Genetic Systems I 165 



Paramecium and the more variable ones in bacteria and their asso- 

 ciated viruses. From the kappa-particle inheritance to the inheri- 

 tance of differences in chloroplasts in plants is but a short step. 

 Chloroplasts are bodies with genetic continuity in the cytoplasm 

 which may mutate to a different form or color. The variant may 

 then continue to reproduce the altered form. Portions of a plant may 

 be affected, producing a mosaic of yellowish or white areas inter- 

 spersed with the usual green. Chloroplasts are inherited in the 

 cytoplasm of the egg but not through the pollen. Some mutants are 

 initiated and maintained by chromosomal gene changes; some are 

 initiated by nuclear change but continue in the absence of the 

 chromosomal factor; finally, there are plastid mutants which ap- 

 pear to be independent of nuclear control. If other cellular com- 

 ponents, such as mitochondria, could be studied as easily as plastids, 

 perhaps they would be found to exhibit the same behavior. It should 

 be noted that virus infection of plants may simulate the same sort 

 of mutations of plastids that occur spontaneously or can be induced 

 by irradiation with X rays or ultraviolet light. 



This digression into what is usually termed cytoplasmic inheri- 

 tance will serve to illustrate the difficulty of drawing distinctions 

 between nuclear and cytoplasmic inheritance and between genetic 

 recombination and infection at the level of cell and microorganism. 

 It should be reiterated that the evolutionary significance of such 

 phenomena as cytoplasmic heredity and infection is largely un- 

 known, but their importance can scarcely be doubted. There are 

 indications from work with both plants and animals that what have 

 been regarded as genetic traits are the effects of viruses or that 

 genetically determined characters are mediated by the presence of 

 viruses. When these processes are considered in relation to the role 

 of microorganisms in the ecosystem or community, very important 

 relationships may emerge. Bacteria and other microorganisms may 

 be involved in the transmission of genetic information in the eco- 

 system in diverse ways, in addition to their role as reducers or 

 decomposers. 



GENETIC SYSTEMS 



OF OTHER ORGANISMS 



The basic features of the genetic systems in the vast majority of 

 living organisms are the same. There is an alternation of haploid 

 and diploid phases, the result of alternating haplosis ( meiosis ) and 

 diplosis ( syngamy ) . In the few viruses and bacteria that have been 

 studied, the genetic system is simpler and more variable. Only in 



