SEX IN UNICELLULAR ALGAE 101 



It is therefore \\ith the Volvocales that this review will be chiefly 

 concerned. 



No attempt has been made to discuss the literature on this sub- 

 ject exhaustively here. The field has already been well surveyed by 

 Fritsch (1935), Moewus (1941), and Smith (1951a); see also Luyet 

 (1950). Particular attention will be given here to studies of cultures 

 and experiments under controlled conditions, where a measure of 

 reproducibility can be expected in the results. The most critical in- 

 vestigations are those which involve material of genetic uniformity 

 and known physiological background, in a constant physical and 

 chemical environment and in the absence of any other living organ- 

 isms. 



SEXUAL DIFFERENTIATION 



Vegetative Cells and Gametes 



In a unicellular organism sexual differentiation may be taken to 

 indicate a difference between the two sexes or mating types; it may 

 also be considered to refer to the differentiation often found between 

 vegetative cells and gametes (see Moewus, 1933). In the simplest 

 case, as in many species of Chlmnydomonas (Klebs, 1896; Smith, 

 1950b), Polytoma (Pringsheim and Ondratschek, 1939), and Dima- 

 liella (Lerche, 1937), all haploid cells may be capable of sexual fusion. 

 In Chlanty domonas gymnogyne (Pascher, 1943) the sexually active 

 cells are morphologically identical with vegetative cells but appear to 

 differ in their more trembling mode of progression. However, in most 

 Chlamy domonas species and in related genera, for example, Chlo- 

 romovas (Korschikoff, 1926), Chlorogoniwn and Haentato coccus 

 (Schulze, 1927), Fhyllocardium (Korschikoff, 1927), and Brachio- 

 vmnas (Moewus, 1944), it may be observed that only the smaller 

 cells, those most recently liberated after cell division, take part in 

 mating. Each young cell may be said to pass through a gametic stage 

 and then to lose its sexual activity as it enlarges prior to asexual divi- 

 sion. In some such cases the cells may be naked during the gametic 

 phase and only develop cell walls later. Nayal (1933) has described 

 in Protosiphon the formation of two types of swarmers, rounded zoo- 

 spores and elongate facultative gametes, both capable of direct devel- 

 opment into vegetative cells. 



