Selection for Interchange Heterozygosity in an Inbred Culture of Blaberus Discoidalis (Serville).

ERMANENT interchange heterozygosity is not a common phenomenon. Its Psuccess depends on conditions of two kinds, those internal and those external to the organism. The failure of interchange heterozygotes is generally attributed to their semisterility. Their fertility depends on such internal factors as the degree of multiple association and the orientation of the multiples at metaphase of meiosis. These, in turn, are affected both by structural properties of the chromosomes and by various genotypically controlled aspects of chromosome behaviour, for example localization and terminalization of chiasmata. These are subject to selection, but the initial reproductive efficiency of an interchange heterozygote will depend largely on the genotypic and chromosomal properties of the type which produce it. Given that these preadaptations are favourable, the objection of semisterility in interchange heterozygotes is largely removed. Preadaptation can thus account for the persistence of interchange heterozygotes in populations: but it does not in itself account for their selective superiority in certain populations. In all the well examined and best understood cases, like Campanula and Oenothera, it has been found that the establishment of interchange heterozygosity is favoured by a change from outbreeding to inbreeding (DARLINGTON and LA COUR 1950). The nature of the breeding system depends both on genetic factors (incompatability systems, sex chromosome mechanisms, etc.) and on those factors external to the organism which govern the distribution and density of populations of a species. Many plants are monoecious and can be, or become, self-fertilizing. In most animals, however, the sexes are separate and inbreeding to this extent is not possible. Furthermore plants are immobile while animals in a more active sense choose their environment (DARLINGTON 1956a). These differences are probably the most important in determining the greater incidence of interchange heterozygosity in plants as compared with animals. We have recently described wild and laboratory populations of the American cockroach (Periplaneta americana) where interchange heterozygotes are at a selective advantage (LEWIS and JOHN 1957b; JOHN and LEWIS 1958). In this species the genetic and structural preadaptations are favourable, the interchange