April 16, 1920] 



SCIENCE 



377 



to be explained shortly. By inoculating plus 

 and minus spores in appropriate spots in tlie 

 culture dish definite patterns may be obtained 

 by the lines of zygospores formed where the 

 opposite sexes meet as shown in Figs. 52-55. 



IMPERFECT HYBRIDIZATION 



It was soon established that in any given 

 diecious species there are only two sexes 

 present but at first there was no assurance 

 that the two sexes of a given species were the 

 eame as those of any other. It has been dis- 

 covered, however, that the opposite sexes of 

 different species are capable of showing an 

 imperfect sexual reaction when grown in con- 

 tact. The photograph on the screen (Fig. 1, 



Fig. 1. Diagram of Petri dish culture showing 

 zygospores (dots) between the (-|-) and ( — ) races 

 of the same species, Mucor C and lines of "im- 

 perfect hybridization" (dashes) at contact be- 

 tween opposite sexes of different species, Muoor 

 C and Mucor V. 



which is taken from photograph in 7) repre- 

 sents a culture of the sexual races of two 

 different species Mucor V and Mucor C. 

 "Where the two sexes of Mucor C meet a 

 line of zygospores is evident as might be 

 expected. The sexual race of Mucor V on 

 the right shows a reaction only with Mucor C 

 minus as indicated by the white line where 

 the two meet and must be considered the 

 opposite sex from Mucor C minus, or plus. 

 In like manner the other sex of Mucor V on 

 the left shows a reaction only with the plus 

 race of Mucor C and must therefore be con- 



sidered minus. A microscopic investigation 

 of the appearance of the white lines of sexual 

 reactions shows the condition represented in 

 Figs. 36-39 (1). The stimulus of contact 

 leads to the formation of progametes. Some- 

 times the gamete is formed by one and more 

 rarely by both the reacting filaments. In one 

 strong reacter the gamete which is formed in 

 this reaction transforms itself frequently into 

 a thick-walled a-zygospore. The stimulus 

 which leads to a dissolving away of the wall 

 between the two gametes and their conse- 

 quent fusion is constantly lacking. Since the 

 sexual reaction between opposite sexes of 

 different species is incomplete it has been 

 termed " imperfect hyhridization." 



acd darker. 



Sporangial growth Hucwr tr. 

 lower aoJ ligliter in 



Sporangial growth less Pbycomycea^f. 

 phoreB more slender. 



Sporangial growth Mi 

 slightly denser ajid 



M. Mucedo 



Circinella umbclliil 

 Synccphalastruni, 

 Cliaetoeladiuni 



Fig. 2. "Imperfect hybridization" reactions 

 shown by solid lines between sexual races of dif- 

 ferent species. 



At first the paired races of different species 

 were designated by letters and symbols. 

 Lines in the diagram (Fig. 2) indicate some 

 of the reactions that have been obtained be- 

 tween races of different species. All races 

 that show a reaction with the " c " race of 

 Mucor V used as a standard but not with its 

 " d " race, are placed in the right column, 



