RA 00110.pdf - OAR@ICRISAT

Andrews 1984), there are operational differences as
a consequence of their morphological differences.
Pearl millet is generally earlier maturing than
maize and tillers freely at low plant densities. Large
amounts of seed (1000 or more) can be produced on
each of several heads (which can be used separately
for different breeding operations), and more tillering
produces an increased range of flowering per plant.
Selfed seed can be produced without manipulating
pollen, merely by bagging heads before stigma
emergence. A cross can be made by a single pollination
of a previously bagged head where the stigmas
have emerged but not the anthers.
In pearl millet, commercial hybrid production
depends on the use of cytoplasmic-genetic male sterility
and hence, is limited to combinations of fertility
restorers and lines into which male sterility has
been introduced by tedious backcrossing. In maize,
hybrids can be made between any two lines by detasselling
either line. This is possible because the
male and female flowers are on separate inflorescences.
These differences are acknowledged in the following
discussion, which deals largely with maize
experiences. In general, apart from the current restriction
of available seed parents for pearl millet
hybrids, pearl millet is an easier organism than
maize in which to conduct recurrent selection.
steps (not necessarily separate from each other) of
• observation of individuals or progenies followed
by selection,
• recombination of the selected fraction, and
• a repetition of the procedure.
A better definition might well include the objective
of maintaining genetic variation in the population in
which recurrent selection is taking place. This then,
requires a structured recurrent selection program.
Decisions are required on selection intensity and
effective population size. Once these choices have
been made, the future of the selection prograrm is in
part determined. The higher the selection intensity,
the greater the rate of gain. The smaller the effective
population size, the sooner the population will plateau
with a lowered selection limit. Thus, selection
intensity and effective population size are in conflict
with each other.
These concepts can be expressed in terms of the
probability that a favorable allele will be fixed in a
population. This probability is determined by the
initial frequency of the allele (q 0 ), the effective population
size (n), and the selection pressure that can be
brought to bear on the allele (s). H i l l and Robertson
(1966) showed that the probability of fixing an allele
could be expressed as
General Principles of Recurrent
Selection
In a broad sense, recurrent selection has been practiced
since people first began domesticating plants.
A definition of recurrent selection should include the
where e = 2.178 and is the base for natural logarithms.
Table 1 contains approximate probabilities of fixing
an allele with q 0 varying from 0.05-0.7, n varying
Table 1. The probability of fixation of a favorable allele 1 .
n
s
0.05
Initial frequency (qo)
0.1
0.3
0.5
0.7
64 1/2
32 1/2
16 1/2
8 1/2
8 1/4
8 1/8
0.9
0.8
0.5
0.3
0.2
0.1
1.0
0.9
0.8
0.5
0.3
0.2
1.0
1.0
1.0
0.9
0.7
0.5
1.0
1.0
1.0
1.0
0.9
0.7
1.0
1.0
1.0
1.0
1.0
0.9
1. Where e = 2.718 and is the base for natural logarithms, qo is the initial gene frequency, n is the effective population size, and s is the
selection coefficient.
Calculated from
1-e -2nsq o
-2ns
1 -e
108