A biochemical mechanism for hybrid vigour

when present in a hybrid strain then when they are separate in their two homozygous parents. Thus ‘subA new hypothesis is proposed that gives a mechanistic, tractive heterosis’ can be expected where, in a very biochemical interpretation of the increased size of het- few crosses, the F 1 hybrids are smaller than the mean erozygous organisms in comparison with their homozy- of the parental strains. An example of this has been gous parents. The interpretation is predicated on the found. concept that growth is restricted by internal genetic factors to less than the maximum possible. It is now Key words: Heterosis, hybrid vigour, subtractive heterosis, suggested that heterozygous organisms possess some biochemical mechanism. factors coding for control mechanisms in which two slightly different alleles occur and this lessens the Introduction rigour of control of their growth. For example, where a regulatory factor is coded for by two different alleles Hybrid vigour or heterosis is the well-known phenomenon then the two forms could have slightly different in plant and animal breeding where the progeny from patterns of regulatory response. The less inhibitory crosses between diVerent, inbred or distantly related version of such a pair of alleles in a heterozygote strains show increased size in comparison with their would produce or allow a larger amount of growth. parents (Shull, 1908). The increment in size over that of Consequently, growth reactions which are constrained the parental strains is usually reduced by inbreeding over to operate below their maximum rate would be succeeding generations as the hybrid strains themselves restricted less in heterozygotes, where two forms of become progressively more homozygous. In 1983 R several regulating influences are present, than in Frankel wrote in the preface to the book Heterosis … homozygotes. Similarly, the heterozygosity would tend ‘the causal factors for heterosis at the physiological and to allow a greater flux along metabolic pathways con- biochemical level are today almost as obscure as they taining restricted, regulated steps as the pathway were 30 years ago’ (Frankel, 1983). The physiological would be less inhibited in heterozygotes. Hybrid organ- bases of the large increase in size of the F 1 s in comparison isms which contain even partially effective factors with their parents has been shown to derive from very exerting control over growth processes can be much smaller percentage increases in relative growth rate expected to grow larger than wild-type homozygous (Ashby, 1937; Gowen, 1952; Hull, 1952), or duration of parental strains with fully effective regulatory mechan- growth. The overall diVerence in growth rates can be isms. This mechanism would apply to plants and further analysed into small percentage diVerences between animals. the components of growth (Hunt and Cornelissen 1997) Thus hybrid vigour is now considered to be a phe- and so any attempt to find an enzyme reaction which is nomenon in which strict regulatory limitation of growth more rapid in the hybrid than in the parents is destined is relaxed by heterozygosity. to measure diVerences in rate of the order of fractions of If growth is limited by the action of a number of a per cent. randomly segregating regulatory factors then recom- A quantitative measurement of an enzyme’s activity in bining different homozygous strains in all combina- vitro cannot be related definitively to the enzyme’s activity tions should occasionally bring together controlling in vivo because the concentrations of the substrates in the latter situation are unknown. Total cell water, for factors which exert a stronger restrictive influence