Agriculture Reference
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mentioned above. A comparison of the ordering of these ESTs in onion with
that known for rice showed little similarity (i.e. scant colinearity). These results
suggest that genomic information from the grass family crops (Order Poales), of
which there is a vast amount, may not provide appropriate genomic models for
crops in the Asparagales Order - e.g. alliums (Martin et al. , 2005).
This makes it necessary to develop more genomic information for these
plants. In view of the large genome and long generation time for onions and
other vegetable alliums, a species in the Asparagales with a smaller genome
and short life cycle is likely to be chosen as a 'model' for developing a detailed
genetic map applicable to the Order. The expected large measure of colinearity
of genes between the genomes of different species within an order would
facilitate the application of genomic information from such a 'model' species to
the crop species. As information is published on allium genetic sequences by
research groups worldwide, it is being collated at the above web site so that
genomic information is accumulating in a coordinated and collaborative way
(Havey et al. , in press).
HYBRID VIGOUR
Crosses between widely divergent onion populations can produce hybrids that
exceed either parent in vigour. For example, Synnevag (1988) crossed a Finnish
multiplier onion with a Norwegian large-bulbed cultivar and obtained hybrids
that outyielded either parent yet had a shorter growing period than the large-
bulbed parent. Such an effect is termed 'hybrid vigour' or 'heterosis'. In such
hybrids the influence of any unfavourable recessive alleles homozygous in the
parent lines may be masked by more favourable dominant alleles from the other
parent. When a number of parent lines are crossed, even if they are themselves
weak and inbred, some of the resulting hybrids can be extremely vigorous, and
may even exceed the performance of a vigorous cross-pollinated line. The fixing
of genotypes showing such hybrid vigour in F 1 hybrid cultivars has become the
dominant trend in onion breeding in recent years. Such hybrids have also been
developed in Japanese bunching onion, chives and leeks. From a commercial
point of view, one advantage of F 1 hybrids is that they will not breed true from
saved seed, but must be produced anew every generation from the appropriate
parent lines, which remain under the control of the breeder or seed company.
GENETIC MALE STERILITY AND HYBRID BREEDING
The most important qualitative genes in the edible alliums are those that cause
male sterility. In male-sterile plants pollen fails to develop and they are therefore
incapable of self-pollination. Hence, any seed produced must result from cross-
pollination. This property has been utilized to produce F 1
hybrid cultivars
 
 
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