Chapter 12 



A COMPARISON OF SEXUAL REPRODUCTION IN YEASTS AND 

 OTHER MICROORGANISMS 



The object of this chapter is to consider the respective advan- 

 tages of different microorganisms for genetical research. Al- 

 though a thorough understanding of the life -cycle of any organism 

 always yields unexpectedly valuable and interesting facts, each 

 investigator chooses the object of his research with a view to its 

 peculiar advantages. Whatever he may find out about the biology 

 of a plant or animal which produces no viable sexual offspring and 

 can only be propagated asexually, he will be unable to make chro- 

 mosome maps or to exploit tetrad analysis. He may succeed in 

 accomplishing a very valuable research in vegetative life -cycles 

 or physiology or biochemistry, but it cannot demonstrate the re- 

 lation between a chromosomal gene and an hereditary character- 

 istic except by employing analogy - a device which is universally 

 frowned upon in scientific circles. An organism with a number of 

 other peculiar advantages may be handicapped by an extremely 

 large number of chromosomes so that the amount of labor involved 

 in making chromosome maps may be prohibitive. It was a fortunate 

 accident that Drosophila unexpectedly had a simple life -cycle and 

 essentially only 3 chromosomes: Sciara which might equally well 

 have been chosen has proved quite difficult to exploit successfully. 

 Many workers are limited in the choice of their material by other 

 considerations. One important factor which cannot be ignored is 

 the amount of capital invested in a given project although we are 

 never permitted to forget the old adage that it does not pay to throw 

 good money after bad. Corn and Drosophila are peculiarly valua- 

 ble because such a great variety of stocks is available and the 

 techniques are so simple and so widely known that a person who 

 had spent half his working life time on one or the other cannot af- 

 ford to change. 



It has been argued that one reason for studying the genetics of 

 maize is the great importance of this type of work from an econo- 

 mic point of view. However, I do not believe it is possible to prove 

 that theoretical genetics has contributed in a large way to the hy- 

 brid corn program. This program has fallen to a large extent 

 within the peculiar province of the plant breeder between whom 

 and the theoretical geneticist there has always been a major schism. 

 Hybrid vigor is a problem for which classical genetics has pro- 



12-1 



