Chapter 21

As early as 1881, de Bary proposed that chemical substances emanating from the oogonia of watermolds triggered the development of antheridial branches and directed their growth toward these cells. This was, on the face of it, an unusually perceptive theory because de Bary did not fully accept the idea that fertilization by antheridia took place in any of the species then known in the family. Klebs (1899) also surmised that the presence of oogonia stimulated antheridial branches to form. Nutrients, Kauffman (1908) conjectured, lead to the synthesis of substances -he referred to them variously as enzymes and hormones -that were set free in the medium and there, at many points of “equilibrium” induced antheridial hyphae to form and to grow toward the oogonia. In his study on “heterothallism” (Whitehouse, 1951, refers to this condition as haplodioecism) in Dictyuchus, J. N. Couch (1926b) sought possible chemical stimulants for the matings obtained in two-member cultures. Since antheridial branches in some of his isolates grew rather long distances before contacting oogonia, he suspected that a diffusible substance was operating in some fashion. However, the mating strains of Dictyuchus had to make actual contact, he found, or the sexual process would not manifest itself. If as many as six hyphae from an antheridium-producing thallus were simply positioned across the hyphae of a “female” thallus sexual stimulation resulted. On the other hand, one female colony immersed in the same culture water with eight “male” thalli would not respond unless there was direct contact, and plants potentially able to form oogonia simply would not do so in “juices” expressed from male mating strains (J. N. Couch, 1926b). In sum, Couch uncovered three lines of evidence to suggest that there were diffusing chemical substances in various individuals of Dictyuchus. First, antheridial hyphae definitely grew toward the oogonia. Second, oogonial initials and antheridial branches sometimes were formed on portions of reacting hyphae far from regions of actual contact, and third, sex cell production was noticeably stimulated in matings among dioecious members of Dictyuchus and Thraustotheca primoachlya (=Achlya primoachlya). About four decades after Couch’s paper appeared, W. A. Sherwood (1966a) assembled experimental evidence that Dictyuchus monosporus in fact did produce a diffusible hormone system. His observations suggested that the mechanism was induced under the combined influence of the male and female strains. Sherwood utilized a perfusion system in “microaquaria” in which the male and female thalli could be separated but kept in alternating order. Female thalli receiving water that had perfused over male and female colonies together in one microaquarium produced oogonia. Antheridia were not induced in the cultures, however, so there were no oospores. Couch’s early work was a pioneering effort, but the reality of hormonal control of sexuality in some watermolds was proven by J. R. Raper (1939a, b; 1940a; 1942a, b). In a series of carefully conducted experiments coupled with a fortunate choice of organisms, Raper demonstrated conclusively that diffusible substances are produced in