SEX IN UNICELLULAR ALGAE 103 



regularly sheds its cell wall before cytogamy (Pascher, 1943), and 

 C. '/noennsii, in w hich only one gamete remains motile after pairing 

 (Lewin, 1950a). (See page 119.) 



The condition in which the two types of gametes are morpho- 

 logically^ distinguishable is generally known as anisogamy or heter- 

 ogamy; wliere one gamete is non-motile at the time of fertilization 

 (and is thereby designated as female), the species is said to be ooga- 

 mous. Sexual differentiation of this sort has been discussed by Fritsch 

 (1935) and Smith (1950a, 1951a), who have pointed out that more 

 than one condition may exist within a single genus. Thus sexual re- 

 production has been observed in only about 10 to 20 per cent of the 

 described species of Chlaiuydoijiouas; of these over forty are isoga- 

 mous species, about eleven are anisogamous, and three show fairly 

 well-marked oogamy (Skuja, 1949). Fhyllo7nonas striata exhibits 

 marked anisogamy (Korschikoff, 1926); Chlorogo?iiufn oogaimim 

 (Pascher, 1931) and Carteria iyengarii (Ramanathan, 1942) are ooga- 

 mous. If we are to consider heterothallic species exhibiting anisogamy 

 or oogamy as derived from heterothallic isogamous forms, we may 

 envisage the evolution of the larger, or female, gamete from either 

 the plus or the mimis mating type, and we need not expect that in 

 all evolutionary lines the same mating type would have become the 

 female. This consideration can hardly be pursued, however, until 

 some of the physiological bases for heterothallism have been eluci- 

 dated. 



HeterothaUism. There are few diploid unicellular algae other 

 than the pennate diatoms, and of the latter almost all species which 

 have been critically examined appear to be homothallic or monoecious 

 (Geitler, 1949). In the case of Navicula balophila, however, Subrah- 

 manyan (1946) has presented circumstantial evidence for genetically 

 controlled dioecism, a condition found in Fiiciis vesiculosus, for in- 

 stance, but almost unknown in the lower Protophyta. Sex in diatoms 

 is more fully discussed (in this volume) by Patrick. 



Among haploid organisms, homothallism (monoecism or synoe- 

 cism) may be defined as the condition in which a complete sexual 

 cycle can take place within a single clone, and heterothallism (dioe- 

 cism or heteroecism) as that state in which two haploid clones of 

 different genotype — different genetic mating type — are required for 

 sexual reproduction. 



A discussion of homothallism, specifically in combination with 



