the breeder to simultaneously develop hybrids while improving the populations.
Genetic issues
Another advantage of this method is that additive genetic
variance of full-sib families is twice that of the half-sib families. The
expected genetic gain is given by:
where sPFS is the phenotypic
standard deviation of the full-sib families.
Application
The scheme has been used in crops
such as maize and sunflower with reported genetic gains of the magnitudes of
2.17% for population per se and 4.90% for the population hybrid.
Optimizing gain from selection in population
improvement
The goal of the breeder is to make systematic
progress in the mean expression of the trait of interest from one cycle to the
next. Achieving progressive gains in yield depends on several factors.
_ Genetic variance. As previously
indicated, additive genetic variance is critical to increase in gains per cycle.
Additive genetic variance can be increased
through increasing diversity in the entries used in
population improvement.
_ Selection intensity. The rate
of gain with selection is increased when selection intensity is increased. The number
of individuals selected for recombination in each cycle should be limited to
the best performers.
_ Generations per cycle.
Breeder’s choice of the breeding system to use in a breeding project is
influenced by how rapidly each cycle of selection can be completed. When
possible, using 2–3 generations per year can increase yield gains. Multiple
generations per year is achieved by using off-season nurseries, or planting in
the dry season using irrigation.
_ Field plot technique. Breeders
select in the field, often handling large numbers of plants. Heterozygosity in
the field should be managed by using proper experimental designs to reduce
random variation. Whenever possible, uniform fields should be selected for
field evaluations. The cultural conditions 348 CHAPTER 17 should be optimized
as much as possible.This practice will reduce variation between replications.
Other factors to consider are plot sizes, number of plants per plot, number of
replications per trial, and number of locations. Implemented properly, these
factors reduce random variations that complicate experimental results.
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