Agriculture Reference
In-Depth Information
F
1
plants for the resistance trait or marker in BC
1
F
1
, BC
2
F
1
and so on generations (or else
in BC
x
F
2
progeny) to ensure that the resistance is present. The number of backcrosses
made will vary depending on the crop species, on the ease of selection in single plants, on
the degree of adaptation of the resistance donor and the maximum manageable population
size of the subsequent selection generations.
Backcrossing provides a very rapid method for delivering new varieties providing
the recurrent parent is suitable and if screening of F
1
plants is effective and reliable.
Backcrossing usually requires few resources other than the crossing itself and F
1
screen-
ing in the early generations. It is best used where the resistance donors are poorly adapted
in the target environment and where several rounds of crossing are required to return to
an adapted genotype.
Backcrossing can be speeded up substantially where MAS is used to select for the
recurrent parent genotype at the same time as the required resistance from the donor. Such
background screening can, for example, result in three backcrosses in maize being the
equivalent of 8 backcrosses where only the donor trait is selected (Frisch
et al
., 1999).
Backcrossing is, however, essentially a conservative approach. It does not allow for
much progress in yield or for traits other than the disease resistance gene being sought.
A variation on backcrossing is
partial backcrossing
where, usually, a single backcross
is made to the better-adapted parent. This is followed by selection through subsequent
generations by the pedigree or other methods. Partial backcrossing provides a compro-
mise between allowing for suffi cient variation amongst progeny to make yield and other
gains, whilst retaining a high chance of maintaining donor resistance in a suffi ciently
large proportion of the progeny. Unless very large segregating population sizes can be
generated, single backcrosses require at least moderately adapted donor parents.
With partial backcrossing there is an opportunity for screening of single plant BC
1
F
1
progeny either by phenotypic response, where the resistance is dominant, or by MAS. Such
screening allows a higher proportion of resistant progeny to be present in the subsequent
generation or, particularly where molecular markers are used, for a higher proportion of
progeny with other desirable traits to be selected. Partial backcrossing is particularly use-
ful where inheritance is multigenic or complex and/or is inherited as a recessive trait and
molecular markers are not available, that is where normal backcrossing is not possible.
It is also best used where resistance screening is costly, slow and/or has low repeatability
such as with root and crown diseases.
6.4.4
Mass selection
This method involves the growing of a large number of early generation progeny in
a disease-prone environment with strong natural selection pressure, so that the most
resistant lines preferentially survive and form a high proportion of the harvest and thus
of later generations. It may involve single or complex crosses and has the advantage
of requiring little technology or bookkeeping. Mass selection is rarely used in isola-
tion from other methods and has been even more rarely successful where used as a
sole means of selection. The method is ineffi cient for the production of varieties where
quality or other required traits are not naturally selected for in the specifi c screening
environment.
