Genetics and Microbiology 31 



frequent recombinants. But as early as 1908, Browning 

 (8) did, in fact, make an impeccably designed test of sex- 

 uality in trypanosomes, using drug resistance for selective 

 markers. In 1946, this forgotten experiment was repeated 

 with some strains of Escherichia coli, and has since pro- 

 vided grist for the mills of several laboratories (30, 46). 

 The main conclusions of the recombination analysis point 

 to the participation of intact cells in a sexual interaction, 

 and it is called sexual because the putative gametes en- 

 compass the whole genetic content of each parent (37). 

 Selective methods were originally necessary because of the 

 infrequency of recombination, which also precluded a 

 parallel morphological decision. 



More recently, especially favorable strains with higher 

 fertility have facilitated microscopic studies, and in appro- 

 priate mixtures of cells conjoined pairs have been seen 

 and isolated with the micromanipulator (27). If left un- 

 disturbed, the swimming pairs will disjoin in an hour or 

 two. Exconjugants from about half the pairs engender 

 detected recombinants (and other sexual progeny are 

 doubtless undetected with the particular markers used). 

 The recombinants issue from only one, the maternal par- 

 ent, so to speak, to imply that conjugation transfers a 

 nucleus from one cell to the other, followed by fertilization 

 and reduction, rather like either half of a paramecium mat- 

 ing. No distinctive zygote structures, aside from the pairs, 

 have been noted. Except that it submits to considerable 

 stretching and torsion, nothing has been seen directly of 

 the conjugal apparatus, probably because of optical limita- 

 tions. A good deal remains to be learned, but I want to 

 emphasize that this experiment is validated primarily by 

 the genetic, not the morphological, observations. 



Although about 5 per cent of E. coli strains are known 

 to be fertile, sexual recombination has not been verified, 

 or at least studied genetically in any recorded detail, in 

 other bacteria, though a number of leads are being in- 



