COMMENTS ON THE ORIGIN AND EVOLUTION OF SEX 34 1 



loidy. Apparently Cleveland is the only author to provide a detailed 

 description of one-division meiosis in the Protozoa. This may well be 

 the priniitiAe tvpc, whether diploidv arose by endomitosis or by 

 fusion of the nuclei of previously independent cells. 



All three of the theories mentioned are based upon the study of 

 organisms with definite nuclei which divide by mitosis. Is this the 

 most primitive condition? There is genetic evidence for recombina- 

 tion of genes ^^■hen different races of bacteria are mixed, suggesting 

 syngamy and meiosis in that group (see discussion by Lederberg and 

 Tatum). Genetic evidence suggests syngamy in viruses (see discus- 

 sion by Visconti). Are we to suppose that bacteria and viruses have 

 nuclei with chromosomes and that not only mitosis but also syngamy 

 and meiosis take place in these organisms? Hutchinson and Stempen, 

 in this volume, have shown that cell fusion may take place in bac- 

 teria; and there is some evidence for mitosis in bacteria (see discus- 

 sion by Lederberg and Tatum). Can viruses have nuclei, chromo- 

 somes, mitosis, and syngamy? Recently Fraser and Williams (1953) 

 claimed to have identified strands of nucleic acid, or chromosomal 

 material from viruses when host bacteria M^ere broken up on an elec- 

 tron microscope screen and photographed. 



If "sex" exists in bacteria and viruses, is the phenomenon a spo- 

 radic and isolated one, or is it more general? If these organisms rep- 

 resent the most primitive types of living things, and "sex" is found 

 to be general among them, did "sex" appear with the origin of life? 

 Is "sex" therefore the universal characteristic of living things that has 

 often been postulated? If this is so, can there be "sex" without nuclei 

 and chromosomes, or did the first living beings have nuclei and chro- 

 mosomes? If the first forms of life did not have nuclei and chromo- 

 somes, could they have had "sex"? In the blue-green algae (Cyano- 

 phyta), in which organized chromosomes apparently are absent, no 

 sexual phenomena have been found. Are the blue-greens to be re- 

 garded as degenerate, having lost chromosomes and "sex," or are they 

 primitive, having been derived from still simpler ancestors in which 

 chromosomes and "sex" did not exist? Dodson (1952) has suggested 

 that the blue-greens represent the ancestral type from which all the 

 other algae have been derived. If this is so, when and where did well- 

 defined nuclei, chromosomes, and "sex" come into existence? Why 

 is syngamy so uncommon in the Chrysomonadina, Cryptomonadina, 

 Euglenoidina, Amoebina, and so on? Have these groups evolved in 



