April 16, 1920] 



SCIENCE 



379 



as well as others, have observed also consider- 

 able differences between the different strains 

 of a single sex of a given species. It is 

 possible that these Japanese investigators have 

 been dealing with races with differences of the 

 order just mentioned. The matter may be a 

 question of what is a species. Burgeff, how- 

 ever, in his brilliant investigations of Phyco- 

 myces (10), has obtained a striking distinct 

 mutant which he has been able to cross suc- 

 cessfully with the normal stock. 



It seems strange that the reaction initiated 

 in the process of " imperfect hybridization " 

 is, usually at least, unable to carry through 

 to completion. We can assume something 

 fundamental, common to all the plus races 

 of the various mucor species, that causes a 

 response when they are brought into contact 

 with a minus race and something in addition 

 that must be present peculiar to the same 

 species in order to extend the reaction to a 

 imion of gametes and their development into 

 normal zygospores. 



These fundamental characteristics of plus 

 and minus must be present also in the herma- 

 phroditic or homothallic species since, as in- 

 dicated at the top of Fig. 2, such hermaphro- 

 ditic species may show sexual reactions with 

 plus and minus races used as testers. The re- 

 action is often strong enough to be indicated 

 by white lines as shown in Fig. 56 (1) where 

 the hermapliroditic Mucor I is reacting with 

 both plus and minus races of Mucor V. 



ZYGOSPORE GEKSIINATION 



It will be of interest to note what occurs at 

 the germination of zygospores formed by the 

 sexual races. A zygospore at germination 

 produces a short germ-tube, terminated by a 

 germ sporangiima. The condition is repre- 

 sented diagrammatically on the screen (4). 

 In the hermaphroditic species investigated, all 

 the spores in the germ sporangium are herma- 

 phroditic and give rise to hennaphroditic 

 plants' as is to be expected. In the diecious 

 species, however, there are two types of zygo- 

 spore germination. In Mucor Mucedo the 

 spores in a germ siKirangium are all of the 

 same sex — plus or minus, never mixed. In 



Phycomyces, on the other hand, the germ 

 sporangium may contain spores of both sexes. 

 The germ tube may be induced to grow out 

 vegetatively before the formation of its germ 

 sporangium. By this means its sexual condi- 

 tion can be tested. The germ tube of Mucor 

 Mucedo has been foimd to be imisexual, of 

 the same sign as its germ sporangium. Segre- 

 gation or differentiation of sex in this species, 

 therefore, must have taken place at or before 

 the zygospore germinates. In Phycomyces 

 sexual differentiation takes place in the germ 

 sporangiiun and induced growth from a germ 

 tube gives rise to a temporarily hermaphro- 

 ditic condition. Such a hermaphroditic or 

 homothallic mycelium is shown in the photo- 

 graph (2). Its yellowish felted growth is 

 strikingly different from the normal plus and 

 minus races which are forming a line of 

 zygospores where they meet at the upper 

 right-hand corner of the culture. Occasion- 

 ally spores in the germ sporangium of Phyco- 

 myces are hermaphroditic and produce such 

 hermaphroditic mycelia. Burgeff has in- 

 geniously produced them by mechanically 

 mixing the protoplasm of plus and minus 

 vegetative filaments and has given them the 

 name of mixochimaeras. He concludes that 

 such mixochimffiras are mixtui'es of plus and 

 minus nuclei. That these sexual mixoehi- 

 mseras are bisexual is shown by their oc- 

 casional production of hermaphroditic zygo- 

 spores and by the fact that the scanty spor- 

 angia which they produce again divide up 

 into plus and minus and occasionally herma- 

 phroditic spores. Often they show a plus or 

 a minus tendency by forming zygospores with 

 the normal minus or plus test races of Phyco- 

 myces. If propagated by cuttings of the 

 mycelium, they eventually revert to normal 

 plus or minus races. 



The diagram (4) has been shown in order 

 to point out certain homologies between sexual 

 differentiation in the mucors and that in 

 other groups of plants. The mucor plant is 

 the gametophyte, the flowering plant the 

 sporophyte. The germ tube with its germ 

 sporangium we have homologized with the 

 sporophyte and Burgeff reports in Phycomyces 



