374 REPRODUCTION 



Burgeft (56) studied Mucor mucedo, a heterothallic Zygomycete 

 which copulates by the fusion of identical gametangia when two 

 strains of opposite mating type are brought together. The two mor- 

 phologically identical types are called for convenience plus (+) and 

 minus (— ); historically, at least, the term heterothallism is appropri- 

 ately applied to this situation. Burgeff showed that at least two of the 

 steps in reproduction are initiated by a chemical, diffusible stimulus. 

 Zygophore development in each strain is begun only when the opposite- 

 strain is close enough for the stimulus to reach receptive hyphae. The 

 somewhat clumsy term "telemorphosis" has been used for this reaction. 

 A second reaction, also initiated by a diffusible metabolite, has been 

 called "zygotropism": zygophores of each strain bend toward those 

 of the opposite strain, preparatory to contact and fusion. 



These reactions in Mucor mucedo have been confirmed (167, 176, 

 225) and found also to prevail in Pilobolus crystallinus (177, 178). 

 Limited evidence suggests that the zygotropic reaction is effected by a 

 volatile product (15, 167). Such a material would have to be of very 

 high activity to induce a differential growth response in an organ as 

 small as the zygophore. 



Banbury (14) reports evidence that zygophores of the same strain in 

 Mucor mucedo exert a mutual repulsion, also by a volatile substance. 

 This recalls the numerous examples of "aversion" or "barrage": a 

 zone of inhibition develops between certain colonies of the same or 

 of different species and not between others. This phenomenon is re- 

 viewed by Cayley (73) and by Takemaru (282); little physiological 

 work has been done, although it has been suggested that a volatile ma- 

 terial is responsible (293). 



A final type of interstrain chemical influence is shown by a slight in- 

 crease in oxygen uptake when (+) and (— ) mycelia occupy the same 

 respirometric vessel (57). 



None of the hormonal activities of the Zygomycetes has been sepa- 

 rated physiologically or biochemically from the others; it is not yet 

 possible, therefore, to describe any such elegant sequential mechanism 

 as that of Achlya spp. 



At this point we may briefly mention the numerous attempts to find 

 physiological or biochemical differences between (+) and (— ) strains 

 of species of the Mucorales. Various workers have claimed that the 

 strains differ in growth rate on certain sugars, sucrase or catalase forma- 

 tion, lipid or carotenoid content, sugar content, reductive capacity, 

 susceptibility to poisons, and so on. Most of these are reviewed by 

 Ronsdorf (250) and by Raper (236). Study of the original papers and 

 of the divergent results obtained leaves one skeptical of all this work. 



