SI X IN UNICELLULAR ALGAE 1()5 



l)c hirLjcK- controlled l)\' the presence of soluble "sex substances" in 

 the medium (Aloewus, 1938b, etc.) (see page 115). From the evolu- 

 tionar\- standpoint, it would seem that a homothallic isogamous spe- 

 cies, with non-genetic mating-typc differentiation within a single 

 clone, would be at a disadvantage, since such a mechanism would 

 prcsumabh' reduce the potential sexual fertility of the species, while 

 in no wise promoting outcrossing or bringing other compensatory ad- 

 vantages. 



Another concept inherent in Hartmann's theory of sexual dif- 

 ferentiation is that of relative sexuality. Several examples of such 

 behavior in different genera of algae, such as Chlamydomonas, Poly- 

 tovia, and Protosipboii) , have been reported by Moewus, and in cer- 

 tain cases viable zygotes have been obtained by mating "strong" 

 gametes with "weak" gametes of the same sex. Extensive physiologi- 

 cal and genetic investigations have been described in many such cases, 

 and these have been reviewed in some detail by Moewus (1939a, 

 1941, 1950a), Chodat (1941), Smith (1951a), and others. Relative 

 sexuality has apparently not been established in any unicellular alga 

 investigated by other \\'orkers. No report of sex reversal in algae has 

 come to the notice of the present author. 



ISOLATION OF SEXUAL STRAINS 



The isolation of unicellular algae capable of sexual reproduction 

 under laboratory conditions has always presented difficulties. These 

 are attributable partly to the fact that the diplophase is rarely iden- 

 tifiable in nature, and therefore, in the heterothallic species, two hap- 

 loid clones of complementary mating type must first be isolated. It is 

 reasonable to assume that, in many species, reproduction is entirely 

 asexual; M^hereas in others the conditions favorable for eliciting a 

 sexual response, whether in homo- or heterothallic forms, are un- 

 known except for fragmentary and largely empirical data for a few- 

 species. Schreiber (1925) had no success in finding mating strains 

 among seventy Chlaviydovionas clones tested. 



Aloewus (1931) succeeded in isolating mating types of a species 

 of Chlamydomonas which he accordingly named C. eiigametos, later 

 retaining the name despite its prior description as C. sphagnophila by 

 Pascher in 1930 (see Moewus 1934, 1935b). A number of sexually 

 compatible clones of Chlainy domonas , probably identical with C. 



