Agriculture Reference
In-Depth Information
Figure 8.1
Winter biennial forms of yellow mustard (
Sinapis alba
) (left) and Indian mustard (
Brassica juncea
) (right)
produced through intergeneric hybridization.
embryo development the ovary is removed and cultured
in vitro
to achieve embryos, which are of a suitable size
to successfully rescue and culture.
Finally, it is not uncommon in hybrid crosses
that rather than resulting in hybrid combinations
the resulting seed develops as matromorphic plants,
which are thus derived from the maternal genotype.
This characteristic has been developed to advantage in
producing homozygous lines (i.e.
Hordeum vulgaris
characters from the donor species, along with the char-
acter it was wished to introduce. In such circumstances
it is necessary to carry out several rounds of backcrossing
to the host species, with selection for the new character
to obtain genotypes, which will have commercial worth.
Any programme involving interspecific or intergeneric
hybridization is therefore likely to be long-term.
×
H. bulbosum
). The seeds from interspecific crosses
should thus be checked to ensure that the matromorphs
are discarded if the desire is to produce hybrids - but
retained if this feature is being used to produce haploids
of the maternal genotype!
Increasing genetic diversity
Many crop species have a relatively narrow genetic base
and it is often advantageous to broaden genetic diversity
by introgressing traits from related weedy species. Sev-
eral crop species (i.e. rapeseed and wheat) have evolved
as allopolyploids, whereby they contain complete chro-
mosome sets from two or more diploid ancestors.
Greater genetic diversity and variation can be achieved
in breeding by resynthesizing the crop species from its
ancient ancestors.
Hybrid sterility
In many cases the F
1
plants resulting from interspecific
crosses are completely (or partially) sterile. A common
technique used to overcome sterility, caused by lack of
chromosome pairing, is to induce chromosome dou-
bling in the hybrid, and hence develop alloploids. When
doubled, it allows each chromosome to have a homo-
logue with which to pair at meiosis, and thus reduce the
infertility problem.
Creating new species
It is possible to create new crop species by intergeneric
hybridization. Despite the possible attraction of this
there are very few instances where new crops have
resulted. Two notable examples include:
Backcrossing
After interspecific hybridization the resulting progeny
will generally contain a large proportion of undesirable
•
Triticale
, which resulted by intergeneric hybridiza-
tion between wheat (
Triticum
) and rye (
Secale
)
