456 RADIATION BIOLOGY 



dence concerning what has happened to genes in the course of their past 

 evokition is furnished by studies by the present writer and Pontecorvo 

 (Muller and Pontecorvo, 1940, 1942a) of the effects of substituting 

 given chromosomes of Drosophila simulans in the place of their D. 

 melanog aster homologues, in an otherwise D. melanogaster genotype. 

 Natural hybrids between these two species are always sterile. Hence 

 the species are essentially unmixable, in the sense that natural recom- 

 binants between them cannot be obtained, and the gene differences 

 between them would therefore appear to be unanalyzable. However, it 

 was found possible to circumvent this difficulty by heavily irradiating 

 D. simulans males and then crossing them to triploid D. melanogaster 

 females, all of whose pairs of chromosomes had been made homozygous 

 for recessive "marker" genes. In this way zygotes could be formed, 

 and recognized by their markers, in which one or more chromosome-pairs 

 were species-heterozygotes and the rest homozygously of melanogaster 

 origin, just as if these zygotes had been second-generation individuals 

 that had arisen through the impossible cross of a sterile hybrid male back 

 to a marked ynelanogaster female. The homozygous melanogaster pairs of 

 chromosomes in such individuals arose from the fact that the egg had in 

 these cases received two of these chromosomes from the triploid mother 

 while the homologous simulans chromosome of the sperm had been broken 

 by radiation and lost by the fusion-bridge sequence. 



By noting which recombination types survived and what characteristics 

 they had, various deductions could then be made concerning the roles 

 played by the genes in different chromosomes in the production of hybrid 

 sterility, low viability, and morphological abnormalities. It was at the 

 same time proved that the species differences responsible for all these 

 effects had their genetic bases located in the chromosomes, there being no 

 cytoplasmically located genetic residuum for the given effects. More- 

 over, through the lucky case of a fertile hybrid, all of whose major 

 chromosomes had been derived from melanogaster, the transfer of the 

 small fourth chromosome into an otherwise melanogaster stock was 

 accomplished. Through the more intensive study of this " introgressive 

 hybrid" stock, the prevalence of interspecific gene differences more 

 cryptic in their expression than those dealt with above, but also chromo- 

 somal in their location, was then demonstrated. 



The above experiments represent, in a way, a further extension of the 

 technique used in those investigations — ranging from the early ones of the 

 Hertwigs (e.g., G. Hertwig, 1911; P. Hertwig, 1917) to the recent ones of 

 A. R. Whiting (1948) — in which the loss of the entire complement of 

 chromosomes in the sperm or egg, induced by heavy irradiation, has 

 resulted in genetic uniparentalism of the maternal and paternal types, 

 respectively, according to whether male or female gametes had been 

 treated. These effects have been produced in both intra- and inter- 



