Agriculture Reference
In-Depth Information
the required homogeneity or show off-types are dis-
carded. At harvest the progeny plots are bulk harvested
as breeders' seed. Breeders who wish to maintain a
degree of heterozygosity in the released cultivar will
include greater numbers of initial single plant selections
in the scheme or they may not be as restrictive in deci-
sions to discard progeny rows or plots. The advantage
of the progeny multiplication method is that it allows
greater control by the breeder and results in greater
homogeneity in the released cultivar.
Marketing of multiline varieties in the United States
has advantages over other types of cultivar as seed com-
panies can sell the seed without a common brand name
if the seed sold is labelled with a '
cultivar not stated
'
label. Multilines can also be sold under more than
one name. For example, the same multiline can be
sold with the brand names 'Browns Appeal', 'Browns
Wonder' and 'Wonder Why' by the same or different
seed sales groups. In other countries, however, multiline
cultivars must comply with the set standards of DUS
required for other inbred cultivars, and this has limited
their use because of the difficulties in obtaining such
homogeneity standards in a mixture.
The same care needs to be taken when producing
breeders' seed from a multiline cultivar as is the case
with a pure-line cultivar. The individual lines form-
ing the mixture are increased independently by either
mass or progeny multiplication methods (above). The
individual components are then mixed in the propor-
tions required, the seed mixed to form the breeders'
seed, from which foundation seed is produced. The
prevalent diseases, yielding ability or other appropriate
factors will determine the proportion of lines within
the mix. It is important when calculating multiline
mixture proportions to take into account the seed size
(if mixing by weight) and also the germination poten-
tial of each line (which may be different for the different
lines).
One major complication relating to seed mixture pro-
portions is the reproductive potential or productivity of
each genotype in the mixture. For example, if the given
proportion of two-parent lines (A and B) in a very sim-
ple multiline is 1 : 1 but the reproductive potential of A
is twice that of B a 1 : 1 mix of breeders' seed will result in
a 2 : 1 ratio of the lines being harvested from foundation
seed and a 4 : 1 ratio being sold to the farmer after one
further multiplication, to certified seed. Similarly, other
environmental conditions may affect the proportions of
mixed lines in the multiline. These changes could be
related to foundation and certified seed being produced
in an atypical environment or where a different disease
spectrum exists.
Some multiline cultivars are mixtures of isogenic
(or near isogenic) lines which differ for a single gene
(usually conferring resistance to a certain strain of a
pathogen). The most common method used in develop-
ing isogenic lines (lines which only differ in their geno-
types by specified genes) in plant breeding is through
DEVELOPING MULTILINE
CULTIVARS
Multiline cultivars (multilines) are mixtures of a number
of different genetic lines (or indeed different species).
Multiline cultivars are almost exclusively comprised of
mixtures of lines from inbreeding species. Multilines
have been developed for a number of different crop
species including barley, wheat, oats, and peanuts.
In turfgrass, intraspecific and interspecific multilines
cultivars are grown commercially.
Multilines have been suggested as one means to min-
imize yield or quality losses due to diseases or pests
that have multiple races and where the race specifici-
ties can change from year to year. Therefore there is
a lower probability that all plants within the mixture
(each with a specific disease resistance gene or resis-
tance mechanism) would be affected as severely, over
a period of years, as a homozygous cultivar. It has also
been suggested that the use of multilines would result in
more durable mechanisms of disease resistance in crop
species.
Research has also suggested that multiline cultivars
are more stable over a range of different environments
than are pure-line cultivars. The reason for this has been
related to the heterogeneous nature of the mixture where
some lines in the mix do well in some years or locations
while others perform better under different conditions.
Therefore multilines show less genotype by environ-
ment interactions, a primary reason for their popularity
with peanut breeders. Similarly, such considerations
have led to mixtures of rye grass and Kentucky bluegrass
being sold commercially. Rye grass has rapid emergence
and establishment and does better than bluegrass in
shaded areas.
