Sexual Reproduction 

 in Diatoms 



RUTH PATRICK, Academy of Natural Sciences of Philadelphia, 

 Philadelphia, Pennsylvania 



In diatoms reproduction is most frequently accomplished by vege- 

 tative division. Ordinary somatic mitosis occurs. This type of repro- 

 duction in a healthy culture of diatoms may occur several times 

 during a twenty-four-hour period. The rate is dependent on the kind 

 of diatom and the cultural conditions present. 



Of much less frequent occurrence are auxospores. An auxospore 

 is a resting cell which usually develops from a zygote. This process 

 has been occasionally observed in many of the genera of diatoms but 

 in only a comparatively few species. 



To date there seems to be little correlation between the type 

 of auxospore formation and the taxonomic relationship of the species. 

 Indeed, several different types of auxospore formation have been 

 observed in different varieties and forms of the same species. 



Often auxospore formation seems to be the result of sexual 

 processes. However, in many cases, as Sonneborn (Calkins and Sum- 

 mers, 1941) has said, perhaps we should "abandon the concepts of 

 male and female in unicellular organisms and view sexual union as 

 brought about by copulation-conditioning factors." 



The cause of auxospore formation seems to be a combination 

 of cell size and external environmental conditions. All species of dia- 

 toms are known to vary in length. According to Geitler (1932, 

 1935), auxospore formation occurs only when the cells of a taxon 

 are of a certain length, characteristic for each taxon. Often this 

 range in length is fairly wide, and again it is very narrow depending 

 on the taxon. This range is very constant for each taxon. If auxospore 

 formation does not occur when the cells are of the correct size, it 

 never occurs. The cells often become smaller and morphologically 

 quite changed, especially in culture. 



Auxospore formation in diatoms has been a frequent subject of 

 research during the nineteenth and twentieth centuries. Although the 



82 



