The genetical basis of heterosis

Abstract

Appreciation of the practical value of hybrid vigour is as old as the mule, but its scientific investigation began only relatively recently. Vigour transcending that of the parents was observed in hybrids by a number of the early hybridizers and, indeed, Mendel himself records it as an incidental observation on his peas; but it remained for Darwin to make the first experiments designed to bring out the essential nature of the phenomenon. The results of his extensive investigations are set out in his book The effects of cross- and self-fertilization in the vegetable kingdom (1876), where he confirms the widespread occurrence of hybrid vigour. His inbreeding and crossing experiments enabled him also to show that the act of crossing was not itself sufficient to produce extra vigour in the offspring. Rather this vigour resulted from the ‘sexual elements (being) in some degree differentiated’. The differentiation he attributed to ‘individuals having been subjected during previous generations to different conditions or to their having varied in a manner commonly called spontaneous’. This is not quite the form of expression we should now choose; but while our genetical understanding amplifies these findings of Darwin, it brings no basic contradiction, for hybrid vigour depends on genetical dissimilarity of the gametes which fuse. Darwin also recognized what we would now call inbreeding depression, and attributed it to ‘the want of such differentiation in the sexual elements’, or to put it in our words, to the absence of genetical differences. The rise of Mendelian genetics in the present century made possible the re-examination of Darwin’s and similar findings in terms of the particulate factors or genes which we have come to recognize. Such a re-examination was made all the more urgent by the prospective practical value of hybrid vigour, especially to the breeder of cross-fertilizing crop such as maize, as the work of Shull, East and Jones had made clear. One question was particularly insistent. Did the extra vigour spring directly from the heterozygosity per se of the hybrids, or did it reflect simply the superior gene content possible in a hybrid where both parental gametes are bringing in desirable genes? The decision is of practical as well as theoretical significance, as on the one interpretation the vigour would be a direct property of heterozygosity and as such would never be fixable, whereas on the other view it could in fact be fixed—once all the desirable genes could be assembled m one set of chromosomes, so that male and female gametes could be produced like each other in that each included all the genes necessary for full vigour. Homozygotes could then be made with properties as desirable as the earlier hybrids.