5 



Recombination in Viruses and Bacteria 



We are accustomed to think that mating must somehow involve sex. 

 Such a concept comes naturally out of our own experience. During 

 the early development of genetics, it was not only assumed that sex was 

 necessary for genetic analysis but also that the recombination of genes 

 necessarily depended upon the fertilization of eggs by sperm. In recent 

 years, novel recombination processes have been discovered which are 

 probably of widespread occurrence, not only among microorganisms but 

 also in the somatic cells of higher organisms including man. 



Before discussing these processes, something will be said about sex and 

 sexual reproduction. The classical definition of sex is morphological. 

 The dictionary says that it is the character of being either male or female. 

 Among animals sex is obvious because of the different structure and habit 

 of male and female individuals. Plants too are sexual. In their repro- 

 duction, the male and female gametes which fuse together in fertilization 

 are the products of different meioses. 



From the point of view of genetics and evolution, however, the essen- 

 tial feature of sex is that it makes possible the recombination of genes 

 from different parents. In this way, new assemblages of genes arise 

 which then become available for the selective action of the environment. 



Some microorganisms such as Neurospora are sexual but hermaphro- 

 ditic; usually all strains can act both as the female parent by forming 

 fruiting bodies and as the male by fertilizing the fruiting bodies of an- 

 other individual. The zygote undergoes a true meiosis and the products 

 of a single meiosis may unite (see Figure 2.9). Some microorganisms 

 such as Chlamydomorms and yeast do not even have a structure such as 

 the fruiting body which can be compared to the egg. The cells of these 

 forms fuse together in a process called syngamy which leads to the pro- 



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