Agriculture Reference
In-Depth Information
of a hybrid must also flower simultaneously so that they cross-pollinate.
Problems of hybrid seed production are discussed in Chapter 6. If a cross is
made between a male-sterile line and an unrelated maintainer line, the
resulting hybrid will itself be male-sterile. Being a hybrid it may well be more
vigorous than the parent lines and, consequently, it may have greater potential
for seed production. This male-sterile hybrid may itself be crossed with another
male-fertile line to produce what is termed a 'three-way hybrid'. The extra
vigour of the male-sterile parent in a three-way hybrid makes seed production
easier, but some of the genetic uniformity of an F
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between two inbreds is lost.
As with two-way hybrids, various three-way hybrid combinations must be
made and trialled.
Since onion populations contain many deleterious recessive genes it is
usually feasible to self-pollinate lines just two or three times before they become
too weak to cultivate. Consequently, the hybrids between such inbreds are not
genetically or phenotypically completely uniform. Also, since the inbred lines
remain quite heterozygous, their genetic constitution may change over time by
drift or selection, and this could be detrimental to the hybrid varieties they
produce.
An alternative to inbreeding as a means of increasing genetic uniformity
(homozygosity) is to utilize, as the parent lines for hybrid production, completely
homozygous diploid plants derived by the chromosome doubling of haploid
plants (Bohanec, 2002). Several seed companies are already using double-
haploid lines in hybrid breeding. Haploid plants can be produced by tissue
culturing unopened flowers on a gel medium containing basal mineral
nutrients, sucrose and the plant growth regulators 2,4-D and BAP. Given
appropriate conditions and concentrations in the culture medium, after around
100 days floral ovaries can give rise to plantlets with half the normal
chromosome number, i.e. haploids (Bohanec, 2002). Some haploids undergo
spontaneous chromosome doubling to produce homozygous diploid plants. The
appearance of both growing plants and bulbs of both haploids and doubled-
haploids is quite normal, except that haploids produce only rudimentary
inflorescences, whereas flowering is normal in the diploids. The yield of haploids
per 100 flowers cultured varies from 0 to 50-60% depending on the genetic
make-up of the parent line, although it is usually just a few per cent in the
typical outbreeding cultivars tested.
The process of haploid regeneration itself must eliminate those genotypes
carrying deleterious recessive genes that would prevent a seedling from
developing. However, deleterious recessives that disrupt flowering and fertility
are not automatically selected out and double-haploid lines usually have low
fertility. However, it should be feasible to improve fertility by cycles of
intercrossing doubled haploids that are fertile, regeneration of new haploids
from these crosses followed by doubling to produce diploids and their selection
for fertility. Field trials comparing double-haploid lines, F
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hybrids between
double-haploids and male-sterile breeding lines, and existing open-pollinated
