Agriculture Reference
In-Depth Information
been found at a very low frequency in a number of onion populations. Also,
plants with the constitution
Nmsms
, i.e. maintainer genotypes, occur with a
frequency of about 5% in most populations (Pike, 1986). If the cross between
such a male-sterile discovery and a fertile plant gives offspring that are all
male-sterile, the breeder immediately has a new male-sterile and maintainer
line. Normally, however, new male-sterile lines must be developed utilizing a
known male-sterile from a non-adapted source - for example, from a breeder in
another locality. Such plants are crossed with a number of individuals from a
locally adapted population and the progeny is grown on until flowering. Any
such cross that produces 100% male-sterile offspring serves to identify the
original pollen donor as
Nmsms
, and therefore as a maintainer line. Normally,
lines that have been inbred for one to three generations are used in such test
crosses, thereby eliminating many undesirable recessive alleles from the
population and increasing the homozygosity of the lines. This ensures that the
ultimate hybrids produced are fairly uniform genetically and are therefore
likely to be uniform in growth and development.
Once a maintainer line has been identified in a locally adapted inbred
population, the male-sterile line must be developed so that it is nearly identical
genetically to it, apart from the presence of the S cytoplasmic factor. This is
done by repeatedly crossing the male-sterile offspring to the original
Nmsms
-
adapted maintainer line. With each generation of such a cross, the genetic
contribution from the original sterile
Smsms
parent is diluted by a factor of two,
so that after four generations of this 'backcrossing' 96.875% of the genes in the
resultant
Smsms
line derive from the original
Nmsms
line, and it is sufficiently
similar to use as a male-sterile line in test crosses to identify vigorous hybrids.
Only certain crosses result in desirable F
1
s, and therefore a number of
adapted male-sterile and maintainer line pairs must be developed for a hybrid
breeding programme to have a good chance of success. The breeder must make
many crosses on to his male sterile lines and grow and evaluate the progeny to
identify which give desirable hybrids. By accumulating data from such test
crosses he/she can build up information which helps predict which crosses will
produce good hybrids. As with maintainer lines, the pollen donor lines used to
produce hybrids are usually inbred to eliminate deleterious recessive and off-
type alleles from the gene pool.
Further details, including many practicalities of breeding onion hybrids
with particular reference to the development of improved, short-day onion
hybrids for Texas, are described by Pike (1986). The development of new hybrid
onions involves a 15-20 year timescale because most seed-to-seed cycles in
onion breeding involve 2 years. Seeds are sown and bulbs are selected,
harvested and stored in year 1. In year 2 the selected bulbs are grown for
flowering and seed production.
Because the lines contributing to hybrids are inbred, they are frequently
weak and they may give a low seed yield. The potentiality for adequate seed
production is essential if a hybrid is to be commercially viable. The parent lines
