238 CELL HEREDITY 



streptomycin-resistant to the same high level as the parental ir-500 

 strain, e\en after vears of continuous cultivation in the absence of strep- 

 tomvcin. The possihilitv that this result derives from the presence of 

 many different chromosomal genes, each exerting an effect on resistance 

 (i.e., multifactorial inheritance), was ruled out by four successive genera- 

 tions of back-crossing of resistant progeny to the sensitive parent of 

 mt~. The same results were obtained in each generation: there was 

 no segregation of sensitivity among the progeny. All were uniformly 

 resistant, although chromosomal genes segregated normally in the ex- 

 pected 1 : 1 ratio. 



In the reciprocal cross, sr-500 mt" x ss mt^, all progenv were found 

 to be uniformly streptomycin-sensitive, and in subsequent crosses the re- 

 sistance factor did not reappear. The difference on the part of the two 

 mating types in their ability to transmit the sr-500 factor was not cor- 

 related with any phenotypic differences in resistance level or stability. 

 One technique for studying nonchromosomal determinants is based on 

 the idea that they may not be well integrated in cellular division, and 

 that, by growing cells at maximal division rates, they might be diluted 

 out; or under difficult growing conditions they might be lost. To test 

 these possibilities in the sr-500 system of Chlamydomonas, vegetative 

 strains of both mating types were grown under a wide variety of nutri- 

 tional conditions at rates from a 5- to a 48-hour generation time. 

 Neither these conditions nor heat shock, cold treatment, or UV-radia- 

 tion gave rise to any streptomycin-sensitive cells. In fact, the stability 

 of the sr-500 property in the absence of streptomycin is impressive. No 

 reversion to sensitivity has been found over a several-year period. 



These results indicate that the sr-500 determinant is well integrated 

 into some replicating mechanism of the cell; and that it must be con- 

 sidered a stable, permanent constituent of the hereditary material, rank- 

 ing with a chromosomal gene. The results of crosses demonstrate that 

 the sr-500 factor is nonchromosomal, on the basis of its nonsegregation 

 in meiosis and its consequent nonmapability. The uniparental trans- 

 mission of 5r-500 by parents of mating-type plus but not of mating-type 

 minus resembles maternal inheritance, as for example in the poky mutant 

 of Neurospora. 



In Neurospora, a slow-growing strain called poky was crossed with 

 wild type, and was found to exhibit nonsegregation in meiosis, uni- 

 parental inheritance, and non-mapability, as in the Chlamydomonas sys- 

 tem. In Neurospora (see Figure 3.2), either mating type can produce 

 fruiting bodies which contribute not only a haploid nucleus but also 

 the cytoplasm to the zygote. The fruiting body can be fertilized by 

 conidia of the opposite mating type which contribute the other haploid 



