244 SEX IN MICROORGANISMS 



chlamydomonad flagellates (see paper by Lewin), or the formation 

 of uninucleate gametes in the Opalinidae. 



Fortunately, we have in the unusual behavior of Dallasia fron- 

 tata (Calkins and Bowling, 1928), the development of just such a 

 brood of gametes (Fig. AM). In this case the cell body divides with 

 each nuclear division, leading to the formation of gametes. Isogametes 

 are formed which, however, unite with sister gametes in a process of 

 autogamy (called paedogamy by Calkins and Bowling) to produce 

 each zygote. Normal conjugation also occurs in this species (Calkins 

 and Bowling, 1929). In typical conjugation, nuclear divisions take 

 place without cell divisions; hence gamete development is telescoped 

 into the cell body of each conjugant. 



Calkins and Bowling (1928, 1929) suggested that the peculiar 

 brood formation in Dallasia is related to "endomixis" and that both 

 might relate to ancestral gamete brood formation. Attention has 

 already been called to the situation in the vorticeUids, where an 

 unequal division of a "neutral" individual produces macro- and micro- 

 con jugants. In many cases, four or eight microcon jugants are pro- 

 duced. These divisions might also reflect the supposed ancestral gam- 

 ete brood formation pattern. Furthermore, it will be recalled that in 

 the microconjugant there is an extra micronuclear division, which 

 may indicate a definite trend toward multiple gamete formation. Just 

 why there should be manifested such an ancestral trend in the micro- 

 conjugant and not in the macroconjugant is an interesting question. 

 A survival value might be postulated since the microconjugant dies 

 if it does not find a mate, whereas the macroconjugant, after a few 

 hours of receptiveness toward microconjugants, reverts to a "neutral" 

 status, capable of continuing the line by normal binary fission (cf. 

 discussion by Finley, 1952). The extra micronuclear pregametic divi- 

 sion in each conjugant of Euplotes can be interpreted as a part of this 

 ancestral trend, as can also the one or more preconjugant cell divi- 

 sions that have been reported for a good many ciliates (see discus- 

 sion of differentiation of conjugants). 



The relationship between the peculiar gametogeny of Dallasia 

 and "endomixis," suggested by Calkins and Bowling, is not so readily 

 discerned. However, since Diller (1936) and Sonneborn (review, 

 1947) have shown that "endomixis" in Faramecium aurelia is au- 

 togamy, and the so-called paedogamy of Dallasia is also seen to be 

 autogamy, then that aspect, at least, is common to both processes. But 



