Agriculture Reference
In-Depth Information
and correlations. In cases where the progeny means pre-
dominate in the prediction equations, which is often the
case, then very good estimation of progeny worth can be
obtained by summing the relative rankings (based on the
phenotypic mean of the cross) for each of several traits.
Consider the potato example shown above where four
traits, yield, tuber size, tuber number and shape were
used to assess progeny worth of eight potato crosses.
The ranking of the eight crosses based on the multi-
variate normal probability (MVP) and those obtained
by summing the relative ranking of each character were:
within more cultivated types (i.e. parents from plant
introduction accessions and germplasm gene-banks)
•
Adapted genotypes which may be new (or old)
cultivars from other breeding programmes
•
Genotypes which have been selected from within the
breeding programme. Often these lines will become
new cultivars but it is not uncommon that advanced
selections (which have only a few slight defects that
would render cultivar introduction infeasible) are
used as parental lines
It may appear strange that recombinant selection
was discussed prior to parental selection (i.e. putting
the cart before the horse). In actual practice there is
no definite order of either selecting parents or select-
ing offspring. A large majority of parents used in plant
breeding scheme are derived from selections within the
breeding programme.
Parental selection is therefore a cyclic operation
where parents are selected, inter-mated, recombinants
screened from segregating populations and these, in
turn, are used as parents in the next round of the scheme.
In deciding which parents are to be used in a breeding
scheme there are two types of evaluations possible:
Cross
Sum Rank
MVP
C1
1
2
=
=
=
C2
2
2
C3
5
6
C4
4
4
C5
8
8
C6
6
5
=
C7
2
1
C8
7
7
As can be clearly seen, ranking each individual trait
and then summing the rankings of each cross can be
a good estimate of the commercial worth of different
cross combinations.
•
Phenotypic evaluation
•
Genotypic evaluation
This information could have been derived from
experiments or assessment trials carried out within
the breeding scheme or by other organizations (i.e.
germplasm databases).
PARENTAL SELECTION
Selection in a plant breeding programme takes two
forms:
Phenotypic evaluation
•
Selection of superior parents
Phenotypic evaluation is often the first stage of parental
selection. New genetic material is continually being
added to the available parental lines within a plant
breeding programme.
It can be of great benefit to a breeder, and will add
increasing knowledge of possible new parental lines, to
grow parental evaluation trials. When a potential new
parent is made known often the information of com-
mercial worth is lacking. Information may be available
from a database management scheme although often
these data are related to performance in different regions
to the target region of the breeding programme.
Selection
of
desirable
recombinants
which
have
•
resulted from inter-mating chosen parents
Selection of the desirable recombinants has been cov-
ered in the foregoing sections of this topic. We will now
consider parental selection.
Parents used in plant breeding programmes are cho-
sen from a wide source of possible genetic material. In
general, however, parents are of three different types:
•
Unadapted or relatively unadapted genotypes which
possess one (or more) character which is not available
