Apeil 16, 1920] 



SCIENCE 



381 



dite. (The large circles in the culture are the 

 iJaces where the races were inoculated.) 

 Tests with plus and minus races show that 

 this normal race is a hermaphrodite with a 

 minus tendency. In the central vertical row, 

 three colonies are growing of a mutant with a 

 plus tendency. That it is also hermaphroditic 

 is shown hy the dark dots representing zygo- 

 spores which it produces, larger and arranged 



Tig. 3. Diagram of Petri dish culture of the 

 hermaphroditic species Muoor genevensis. Circles 

 represent points of inoculation, dots represent 

 zygospores. Two colonies at right represent the 

 normal parent stock; vertical row in center repre- 

 sents three colonies of a mutant with a ( + ) tend- 

 ency; two colonies at left represent a mutant with 

 ( — ) tendency. 



in sectors more often than in the parent race. 

 Since it has an opposite sexual tendency from 

 the normal race, it forms a line of zygospores 

 where it grows in contact with it. It also is 

 forming a line of zygospores with the mutant 

 on the left, two colonies of which are shown. 

 This last mentioned mutant has, like its 

 normal parent form, a minus tendency. 

 Aside from its dense yellow growth, it is 

 characterized by its well-nigh complete sup- 

 pression of hermaphroditic zygospores on its 

 own mycelium. If the suppression were com- 

 plete and the race constant, we might be able 

 to describe the origin of a diecious race from 

 a hermaphroditic species. "With the exception 

 of the dwarf mutant which has been kept 

 running since 1913, but which does not pro- 

 duce sporangia, and another possible exception. 



all the mutant variants found in this species 

 eventually reverted to the normal type. 



The tendency to reversion has been observed 

 by Burgeff in his mutants of Phycomyces 

 and attributed by him to a more rapid 

 growth of the normal nuclei over the rmitant 

 nuclei in mixochimseras which he considers 

 such variants to be. He was able to bring 

 his mutants into a true-breeding condition 

 by crossing them with the normal stock of 

 opposite sex and obtaining the desired purity 

 through segregation in the germ sporangia. 

 Sex and mutant characters he found to segre- 

 gate independently so that starting with a 

 mutant in a plus race he was able eventually 

 to obtain it in the minus condition. 



DISTRroUTION OF SEXUAL TYPES IN NATURE 



A study has been made of the distribution 

 in nature of the different sexual types. So 

 far as the number of species is concerned, the 

 diecious or heterothallic forms greatly pre- 

 dominate. If the table on the screen (5) 

 were made to-day, we probably should have to 

 more than double the list of species definitely 

 known to have been mated. If unmated 

 strains with sex determined by the " imperfect 

 hybridization " reaction were added, the num- 

 ber woidd be still further increased. Of the 

 homothallic (hermaphroditic) forms, very few 

 would have to be added and the hermaphro- 

 ditic forms it will be observed are those in 

 which the sexual condition is readily deter- 

 mined by mere raieroscopic inspection and 

 their zygospores therefore less likely to escape 

 notice. 



Table II. (3) above shows the distribution 

 of races of Rhizopus obtained from different 

 sources. As will be seen, the sexual strains 

 are not at all local in their distribution. 

 Those listed as neutral failed to give any 

 reaction with the plus and minus test races. 

 More extensive tests have recently been made 

 with Bhizopus and other species as will ap- 

 pear in a later summary. 



Collections of races of several different 

 species have been made from diverse sources 

 and the races within a given species tested 

 for reactions inter se. Perhaps a form provi- 



