Agriculture Reference
In-Depth Information
6.4.5
Recurrent selection
This involves the repeated inter-crossing of lines in a mixed population of parents with
mass selection of progeny for the required trait occurring at each generation. This method
often involves the use of male sterility to reduce the cost of multiple crossing of lines.
Recurrent selection has historically been promoted as a means for combining multiple
minor genes with minimal resources and thus useful for breeding in developing coun-
tries. A signifi cant disadvantage of the technique is that useful gene linkage blocks are
often disrupted through the multiple crossing processes. To reduce this, intense selection
is required to maintain good agronomic performance and quality. This however, usually
requires signifi cant resources that have often not been available. Evidence suggests this
method can be successful for some diseases of high heritability such as foliar pathogens,
but that it is much less likely to result in well-adapted quality germplasm. It is very unlikely
to be successful for root diseases where heritability/repeatability of selection is low.
6.4.6
2
progeny method
This is an example of a method that combines components of two of the above general
methods. It combines the principal of mass selection for one generation with pedigree
breeding over subsequent generations, but with populations rather than single plants
being subject to assessment.
The principal aim of the F
2
progeny method (Rathjen & Pederson, 1986) is to post-
pone heavy selection for a trait such as disease resistance until F
2
-derived lines can be
replicated in trials over space and/or time. This allows primary selection to occur on
the greatest diversity of lines using more objective measurements of adaptation. The F
2
progeny method avoids selection for disease resistance in the F
2
, when recessive genes
are poorly expressed, but may allow for mass selection based on other traits such as yield.
This method therefore has a great advantage for traits of low heritability. It also reduces
bookkeeping and work in the early generations.
Many minor disease resistance genes are recessive making this method effi cient for
selecting lines with multiple numbers of such genes. It is also an appropriate method for
selecting for resistance to diseases where screening may require a larger number of plants,
perhaps one or more whole plots, for each assessment. The method can also be combined
effectively with single backcrosses and top-crosses where resistance enrichment can take
place with marker screening in the BCF
1
generation.
F
6.4.7
Avoiding the most susceptible varieties
This involves ensuring that no variety of a crop is cultivated that is highly susceptible to a
pathogen that cannot be easily controlled by other methods. While this is not a breeding
method as such, it involves a different principle and is therefore treated separately. It has
frequently been observed that some varieties or breeding lines are extremely susceptible
to one or more pathogens. They do not just display a susceptible reaction type, but often
develop epidemics much faster and may become infected under a wider range of envi-
ronment conditions than other susceptible varieties. The effect on inoculum production
is illustrated in Figure 6.2 (Wallwork, 2007). Whilst only using hypothetical data, this
