102 SEX IN MICROORGANISMS 



\n Chlamy dojnonas paupera (Pascher, 1931-32) differentiation is 

 carried a stage further. The species is homothallic and isogamous; the 

 gametes after liberation develop first a male sexual potentiality and 

 later, if not involved in sexual fusion, become female. Unmated gam- 

 etes in culture ultimately die vi^ithout growing into a form capable of 

 asexual reproduction (that is, they appear incapable of parthenogene- 

 sis), so that here one finds a dichotomy between obligate gametes and 

 asexual cells. A similar condition has been described in Tetraspora 

 (Geitler, 1931), in Stephanosphaera (Moewus, 1933), and in the mi- 

 crogametes of C. praecox (Pascher, 1943). (According to Moewus, 

 1941, in some algae the ability of gametes to reproduce asexually is 

 controlled by a single, partially sex-linked gene. In C. eiigametos, in 

 which all motile cells are potentially gametes, he claimed to have 

 obtained by x-irradiation numerous mutants in which the gametes 

 lacked this ability to divide vegetatively. This paradoxical situation 

 awaits explanation.) Since both gametes and vegetative cells are pro- 

 duced after haploid mitosis, it is unlikely that they could differ geneti- 

 cally; and in a case of this sort it would be of great theoretical inter- 

 est to determine what synthetic faculty or cytoplasmic moiety may 

 be lost in gametogenesis, thereby rendering the sex cells incapable of 

 further multiplication in the haploid condition. 



Outbreeding Mechanisms 



In sexually reproducing organisms the frequency of sibling mat- 

 ings may be reduced ( 1 ) by sexual dimorphism between the gametes 

 (by anisogamy or oogamy) or (2) by genetically inherent factors of 

 self-sterility, or intraclonal sexual incompatibility. These two out- 

 breeding mechanisms are not mutually exclusive, and homothallic as 

 well as heterothallic species may be found to exhibit isogamy, ani- 

 sogamy, or oogamy. Multipolar heterothallism has not been demon- 

 strated in any alga. Subdioecism or subheteroecism has been described 

 in Chlamydomo7ias and other genera by Aioewus (1934 et seq.). 



Sexual Dimorphism. In some species {Chlamydomonas el on gat a 

 for example) the gametes may be of various sizes but capable of pair- 

 ing in all combinations — a condition referred to by Korschikoff 

 (1923) as ataktogamy. In others the gametes are morphologically 

 similar but exhibit what may be described as physiological anisogamy; 

 for example, C. gymnogyne, in which only one gamete in each pair 



