SEXUALITY 



21 



Another yeast isolated by Pearse and Barker 1 from cider and 

 designated Yeast G presents a copulation which is intermediary be- 

 tween isogamy and heterogamy. In this species the two gametes 

 are cells of the same dimensions which do not show morphologically 

 any sexual differentiation. But the contents of one, which may be 

 regarded as the male, pass into the other, which may be regarded as 

 the female. The ascospores originate from 

 this last and they are always to the number 

 of two. (Fig. 23.) 



More recently, a strictly heterogamic proc- 

 ess was observed in a new species isolated from 

 fermentation products of wine from Bili. 2 This 

 yeast, which we have named Zygosaccharo- 

 myces chevalieri, has ascs which result from a 

 copulation between two cells of different di- 

 mensions. (Fig. 24.) One is very small and 

 represents the male gamete. It is young, 

 while the other, representing the female gamete, is larger and much 

 older, having attained its full development. The two cells unite by 



means of a copulation canal 

 and the contents of the male 

 gamete pass into the female 

 gamete in which the protoplas- 

 mic and nuclear fusion takes 

 place. After this has taken 

 place, the female gamete sepa- 

 rates itself by means of a wall, 

 and produces from 1 to 4 

 ascospores. During this the 



Fig 24. -Various Stages in the Copulation mem brane of the male gamete 

 and Formation of the Asc in Yeast Bill 2. 



23. Copulation in 

 Yeast G" of Pearse 

 and Barker (according 

 to Pearse and Barker). 



22 



23 



1-3, gametes sending out tubules destined for copula- 

 tion; 4-8, union of two gametes by a copulation 

 canal; 9-18, the contents of the male gamete mixes 



***** iis mbTa and 



. . 



1S absorbed. It IS alSO rare to 



nK^ Qn oHnlt an acrmn 

 ODServe an aClUlL d,SC again 



united to a male gamete. 



Recent investigations have 



shown that the heterogamic 



copulation occurs frequently. In Zygosaccharomyces priorianus it has 

 been shown that copulation is accomplished sometimes, but rarely, be- 

 tween an adult cell and a bud formed by it. In Debar omyces globosus 

 Guilliermond has shown that there takes place a heterogamic copula- 



1 Pearse, B., and Barker, P. The yeast flora of bottled ciders. Jour. Agricul- 

 tural Science, 3, 1908. 



2 Guilliermond, A. Sur un exemple de copulation heterogamique observe 

 dans une levure. Comp. Rend, de la Soc. Biologic, 70, 1911. 



