Principles of Plant Genetics and Breeding

BREEDING SOYBEAN 525
An advantage of using parent–offspring regression to estimate heritability is that it is straightforward (Hanson 1963, p. 129).
However, F2 individuals and derived F3 progenies are grown in separate environments (years), with no estimate of environmental
variance and no estimate of genotype × environment interaction. Depending on the variability of the trait measured, biases can be
significant (Hanson 1963, p. 129).
In summary, the development and utilization of multiple generations (P1 , P2 , F1 , F2 , F3 , BC1P1 , BC1P2 , BC1P1S1 , BC1P2S1 , etc.)
can provide flexibility and opportunities for multiple estimates of inheritance factors, both qualitative and quantitative. Such
populations are also complementary for developing breeding populations.
In conjunction with the above early generation studies, F2 plants were grown for the purpose of producing recombinant inbred
lines (RILs) by single-seed descent. The use of RILs in genetic studies requires that the finished inbred lines be an unselected sample
of the F2 population; ideally, an inbred line for each F2 plant grown (Hallauer & Miranda 1981, p. 89). If inbreeding depression
is a problem, resulting in the death of some plants before becoming inbred, a random non-selected RIL population may be
difficult to obtain. However, this is usually not the case with self-pollinated crops such as soybean. The net result of using RILs to
estimate inheritance, as opposed to an F2 population, is that the Φ2 A among RILs is twice as large as the Φ2 A among F3 families
or among F2 individuals. Because each RIL is theoretically completely inbred, Φ2 D is zero (there are no intralocus genotypic interactions),
which also means that there are no unusable Φ2 AD interactions. Hence, selection among RILs is expected to be more
effective than selection among F2:3 families (Bernardo 2002, p. 180).
However, Hallauer and Miranda (1981, p. 91) noted that the use of RILs has two serious handicaps. First, as already noted, RILs
require the development of a set of unselected inbred lines that are representative of genotypes of a reference population (the F2 ).
This can be difficult if, during the selfing process, inbreeding produces weak plants that die and if high disease pressure kills
plants. Second, the time required to develop RILs is much greater than that for developing and testing F3 progeny rows. Hence, it
is wise to make early generation estimates of predicted gain from selection so as to determine if early generation selection can be
profitable. Early generation selection can be highly desirable, if it is possible, enabling a greater allocation of resources to the most
promising families (Bernardo 2002, pp. 119, 180, and 181).
However, where heritabilities are low (due to environment, genotype × environment interactions, Φ2 D , etc.), RILs can provide
useful estimates of genetic parameters and can be complementary to the breeding program. Replicated field trials across multiple
locations and years with RILs will likely result in better estimates of heritability, better estimates of genetic gain from selection,
and in the potential development of improved lines.
The development and utilization of RILs can facilitate the construction of a genetic map based on molecular marker linkages in
the RIL population. If sufficient markers are located at enough strategic points, markers can be detected that are linked to genetic
factors controlling the expression of quantitative traits. This was an important consideration in the development of RILs segregating
for tolerance to charcoal rot and will allow for the development and release of charcoal rot-tolerant varieties, along with the
release of molecular markers tightly linked to genetic factors affecting tolerance. Breeders can then create populations and test
them in their own unique environments, while selecting for charcoal rot tolerance using molecular markers.
The most cost-effective way to reduce losses from plant diseases and stresses is through the use of cultivars with tolerance to the
appropriate stresses. It is anticipated that this will be the case with charcoal rot of soybean. However, at this time, insufficient
information is available to determine how tolerance to charcoal rot is inherited: as a single gene, as multiple genes with high heritability,
or as multiple genes with low heritability. But, because of proper planning and execution, sufficient quantities of the
appropriate segregating populations have been developed to effectively determine the inheritance of tolerance to charcoal rot
and to maximize the available genetic potential for developing improved soybean varieties with tolerance to charcoal rot.
References
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