340 SEX IN MICROORGANISMS 



the Suctoria it usually takes the modified form of conjugation. 

 Among the Mastigophora, in the Phytomonadina, which includes the 

 rather highly evolved Volvocidae, syngamy is extensively repre- 

 sented, whereas examples are very few and often questionable in the 

 more primitive Chrysomonadina; syngamy is apparently absent in 

 the Cryptomonadina, is seldom found or not well authenticated in 

 the Euglenoidina, and not reported for the Chloromonadina. In the 

 animal flagellates syngamy was not well-established until Cleveland 

 described the sexual cycles of the flagellates living in the woodroach, 

 Cryptocerciis. 



Since the higher groups of Protozoa are supposed to have evolved 

 from animal flagellates, and the latter from plant-like flagellates, we 

 may well ask whether or not sexuality (syngamy) has been devel- 

 oped independently in the highly evolved groups mentioned above, 

 or was passed on from the plant-like flagellates to animal flagellates, 

 and from the latter to the Sarcodina, Sporozoa, and Ciliophora. Are 

 the different orders of plant-like flagellates sufficiently closely related 

 to have been derived from a common ancestor, or did they evolve 

 somewhat independently? In either case, what were their ancestors? 

 Some of the primitive fungi, at least, with primitive sexuality (see 

 discussion by Raper) are thought to have evolved from a "flagellate 

 complex" (Cantino, 1950), and now we find that there is evidence 

 for sexuality in the bacteria and possibly in the viruses. What were 

 the ancestors of the bacteria; of the viruses? 



Is it profitable to suppose that "sex" arose only once in some 

 ancestral group and has been handed down to all descendants, or is 

 it more probable that it has arisen sporadically in many different 

 groups? In either case, can we consider any of the three previously 

 mentioned theories of the origin of "sex" as having exclusive validity, 

 or partial validity, or any validity at all? 



According to Cleveland, the first step in the evolution of "sex" 

 could have been the production of diploidy by endomitosis, in which 

 chromosomes are duplicated but not the ccntrioles. This could be 

 followed by meiosis, which would restore haploidy. As this could 

 happen in a single individual, no gametes would be involved. In case 

 diploidy had become established by endomitosis, meiosis could be 

 followed by autogamy, \\'ithout the production of separate gametes. 

 The other two theories mentioned would have diploidy result from 

 cell fusions. If nuclear fusion followed, meiosis could restore hap- 



