44 MOLECULES, VIRUSES, AND BACTERIA. 



Recombination with equal parental participation 



In a search for characters that could be defined in biochemical 

 terms, Beadle and Tatum ( 1941 ) turned to the ascomycete Neurospora 

 crassa, which can be cultivated on a medium containing sugar, inor- 

 ganic salts, and biotin. They selected mutations which suppressed the 

 ability of the mold to synthesize one or another growth factor, an 

 amino acid, or a nitrogenous base. Many such characters proved to 

 show simple inheritance and to be due to the loss of the ability to 

 synthesize a single enzyme. 



Geneticists quickly' saw the great advantage of this material for 

 use in recombination studies. Hybridization is of a classical type, in that 

 two haploid nuclei fuse to form the zygote. However, the diploid 

 nucleus does not persist as such but immediately undergoes meiosis, 

 forming the sexual ascospores. Each spore, upon germination, produces 

 a mycelium populated with haploid nuclei of a single type. Thus all of 

 the products of a single meiotic division can be recovered in a pure 

 haploid state. In some species of molds, Neurospora for one, these 

 spores are even aligned in proper order in the ascus. Thus, by dissect- 

 ing an ascus and germinating the spores separately, each meiotic 

 strand of a given chromosome can be recovered and characterized. As 

 a consequence, recombination can be studied with far greater accuracy, 

 and mapping can be performed on fewer progeny than in the diploid 

 organisms of classical genetics. A second very important quality of 

 such material is that, with nutritional markers, selective media can be 

 employed, and under appropriate conditions very rare recombination 

 events can be detected and studied (Pontecorvo, 1952). Thus the 

 "resolving power" of recombination analysis is enormously enhanced; 

 the microbial geneticist can often analyze segments of chromosomes 

 which are 1/10,000 to 1/1,000,000 of a Morgan unit long. In other 

 words, he can detect events which occur in one out of 10,000 to 1,000,- 

 000 meiotic divisions. 



Similar resolution is obtained in quite another genetic system— that 

 of crosses between bacteriophage particles. A bacterium infected with 

 two or more very similar but recognizably diflFerent phage particles 

 will give rise to viral progeny some of which are like the parents and 

 some of which are hybrid. By using markers affecting host specificity, 

 one can, here again, select rare recombinant types ( Benzer, 1955 ) . The 

 principal difference between phage systems and Mendelian ones, such 

 as Neurospora and Aspergillus, is that between the moment of infec- 

 tion of a cell with two viral particles and the moment of liberation of 

 viral progeny through lysis of the bacterium, a number of matings oc- 

 curs between increasing numbers of intracellular particles. Further, not 

 all of the genomes formed during growth of the viral population give 



