There are four basic recurrent selection schemes,
based on how plants with the desired traits are identified:
Simple recurrent selection. This
is similar to mass selection with one or two years per cycle. The procedure does
not involve the use of a tester. Selection is based on phenotypic scores. This procedure
is also called phenotypic recurrent
selection.
Recurrent selection for general
combining ability. This is a half-sib progeny test procedure in which a wide
genetic based genotype is used as a tester. The test cross performance is
evaluated in replicated trials prior to selection.
Recurrent selection for specific
combining ability. This scheme uses an inbred line for a tester. The test cross
performance is evaluated in replicated trails before
selection.
Reciprocal recurrent selection. This scheme is
capable of exploiting both general and specific combining ability. It entails
two heterozygous populations, each serving as a tester for the other. Two genetically
different populations are altered to improve their crossbred mean. To achieve
this, individual plants from two populations are selfed and also crossed with
plants from the reciprocal female tester population.
Intrapopulation improvement methods
Common intrapopulation improvement methods in use
include mass selection, ear-to-row selection, and recurrent selection.
Intrapopulation methods may be based on single plants as unit of, or family.
Individual plant selection methods
Mass selection
Mass selection for line development is different
from mass selection for population improvement. Mass selection for population
improvement aims at improving the general population performance by selecting
and bulking superior genotypes that
already exist in the population.
Key features
The selection units are individual plants.
Selection is solely on phenotypic performance. Seed from selected plants are bulked
to start the next generation. No crosses are made, but progeny test is
conducted. The process is repeated until a desirable level of improvement is observed.
Genetic issues
The effectiveness of the method depends on the
heritability of the trait since selection is solely on the phenotype. It is
also most effective where additive gene action operates. Effectiveness of mass
selection also depends on the number of gene involved in the control of the
trait of interest. The more additive genes that are
involved, the greater the efficiency of mass
selection. The expected genetic advance through mass selection is given by the
following
_where sp is the phenotypic
standard deviation in the population, s2 A is the additive variance, s2 D is
the dominance variance, and the other factors are interaction variances. DGm doubles
with both sexes. This large denominator makes mass selection inefficient for low
heritability traits. Selection is limited to only the female parents since
there is no control over pollination. Procedure
_ Year 1. Plant the source
population. Rogue out undesirable plant before flowering, and then select several
hundreds of plants based on phenotype. Harvest and bulk.
_ Year 2. Repeat year 1. Grow
selected bulk in a preliminary yield trial, including a check. The check is the
unselected population, if the goal of the mass selection is to improve the
population.
_ Year 3. Repeat year 2 for as
long as progress is made.
_ Year 4. Conduct advanced yield
trial. The mass selection may be longer, depending on progress being made.
Advantages These are highlighted in Chapter 16. Disadvantages
_ Using phenotypic selection
makes selection of superior plant often difficult.
_ Lack of pollen control means
both desirable and undesirable pollen will be involved in pollination of the
selected plants.
_ If selection intensity is high the
possibility of inbreeding depression is increased, as well as the probability
of losing individuals with desirable combinations.
Modifications
_ Stratified or grid system.
Proposed by C.O. Gardener, the field is divided into small grids with little
environmental variance. An equal number of superior plants is selected from each
grid for harvesting and bulking.
_ Honeycomb design. Proposed by
A. Fasoulas, the planting pattern is triangular rather than the conventional rectangular
pattern. Each single plant is at
the center of a regular hexagon, with other six
equidistant plants, and is compared to the other six equidistant plants. There
are modifications that are sometimes complex to apply
0 comments:
Post a Comment