Abstract

The problem of heterosis has been scrutinized from two points of view: the general experimental methodology and the interpretation of the facts in existing theories.Concerning the general experimental methodology, the relativity of the concept of heterosis is shown with respect to age and time of development of the parental strains, to the variations of the environmental conditions and to the direction of the reciprocal crosses. Therefore, in a large number of cases, experiments have taught not to be exhaustive.Concerning the existing theories, heterosis is considered to be the result of a heterozygosity which can express itself at three levels, which are defined as heteroallely, heterogenomy and heterocytomy. Some works, which are well-representative for one of these three levels, are analysed, and within the results and theories of these works is looked for a point of agreement, in order to render a general hypothesis of heterosis possible. This point of agreement appears to be the cytoplasm, or more precisely, a nucleo-cytoplasmic equilibrium. In this way alone it is possible to disclose the existing continuity from crosses, as well as in the expression of the ovula-spermatozoid compatibility, which varies from lethality to hybrid vigour. From these evidences a general theory of heterosis is presented, by which heterosis is explained as the result of a disturbed nucleocytoplasmic equilibrium: it considers heterosis as a particular case of a more general law,i.e. the ovula-spermatozoid compatibility.This hypothesis is also discussed with respect to the diverse existing theories of cytoplasmic heredity. Some unsolved questions bearing on these problems are put forward, and the way to their solution has been indicated