CHAPTER XIV 

 SEXUALITY IN UNICELLULAR ORGANISMS 



T. M. SONNEBORN 



Among unicellular organisms, many different sexual conditions 

 have long been known; descriptions of these are readily accessible (e.g., 

 Calkins, 1926). In recent years, surprising discoveries concerning 

 sexuality have been reported in some of the commonest and most studied 

 unicellular organisms, such as Volpoma and Chlamydomonas among the 

 flagellates, and Paramecium and Euplotes among the ciliates. In this 

 chapter an attempt will be made to set forth this recent work and to 

 examine critically the current interpretations of it. 



The work on the flagellates began earlier and has been carried fur- 

 ther than the work on the ciliates; it will therefore be presented first. 

 Although in both classes of organisms investigations of similar nature 

 have been pursued on a number of forms (see especially Moewus, 1935a, 

 1935b, 1935c, 1937a), they have been carried further on Chlamydo- 

 monas among the flagellates and on Paramecium among the ciliates. As 

 these show essentially the same general relations as do other species of 

 the same classes, the following account will be confined in the main to 

 these two genera. 



Sexuality in Chlamydomonas 



The recent work on Chlamydomonas has appeared in a series of ex- 

 tensive and detailed studies by Moewus since 1932 (Moewus, 1933, 

 1934, 1936, 1937b, 1938a, 1938b, 1939a, 1939b, 1939c; Hartmann, 

 1932, 1934). From the beginning, its great importance with relation to 

 problems of sexuality has been apparent and, as the work progressed, 

 these relations have been repeatedly emphasized by Hartmann, Moewus, 

 and others. However, in order to approach the facts without theoretical 

 bias, they will be restated here in a purely descriptive way. 



Six species of Chlamydomonas have been most fully investigated: C. 

 braunii, C. dresdensis, C. eugametos, C. paupera, C. paradoxa, and C. 



