370 CELL HEREDITY 



sliould Ix' 10~*' 10 '* = 10 '', and /\.s7)(^'r^i//j/.s should remain preponder- 

 aiitU haploid dcspitf a pcrsisti'iit tciidcncN ot liapli)icl mitlci ti) fust', in 

 noriiialK diploid oriianisms. this l)alaiiti' is otluTu isc. in xcrtehratcs, at 

 least, there is a strong tendency for (hploidization of haploid nuclei. It 

 apparenlK results from endomitosis, a douhling of the ciiromosomes 

 without nuclear di\ ision. Parthenogenetic haploids that reach maturity 

 inevitably douhk' their chromosome number in the process. The par- 

 thenogenetic turkeys at Beltsville, Md. are male because in fowls the 

 male is XX in composition. When sperm from a parthenogenetic male 

 fertilize the X- and Y-bearing eggs of a female, both male and female 

 offspring result. Parthenogenetic frogs, on the other hand, are XX 

 and female. 



In some forms, such as Neurospora crassa, diploids cannot be formed 

 in heterokaryons composed of nuclei of the same mating type. Presum- 

 abl\ this is because of an incompatibility that presents such nuclei from 

 fusing, eyen vyithin a fruiting l)ody \\ here karyogamy does occur between 

 nuclei of opposite mating type. In some yeasts, such as Saccharomyces 

 ccrevisuie, there is normally a diplophase in the life cycle and, during it, 

 somatic recombination is known to occur, stimulated especially by ultra- 

 x'iolet light. This recombination does not inyoKe chromosome reduction 

 to any detectable extent, the tendency for karyogamy throwing the bal- 

 ance in the other direction. 



SOMATIC MUTATION AND SELECTION 



Diploid organisms fre(juently undergo changes in chromosome number 

 in the deyelopment of some specialized tissues. In the mammalian liver 

 and in the roots of some plants, for example, nuclei form a polyploid 

 series from n through 8n. These conditions seem to arise at the end 

 of reproductiye life and before the cell s specific function is achieved, 

 but their significance is not evident. Nor is it entirely clear w hat func- 

 tion is achieved by the polyploidy and aneuploidy usually found in 

 tumors and in cells that have lieen established in tissue culture. Here 

 there is often considerable variation in chromosome number, and the 

 stem-line cells from which the variants are produced have a chromosome 

 complement different from normal. The aberrant chromosome comple- 

 ments usually include chromosome rearrangements and may be the sec- 

 ondary consetjuences of an adapted condition. The in vitro transforma- 

 tion of chicki-n fibroblasts into Rous sarcoma cells by treatment with 

 virus is not at first accompanied by recognizable chromosome abnormali- 

 ties. But they do later occur, perhaps reflecting the achievement of the 



