Sexual Differentiation in the Bread Molds. 131 



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Sexual differentiation in the bread molds. 



By A. F. BLAKESLEE (by invitation). 



[From the Laboratory of Experimental Evolution, 

 Cold Spring Harbor, N. Y.] 



The Mucorales, represented by the bread molds, and their 

 relatives, are characterized by a sharp sexual differentiation. 

 Certain sexually primitive forms are hermaphroditic with equal 

 gametes. From such forms as a possible starting point sexual 

 differentiation seems to have proceeded in two directions — first 

 toward a constant inequality in the size of the gametes seen in a 

 few hermaphrodites; secondly toward a difference in the indi- 

 vidual plants themselves. In these diecious forms, the inter- 

 action of two sexual races are necessary for the production of 

 sexual spores. 



Not only will the opposite sexes of a single species unite to 

 form sexual spores but a reaction can also be induced between 

 the opposite sexes of different species which shows itself as an 

 "imperfect hybridization". An imperfect hybridization reaction 

 between the sexual races of diecious species and heterogamic 

 hermaphrodites leads one to believe that the ( + ) or vegetatively 

 more active race is female and the ( — ) or less active race is male. 



Tests of the sexual reaction of races of a large number of 

 different species have been made and no race of the diecious 

 forms has yet been found by the writer which reacts, if at all, 

 other than as a male or a female. Variations, however, occur in 

 the sexual vigor of different races and some are apparently neutral. 

 Such a neutrality has been induced by environmental factors. 

 Mutations have been observed in an hermaphroditic form pro- 

 ducing hermaphroditic races with male, female and neutral 

 tendencies. 



