1 Congruency backcross
Congruency backcross technique is a modification of
the standard backcross procedure whereby multiple backcrosses, alternating
between the two parents in
the cross, are used. The technique has been used to
overcome the interspecific hybridization barrier of hybrid sterility, genotypic
incompatibility, and embryo abortion
that occurs in simple interspecific crosses. The crosses
and their genetic contribution are demonstrated in Table 16.1.
2 Advanced backcross QTL
The advanced backcross QTL developed by S.D. Tanksley
and J.C. Nelson allows breeders to combine backcross with mapping to transfer
genes for QTL from unadapted germplasm into an adapted cultivar. This method
was developed for the simultaneous discovery and transfer of desirable QTLs
from unadapted germplasm into elite lines. It has been briefly discussed
previously.
Multiline breeding and cultivar blends
N.F. Jensen is credited with first using this
breeding method in oat breeding in 1952 to achieve a more lasting form of
disease resistance. Multilines are generally more expensive to produce than
developing a synthetic cultivar, because each component line must be developed
by a separate backcross.
The key feature of a multiline cultivar is disease
protection. Technically, a multiline or blend is a planned seed mixture of
cultivars or lines such that each component
constitutes at least 5% of the whole mixture. The pure lines are phenotypically
uniform for morphological and other traits of agronomic importance, in addition
to the genetic resistance for a specific disease. The component lines are grown
separately,
followed by composting in a predetermined ratio.
Even though the term multiline is often used interchangeably with blend,
sometimes the former is limited to mixtures involving isolines or near isogenic
lines. The purpose of mixing different genotypes is to increase heterogeneity
in the cultivars of self-pollinated species. This strategy would decrease the
risk of total crop loss from the infection of one race of the pathogen, or some
other biotic or abiotic factor. The component genotypes are designed to respond
to different versions or degrees of an environmental stress factor.
One of the earliest applications of multilines was
for breeding “variable cultivars” to reduce the risk of loss to pests that have
multiple races and whose incidence is erratic from season to season. Planting a
heterogeneous mixture can physically impede the spread of disease in the field
as resistant and susceptible genotypes intermingle.
Mixtures may be composited to
provide stable performance in the face of variable environment. Mixtures and
blends are common in the turf-grass industry. Prescribing plants for conditions
that are not clear-cut is challenging. Using mixtures or blends will increase
the chance that at least one of the component genotypes would match the
environment.
In backcross breeding, the
deficiency in a high yielding and most desirable cultivar is remedied by gene
substitution from a donor. Similarly, the deficiency of an adapted and
desirable cultivar may be overcome by mixing it with another cultivar that may not
be as productive but has the trait that is missing in the desirable cultivar.
Even though this strategy will result in lower yield per unit area in favorable
conditions, the yield will be higher than it would be underadverse conditions
if only a pure adapted cultivar was planted.
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