Alma Whiffen Barksdale

THE UPTAKE OF EXOGENOUS HORMONE A BY CERTAIN STRAINS OF ACHLYA

Hormone A, the substance that initiates the sexual reaction between thalli of Achlya ambisexualis Raper, is found in measurable amounts in cultures of the female strains of this species, whereas little or no hormone A can be detected in cultures of the male strains reacting to this substance (Raper, 1940, 1942; Raper and Hagen-Smith 1942). While studying the production of hormone A by various strains of Achlya, I found that the concentration of hormone A in a culture containing a mixture of male and female mycelia equalled, not that of the female strain grown alone, but that of the male strain. A search for an explanation of this failure of hormone A to accumulate in cultures containing a mixture of male and female mycelia led to the discovery that the male thalli of A. ambisexualis and the thalli of certain other species of Achlya, as well, possess the ability to remove hormone A from the surrounding medium.

THE SEXUAL HORMONES OF THE FUNGUS ACHLYA

Achlya is a genus of the aquatic fungi that inhabit the fresh water of lakes and rivers. These fungi are composed of filaments that grow in length at one end only, nuclear division being confined to this growing end. The filaments and their branches lack cross-walls except for those that delimit the reproductive organs. There are two kinds of reproductive organs: asexual and sexual. The motile, asexual spores are produced in a sporangium that is formed by the wallingaff of the growing tip of a vegetative filament. Concomitant with the formation of the septum, growth and nuclear division cease in the sporangium. The sex organs, which are morphologically distinct, consist of a tubular or lobed male organ, the antheridium, and a spherical female organ, the oogonium. Sexual reproduction, involving the production and conjugation of the sex organs, is initiated and regulated by hormones that are sequentially secreted by the sexual partners into their aqueous milieu. The female continuously secretes a substance, called hormone A, to which the male reacts by producing branches on which antheridia are borne. The sexually activated male then secretes a substance, called hormone B, to which the female reacts by producing branches on which oogonia are borne. After the oogonia are formed, the antheridial branches grow to the oogonia and wrap around them. The antheridium is now cut off by the formation of a cross-wall near the tip of the antheridial branch where it rests on the oogonial wall. Fertilization is accomplished by the growth of tiny tubes from the antheridium to the eggs and the discharge of the antheridial nuclei through these tubes into the eggs. The hormones regulating sexual reproduction can be studied conveniently in the dioecious species, Achlya bisexualis Coker and A. Couch and Achlya ambisexualis Raper, which are comprised of male and female individuals. The hormones produced by one species are equally effective on the other. The sexual reactions occurring between males and females in these species are illustrated in the accompanying Figures. Matings may be accomplished either on the surface of an agar plate or in a shallow layer of liquid medium. In FIGURE 1, a male and a female of A . bisexualis have been inoculated about an inch apart on a nutrient agar plate. Where the margins of the two colonies meet, the male on the left has been induced to form antheridial branches. The female on the right has produced oogonial initials in response to the male. In FIGURE 2, taken about six hours later, the antheridial branches are growing to and around an oogonial initial. FIGURE 3 is a photograph of an oogonium and antheridium of A . ambisexualis showing the act of fertilization. Two fertilization tubes can be seen extending from the antheridium to the eggs. Hormone A, the substance secreted by the female, is the only hormone of Achlya to be isolated in pure form. The activity of hormone B and other postulated hormones of Achlya is known only from studies of culture filtrates and concentrates made in the years 1936 to 1950 by J. R. Raper (1936, 1939, 1940a