Journal of Experimental Botany, Vol 49, 1063-1071, Copyright © 1998 by Oxford University Press
B Milborrow
A new hypothesis is proposed that gives a mechanistic, biochemical
interpretation of the increased size of heterozygous organisms in
comparison with their homozygous parents. The interpretation is predicated
on the concept that growth is restricted by internal genetic factors to
less than the maximum possible. It is now suggested that heterozygous
organisms possess some factors coding for control mechanisms in which two
slightly different alleles occur and this lessens the rigour of control of
their growth. For example, where a regulatory factor is coded for by two
different alleles then the two forms could have slightly different patterns
of regulatory response. The less inhibitory version of such a pair of
alleles in a heterozygote would produce or allow a larger amount of growth.
Consequently, growth reactions which are constrained to operate below their
maximum rate would be restricted less in heterozygotes, where two forms of
several regulating influences are present, than in homozygotes. Similarly,
the heterozygosity would tend to allow a greater flux along metabolic
pathways containing restricted, regulated steps as the pathway would be
less inhibited in heterozygotes. Hybrid organisms which contain even
partially effective factors exerting control over growth processes can be
expected to grow larger than wild-type homozygous parental strains with
fully effective regulatory mechanisms. This mechanism would apply to plants
and animals.Thus hybrid vigour is now considered to be a phenomenon in
which strict regulatory limitation of growth is relaxed by
heterozygosity.If growth is limited by the action of a number of randomly
segregating regulatory factors then recombining different homozygous
strains in all combinations should occasionally bring together controlling
factors which exert a stronger restrictive influence when present in a
hybrid strain then when they are separate in their two homozygous parents.
Thus 'subtractive heterosis' can be expected where, in a very few crosses,
the F1 hybrids are smaller than the mean of the parental strains. An
example of this has been found.Keywords: Heterosis,
hybrid vigour, subtractive heterosis, biochemical mechanism.
ARTICLES
A biochemical mechanism for hybrid vigour
School of Biochemistry and Molecular Genetics, University of New South Wales, Sydney, NSW, Australia 2052; e-mail: b.milborrow@unsw.edu.au
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