Vol. 43 T O R R E Y A July 1943 



The Evolution and Determination of Sexual Characters in the 

 Angiosperni Sporophyte* 



Charles E. Allen 



One result of genetic study which bears definitely upon evolutionary 

 theory is the demonstration that the determination of an apparently simple 

 character depends upon the activity of many genes. It is indeed suggested 

 that the interaction of all the genes of an organism may be essential to the 

 appearance of any character ; but for the present this broader conception 

 remains in the realm of speculation. 



Another contribution from the same source is the demonstration that 

 similar or apparently identical phenotypes may be determined by diverse 

 genie complexes. It follows that very different gene mutations in distinct lines 

 of descent may result in the appearance of similar characters — a fact which 

 in another aspect students of phylogeny have long stated in terms of parallel 

 or convergent evolution. 



Turning to a special class of characters, it is evident that sexual dif- 

 ferentiation has arisen independently in many different plant and animal 

 lines. There is no reason for assuming that the changes in the genetic 

 mechanism which resulted in this differentiation were identical, or even 

 closely similar, in diverse lines. 



To this consideration is to be added that, after the first step in sexual 

 differentiation, additional mutations occurred, independent and varying in 

 different lines. These later steps resulted in differentiation between the organs 

 in which gametes are produced ; in differentiation of individual gamete- 

 producing plants or animals as respectively female and male ; and, in certain 

 pteridophytes and in all seed plants, in a backward extension of sexual 

 differentiation to involve structures of the parental spore-bearing generation. 



A priori, then, it is not to be expected that the genetic mechanisms which 

 determine sexual potentialities or which influence sex-expression should be 

 the same in different groups of organisms. Yet it is characteristic of discus- 

 sions in this field that unitary theories of "sex-determination" have been 

 developed ; each based upon phenomena observed in one or in a few related 

 species, but each seeking to apply one mechanism to all groups of sexually 

 differentiated organisms. There is, to be sure, one set of facts which may 

 seem to support this conception of uniformity : namely, the occurrence in 

 widely separated phyla of apparently similar bodies— the sex chromosomes — | 

 which are a part of the genetic mechanism whose nature is being sought. 



* Read at the 75th Anniversary Celebration of the Torre}' Botanical Ckib at Columbia 

 University, Monday, June 22, 1942. 



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