Sex in Unicellular Algae 



RALPH A. LEWIN,* Osborn Botanical Laboratory, 

 Yale University, New Haven, Connecticut 



At the outset it would be well to define the scope of this review. 

 AVaddington (1939) interprets "sex" in terms of sexual differentia- 

 tion, Sinnott and Dunn (1939) extend it to cover gametogenesis and 

 zygote formation, while Darlington (1937) considers sexual repro- 

 duction as embracing not only syngamy but also meiosis, as comple- 

 mentary processes essential for the completion of a sexual cycle. In 

 many protists there is no differentiation between cells capable of 

 acting as gametes; in only a few has the sexual cycle been followed 

 through genetically; while in Escherichia coli knowledge of almost 

 all aspects of sex but the purely genetic is for the most part inferen- 

 tial. For these reasons, in a field where information is so sparse and 

 fragmentary, the author hopes that a certain latitude of interpretation 

 will be allowed. 



Smith (1951a) considers the class of algae to be divided into 

 seven divisions. Among these the Cyanophyta are non-sexual, the 

 Rhodophyta and Phaeophyta are virtually devoid of unicellular rep- 

 resentatives, whereas in the Chrysophyta, the Pyrrophyta, and the 

 Euglenophyta sexual reproduction is unknown in all but a few excep- 

 tional cases. These flagellates are further discussed by Wenrich (see 

 "Sex in Protozoa: A Comparative Review " in this volume). A search 

 for experimental material must be thus largely restricted to the dia- 

 toms and the green algae. It is perhaps regrettable that the ubiquitous 

 species of Chlorelk, Scenedesmus, and Stichococcns, so favorable in 

 other ways for laboratory study, have not been known to exhibit 

 sexual reproduction. The desmids offer promising material; but the 

 success of Pringsheim (1919) in controlling zygote formation and 

 germination in Cylindrocystis has apparently not been followed up. 



* Present address: Maritime Regional Laboratory, National Research 

 Council, Halifax, N.S., Canada. 



Drs. S. H. Hutner, J. C. Lewin, and L. Provasoli have offered many crit- 

 ical suggestions which have been considered in the preparation of this review. 



100 



