LIFE CYCLES, SEXUALITY, AND SEXUAL MECHANISMS 53 



lateral hvphal branch, a stalked cxigoniuni with an anthcridial cell 

 either differentiated in the stalk or arising from it; (2) $ and 9 ele- 

 ments may arise from adjacent sections of main hyphae; and (3) $ 

 and 9 elements may arise from different main hyphae, each main 

 hypha being differentiated in its entirety as c^ or as 9 (Coker, 1923). 



Differentiation of sexual elements within a single thallus is usu- 

 ally reversible, either to the vegetative state or in some cases to sexual 

 organs of the opposite sign. The vegetative development of unfused 

 9 gametes of Alloinyces (Emerson, 1941) and the ability of the 

 differentiated sexual organs and even of isolated 9 gametes of homo- 

 thaUic species of Achlya to regenerate normal hermaphroditic plants 

 are typical examples of such reversibility (unpublished observations). 

 A more extensive reversibility, from sexual organs of one sign to 

 organs of the opposite sign, is fairly common in the homothallic water 

 molds. The production of antheridial hyphae from oogonia and the 

 occurrence of small oogonia intercalated in antheridial hyphae have 

 been observed in various species (Coker, 1923; Humphrey, 1892; 

 Maurizio, 1899). It has also been demonstrated in several homothallic 

 species that sexual hormones from strongly sexed plants caused oogo- 

 nial intials to redifferentiate and produce antheridial hyphae (Raper, 

 1950). 



One further point in connection with true homothallism is of 

 general biological interest. Sexual fusion normally occurs between 

 elements carrying sister (genetically identical) nuclei. This would 

 imply, a priori, that most fungi are deprived of the benefits occurring 

 in the recombinations of genetic factors following sexual fusion be- 

 tween dissimilar elements. Two facts would tend to mitigate this dep- 

 rivation: (1) the separate histories, often extended, of the two sister 

 nuclei brought together in the sexual act allow considerable oppor- 

 tunity for the accumulation and recombination of minor differences 

 due to induced or spontaneous mutations (Pontecorvo, 1947, 1950; 

 Pontecorvo and Roper, 1952; Roper, 1952); and (2) juxtaposed 

 thalli having totally different origins allow for extensive cross breed- 

 ing and hybridization in forms wdth motile or non-motile differen- 

 tiated gametes (Emerson, 1941, 1950) and for occasional cross breed- 

 ing and even hybridization in forms lacking free gametes (Raper, 

 1950; Salvin, 1942). The extent to which either or both of these 

 phenomena might duplicate in nature the benefits of enforced cross 



