48 SEX-DETERMINATION 



Lower Algae. Hartmann and his colleagues (1932) have 

 shown that different strains of the unicellular Chlamydo- 

 monas eugametos can be classified into two groups, desig- 

 nated plus and minus. Plus cells never unite with plus and 

 minus never unite with minus. In certain conditions a plus 

 cell fuses with a minus cell to form a diploid zygote. This 

 undergoes two meiotic divisions and gives rise to 4 haploid 

 zoospores, two of which belong to the plus and two to the 

 minus types. They behave therefore as though they were 

 different and contrasted sexual forms. 



Paramecium. Sonneborn (1947) and others have shown 

 that several species of Paramecium and one species of 

 Euplotes are divisible into mating types between which, 

 but not within which, conjugation takes place. Within a 

 species there can be anything up to eight of these mating 

 types. If, therefore, these mating types are regarded as 

 sexual types they provide examples of the phenomenon of 

 multiple sexuality which is encountered also among the 

 algae and fungi. 



Fungi. Blakeslee (1904) showed that in the bread mould 

 Mucor zygospores are sometimes formed by the union of 

 hyphae from the same mycelium, being homothallic, but 

 that in most instances zygospores are formed only when two 

 distinct mycelia come together, these mycelia being sexually 

 different or heterothallic. Within a given heterothallic 

 species every individual can be assigned to one of two types 

 plus and minus. Two plus individuals will not unite 

 sexually, neither will two minus individuals. It is when 

 plus and minus mycelia come in contact that zygospores are 

 produced. 



Bryophytes. In most of the bryophytes the gametophytes 

 are haploids, being of two kinds, one female (XA) and the 

 other male (YA). In 1917 Allen described the large X and 

 the small Y of Sphaerocarpus donnellii in which the haploid 

 female gametophyte has seven autosomes and a very large 

 X, the male gametophyte seven autosomes and a very 



