Agriculture Reference
In-Depth Information
other plot evaluation trial and are best grown at more
than a single environment. Progeny evaluations are also
often repeated over a number of seasons to obtain more
representative evaluations of the likely performance over
different years.
Depending on the results from the progeny evalua-
tion trials, clones that show greatest general combining
ability will be used as parents for the synthetic cultivar.
These clonal parents are mated in a number of combina-
tions to produce experimental synthetics. The number
of clones that are selected and the number of parents
that are to constitute the synthetic will determine how
many possible combinations are possible. If there were
only four clones selected then there would be a total of
11 different synthetic cultivars (that is, the six possible
2-clone combinations, the four 3-clone combinations
and the one 4-clone combination). With 6, 8, 10 and
12 parents the number of possible synthetics would be
57, 247, 1013 and 4083, respectively. Therefore it is
useful to try to predict the performance of synthetic
cultivars without actually producing seed. One formula
used is:
It is therefore more common in situations where
many parents are used in a synthetic cultivar to produce
Syn.1 seed using a polycross procedure. The selected
parents are grown in close proximity in randomized
block designs with high replication. The crossing block
is obviously grown in isolation for any other source
of the crop to avoid cross contamination. In cases
where insect pollinators are necessary to achieve cross-
pollination, attempts are made to ensure that these
vectors are available in abundance. For example, alfalfa
breeders introduce honey bees or leaf-cutter bees to
pollinate synthetic lines.
Seed from Syn.1 is open-pollinated to produce Syn.2,
which is subsequently open-pollinated to give the Syn.3
population, etc. The classes of synthetic seed are cate-
gorized as breeders' seed, foundation seed and certified
seed. In this case breeders' seed would be Syn.1, foun-
dation seed Syn.2 and the earliest certified seed Syn.3.
In
summary,
the
characteristics
of
a
synthetic
cultivar
are:
•
Synthetic cultivar species need to have potential to
have parental lines, which reproduce from source
material (either clonally or as an inbred line), and
hence the synthetic cultivar can be reproduced from
these base parents;
F
1
−[
(
F
1
−
P
)
]
/
n
where
F
1
is the mean performance of all possible single
crosses among
n
parents, and
P
is the mean performance
of the
n
parents. It should, however, be noted that there
is an assumption of an absence of epistasis (interaction
between alleles at different loci) in order to obtain a
good estimate of synthetic performance and predictions
are therefore often far removed from what is actually
observed. Such predictive methods should therefore be
used with caution.
Synthetic cultivars have also been developed from
out-pollinating species that are semi-tolerant to
inbreeding (e.g. maize and rapeseed). In these instances
the clonal nursery is replaced by a collection of inbred
(or near-inbred) breeding lines, and any other reference
(above) to clones is replaced by inbred lines.
•
To develop synthetic cultivars, the contributing
parental material is tested for combining ability
and/or progeny evaluated;
•
Pollination of synthetic cultivar species cannot be
controlled, and there has to be some natural method
of random mating between parents (e.g. a pollinator
or wind pollination);
•
The
source
parental
material
is
maintained
for
further use;
•
Open-pollinated populations have limited life and are
then reconstituted on a cyclic basis.
DEVELOPING HYBRID CULTIVARS
Seed production of a synthetic cultivar
Crops that are commonly produced and sold as hybrids
are: brussels sprouts, kale, maize, onions, rapeseed,
sorghum, sunflower and tomato.
Although attempts have been made to develop hybrid
cultivars from almost all annual crop species, it is impos-
sible to consider the evolution of hybrids without a
brief history of the developments in hybrid corn. At the
beginning of the 20th century it became apparent that
The parents used to produce a synthetic cultivar can
be inter-crossed by hand-pollination to produce the
first generation synthetic (Syn.1). The aim is to cross
every parent in the synthetic with all others (i.e. a half
diallel cross). This can be difficult with some synthetics
(e.g. alfalfa) where the number of parents included is
around 40.
