30 Perspectives in Microbiology 



is sexual fertilization: a union of an entire gamete nucleus 

 from each parent to form the hybrid, zygote nucleus. There 

 are, of course, innumerable secondary physical and psychic 

 paraphernalia to support the act of fusion, but this and 

 not the accessories is the essence of sex for genetic pur- 

 poses. Sex occurs universally among higher plants and ani- 

 mals and is nearly as prevalent among protozoa and fungi, 

 but, with sporadic exceptions, has been reported absent 

 from the bacteria. 



Among lower plants, we also discern another mode of 

 recombination — heterokaryosis — in which the interaction 

 of intact nuclei falls short of fusion. Instead, diverse nuclei 

 multiply sui generis in a common cytoplasmic pool, where 

 their functional contributions are so intermingled as to 

 simulate hybridity. By sporulation, accident, or surgery, 

 however, the nuclei of the heterokaryon may be segregated 

 to reveal their lasting integrity. In the higher fungi, a 

 binary heterokaryon is a regular feature of the life cycle, 

 is maintained by conjugate mitosis, and may be ultimately 

 terminated by sexual fusion, whereas in the ascomycetes 

 and the phycomycetes, the nuclei of heterokaryons multi- 

 ply independently. A similar type of heterokaryosis almost 

 certainly occurs in actinomycetes (27); among nonfilamen- 

 tous bacteria it could only be transitory, but cannot be 

 ignored in the momentary control of phenotypes in muta- 

 tion and recombination procedures. 



By 1945, morphological approaches to the question of 

 bacterial sexuality had raised so many vexatious contro- 

 versies that, to quote Dubos' book (11) again: "If bacteria 

 do really reproduce by sexual methods, it should be possi- 

 ble to cross closely related species and strains . . . most 

 workers who have attempted to cross related strains have 

 reported only failure." 



Hindsight suggests that the chief omission of previous 

 experiments (14, 43) was a set of clear-cut, unit markers in 

 a selective system that would allow the detection of in- 



