VAN NIEL SYSTEMATICS OF THE BACTERIA AND BLUEGREEN ALGAE 93 



incorporation of two additional, and equally negative, criteria, viz., the absence 

 of plastids in the cells, and the absence of sexual reproduction. 



However attractive Copeland's system may have appeared a decade ago, 

 recent developments have raised difficulties great enough to threaten the very 

 basis of the characterization of the kingdom. The most important of these deal 

 with the problem of the "bacterial nucleus." 



Even in 1938 there were some indications that bacteria contain discrete struc- 

 tures that might be considered, on the basis of their behavior and chemical nature, 

 as nuclei (Badian, 1933; Stille, 1937; Piekarski, 1937). Studies of this sort have 

 been continued, with improved methods and instruments, especially by Delaporte 

 (1939), Eobinow (1944, 1945), Knaysi (1947, 1951), Boivin (1948), Welsch and 

 Nihoul (1948), Tulasne and collaborators (1947, 1949), and DeLamater (1952); 

 the results support the previous allegations. Even though a convincing demon- 

 stration of nuclei has not yet been accomplished for more than a few bacterial 

 and myxophycean types, it may be confidently expected that future work will 

 fill the existing gap. It is thus becoming increasingly clear that these organisms 

 cannot be incorporated into Copeland's kingdom of "microorganisms without 

 nuclei." 



Similar remarks, while not yet as definitive, may well apply to the two addi- 

 tional criteria mentioned above. The finding in cells of the photosynthetic bac- 

 terium, RJio do spirillum ruhrum, of uniform spherical particles in which all the 

 pigment is contained (Schachman, Pardee, and Stanier, 1952) indicates that plas- 

 tidlike elements are not lacking in the bacteria; according to Calvin and Lynch 

 (1952) a, very similar situation is apparently encountered in the bluegreen 

 alga, Synechococcus. 



Last, there is the matter of sexual reproduction in these organisms. While 

 there are some published reports alleging the occurrence of fusion of individual 

 cells in bacterial cultures (Potthoff, 1922, 1924), these had not been taken too 

 seriously, and it is fair to state that the actual conjugation of two cells with the 

 formation of a zygote has yet to be observed by continuous microscopic examina- 

 tion. But the startlingly novel report by Lederberg and Tatum (1946; see also 

 Tatum and Lederberg, 1947, Lederberg, 1947) of the occurrence of "recombina- 

 tion effects" in mixed cultures of bacterial mutants has changed the picture. The 

 observed phenomena cannot be ascribed to "back mutations"; they are, however, 

 readily interpretable on the basis of a postulated conjugation, followed by recom- 

 bination of genetic factors during the mitotic division of the nucleus of the con- 

 jugant. It is true that the recent studies of Hayes (1952) have shown that similar 

 recombinations occur in mixed cultures of mutants in which one of the partners 

 has been rendered nonviable. This suggests that an unequivocal interpretation 

 of the recombination effect as the result of a primary conjugation is not possible. 

 On the other hand, there exists at present a healthy skepticism with regard to 

 the earlier belief that sexual phenomena do not occur among the bacteria. 



Thus it is clear that the criteria for a kingdom of organisms without nuclei 

 do not apply to the bacteria and bluegreen algae. This does not mean, however, 

 that the notion of establishing a separate kingdom for these organisms should be 

 abandoned. As mentioned before, there are good reasons for subscribing to the 

 idea that we must reckon with the existence of organisms that are neither plants 

 nor animals and represent the descendants of precursors of both these groups. 



