12-7 THE YEAST CELL 



that the addition of cytoplasm produced a stable change in the cul- 

 ture. The operation produces an X nucleus in X+Y cytoplasm, or 

 a Y nucleus in X+Y cytoplasm. These cultures can be compared 

 with the original haploids: X nuclei in X cytoplasm and Y nuclei 

 in Y cytoplasm. The additional cytoplasm produced stable mutants. 

 Harder interpreted this as a cytoplasmic effect. In view of the 

 fact that McCarty, Taylor, and Avery; and Boivin, Delaunay, Ven- 

 drely, and Lehoult have shown that simple chemical substances 

 can produce stable transformations, Harder' s conclusions may re- 

 quire qualification. The experimental procedure which Harder used 

 to produce his "operated" variants is quite simple, and it would 

 seem that this is one of the important discoveries in the Hymeno- 

 mycetes that could be profitably repeated. 



In spite of the abundance of genetical work done on the hymeno- 

 mycetes, no chromosome maps are available; this is a strong in- 

 dication that many technical difficulties must be overcome before 

 they can compete v/ith yeasts as objects of choice for genetical 

 analysis. 



PARAMECIUM 



Paramecium has the peculiar advantage that cytoplasmic ex- 

 change occurs to a variable and roughly controllable amount during 

 each copulation. Tetrad analysis in Paramecium is impossible 

 for immediately after meiosis three of the haploid nuclei disinte- 

 grate. At this time two paramecia are copulated and the remaining 

 haploid nucleus in each undergoes a mitotic division. Then an ex- 

 change of nuclei occurs between each conjugant, simultaneously 

 with a cytoplasmic exchange, and in the two conjugants the nuclei 

 fuse producing two diploid paramecia with presumably the identical 

 diploid nuclei. Chen has shown, in a remarkably competent cyto- 

 logical investigation, that Paramecium has about 80 chromosomes 

 and, therefore, no possibility exists of unravelling chromosomal 

 aberration if such may occur. In nearly 20 years of study Sonne- 

 born and his numerous coworkers have demonstrated apparent 

 Mendelian inheritance for only one pair of alleles so the prospect 

 of chromosome maps seems out of the question. The exchange of 

 cytoplasm between the conjugants has led to a number of very in- 

 teresting discoveries and a series of different explanations have 

 been proposed involving relations between the gene and its com- 

 ponents or supposedly autonomous cytoplasmic entities. However, 

 since some workers believe that all cytoplasmic activity, except 

 that of parasites and what one might classify as symbiotic plastids, 

 like the chloroplasts, is directly under genie control the possibili- 

 ty of demonstrating the independent inheritance of the cytoplasmic 

 entities and the chromosomal genes is remote if not impossible 



