518 Heredity in Protozoa 



the type A conjugant remain type A, while the other conjugant produces 

 mostly type A and few or no type B lines. If a different type B strain is 

 mated with type A, the proportions may be exactly reversed. The behavior 

 of micronuclei affords no apparent basis for all these results. 



GENETIC SIGNIFICANCE OF ENDOMIXIS, 

 AUTOGAMY, AND CYTOGAMY 



Endomixis 



As described in Paramecium aurelia (102), endomixis involves: 

 (1) a periodic disintegration and resorption of the macronucleus; (2) 

 division of the micronuclei to produce eight daughter nuclei, all but two 

 of which usually disintegrate; (3) a fission which produces two ciliates, 

 each with a functional nucleus; (4) division of this nucleus to produce 

 four, two of which become macronuclei; (5) division of both micronuclei 

 at the next fission to restore the normal equipment. 



From the genetic standpoint, the significance of such a process is un- 

 certain. The absence of meiosis and nuclear fusion should eliminate 

 recombinations of genes, and the genetic implications of macronuclear 

 replacement under such conditions are unknown. Although genetic effects 

 have been attributed to endomixis in P. aurelia (5, 66, 92), more recent 

 data afford no evidence for the occurrence of endomixis in this species 

 (81). The "endomixis" induced by Sonneborn (73) in P. aurelia appar- 

 ently represents some other type of nuclear reorganization. 



Autogamy 



In contrast to endomixis, autogamy in P. aurelia (11) involves not 

 only replacement of the macronucleus, but also meiosis and subsequent 

 fusion of haploid nuclei in the same ciliate. Certain other details are of 

 possible genetic significance.^ The fact that more than two potential 

 gametic nuclei are usually produced suggests that the new^ synkaryon may 

 be either homozygous or heterozygous (Fig. 9. 5). The former condition 

 would result if both gametic nuclei are produced from the same haploid 

 second-division nucleus, or if the original diploid micronuclei were homo- 

 zygous. If the stock is heterozygous and the gametic nuclei have different 

 origins, the resulting synkaryon would often be heterozygous. In other 

 words, there is no cytological assurance that autogamy invariably results 

 in a homozygous synkaryon. In contrast to this lack of cytological evi- 



^ Two, three, four, or five nuclei may undergo the third prezygotic division, the 

 division which produces the potential gametic nuclei. "I do not have clear-cut cases of 

 just a single one of the eight nuclei going ahead to form the gametic nuclei, but prob- 

 ably this condition does occur at times." Furthermore, ". . . in most cases at least 

 four potential gametic nuclei are formed in the region of the paroral cone" (11). 



