HOMOTHALL1SM AND HETEROTHALLISM 319 



HOMOTHALLISM AND HETEROTHALLISM 



From the above generalizations we may pass on to their sig- 

 nificance. For a long time it was held that the spore or the indi- 

 vidual derived from any spore is totipotent. This concept, it may 

 be interjected, as employed in studies of monosporous cultures, has 

 been both a great deterent to progress and a potent factor in pro- 

 moting progress in the acquisition of knowledge of fungi. It has 

 hindered progress because many workers have regarded a mono- 

 sporous culture of a fungus as a whole organism, whereas it may 

 be, as we now know, only a "hemi-organism." On the other hand, 

 the concept has promoted knowledge because by use of mated 

 monosporous cultures it has been possible to learn that each 

 individual may not be totipotent but may require another comple- 

 mentary culture. In 1904 Blakeslee (1904) first proved, for a 

 number of species of Zygomycetes, that zygospores can be ob- 

 tained only when mycelia of opposite sex are mated. If he grew 

 mycelium from a single conidium, sporangia and conidia were 

 formed in abundance, but gametangia and zygospores were not 

 produced. To those organisms requiring two thalli of opposite 

 sex potentialities for fertilization, he applied the term heterothallic. 

 One strain or race he called plus ( + ), and the other minus ( — ). 

 Sex in these species is segregated in bipolar fashion at meiosis. On 

 the other hand, in Sporodinia grandis mycelia from single conidia 

 produce zygospores and are therefore hermaphroditic, and sex 

 segregation is entirely lacking. Subsequently both heterothallism 

 and homothallism have been found to occur side by side in genera 

 in all the larger groups of fungi. 



In Phycomycetes. BurgefT (1928) isolated from Phy corny ces 

 blakesleeamis a number of variant or mutant races to which he 

 gave such form names as arbusculus, mucoroides, gracilis, and 

 pallens. When various crosses between the original P. blake- 

 sleeamis and any one of the forms were made, the progeny ap- 

 peared like that of the original except in the crosses with mucor- 

 oides. The type of progeny, therefore, is determined by a single 

 factor that is recessive in the form and dominant in the original; 

 this was true in all crosses with mutants except in the form 

 mucoroides. Burgeff also noted certain linkages with the factor 

 for sex. For example, in his crosses of arbuscula with the normal, 



