342 SEX IN MICROORGANISMS 



such a way that "sex" has been largely lost? If so, why was it lost? 

 How can these organisms flourish without "sex," which Nature seems 

 to have found so advantageous in the evolution of higher forms of 

 life? Possibly "sex" is more prevalent than present evidence indicates 

 and more research will bring more examples to light. However, there 

 is ample evidence that many unicellular organisms can multiply asex- 

 ually for indefinite periods of time without syngamy. Frankly, we 

 do not have satisfactory answers to many of the questions asked 

 above. Future research may possibly provide the answers. 



SEX DETERMINATION AND SEX DIFFERENTIATION 



Since the formation of gametes and their fusion to produce new 

 individuals is widespread among microorganisms, what light can be 

 shed on how gamete determination and differentiation have arisen? 

 The very expression "sex determination" implies a difference between 

 the sexes, and much of modern study of lower and higher organisms 

 indicates that sex determination is genetic and therefore presumably 

 due to different genie composition of the chromosomes; and indeed, 

 in many animals, there are visible differences in the chromosomes of 

 the two sexes, as pointed out by McClung in 1902. 



The terms male and female indicate a morphological difference 

 between the gametes which unite to produce a zygote and, in higher 

 organisms, differences in the individuals which produce the gametes 

 and in the organs in which they are developed. In cases of hermaphro- 

 ditism the sex organs of both sexes occur in the same individual. Ap- 

 parently most of the numerous theories of the nature of sex suggested 

 by the quotation from Geddes and Thomson (1889) dealt with the 

 development of maleness and femaleness. 



In cases of "isogamy" morphological differences between uniting 

 gametes are not apparent, although physiological differences are 

 presumed to exist. For example, the attraction that brings gametes 

 together may be centered primarily in one gamete or may involve 

 both. According to Hartmann's theory of relative sexuality, both 

 male and female potencies are present in each gamete, and the way 

 in which they unite depends upon the relative amounts of these 

 potencies possessed by each gamete. However, with most organisms, 

 male and female characteristics are fairly stable and definite (see re- 

 view by Smith, 1951). 



