Agriculture Reference
In-Depth Information
(iii) storage life. Selected lines were then intercrossed to make a new 'synthetic'
outbreeding population. Selection was based on the average performance of
the inbred lines rather than on individual bulbs. Those lines that were
significantly better than a control cultivar in at least one set of properties, and
not significantly worse for any other, were intercrossed. The control was one of
the original well-adapted cultivars used as starting material. Equal weights of
seed saved from each parent in the intercross were bulked and grown to bulb,
and plants from 100 selected bulbs were mass pollinated to bulk-up the seed of
the resulting 'synthetic variety' for use in trials replicated at three locations.
The cycle of inbreeding and selection and synthesis was then repeated, using
the population from the first cycle as a starting point.
Yields were increased by 5.5%, and the percentage of bulbs sprouting in
store by April decreased by 7.5%, per cycle. The yield improvements were
similar to those reported for maize under a similar breeding regime. This kind
of selection programme can be fairly 'open-ended'. The number of generations
of inbreeding and the number of cycles of selection can vary, and promising
new lines can be brought into the sequence at any stage (Dowker, 1990). As
well as inbreds, lines that are partial inbreds - like the half-sibs that result from
growing the seed from a single mother plant in isolation - can also be used as
'lines' in such a programme. Line selection has replaced mass selection in leeks
from the 1960s onwards and has led to modern cvs largely replacing the older,
mass-selected landraces (de Clercq and van Bockstaele, 2002).
Hybrids
If a particular male-sterile line crosses with a particular pollen donor to
produce a vigorous F
1
hybrid with desirable traits, then we have produced a
hybrid cultivar. To reach this point usually involves many years of careful work.
First, there is the problem of how to reproduce a male-sterile mother line
incapable of pollinating itself. In early work Jones and Clarke maintained their
male-sterile vegetatively using the bulbils produced on the flower head.
However, bulbils are difficult to store and viruses tend to accumulate in the
plants. To overcome these problems they developed male-fertile 'maintainer
lines' with the genetic constitution
Nmsms
for male sterility but, in other
respects, genetically near the same as the male-sterile line. Such a line will, as
shown above, produce pollen that can fertilize the
Smsms
male-sterile line, the
offspring will remain
Smsms
and therefore male-sterile but will not vary
otherwise genetically from the parent male-sterile. Thus, the
Nmsms
line makes
it possible to produce successive generations of seeds of the male-sterile line,
hence its name 'maintainer line'.
Male-sterile and maintainer lines must be developed, which have the
potential to produce good hybrids. This means that these lines must be well
adapted to the locality where the hybrid is to be grown. Male-sterile plants have
