Studies on the Development of Asexual Reproductive Structures in Pilobolus

SUMMARY

The formation of trophocysts and sporangiophores in a strain of Pilobolus kleinii which formed trophocysts only after the mycelium had been exposed to light was investigated by altering physical and chemical factors in the environment. A determination of the action spectrum showed that light with wavelengths between 380 and 410 mμ was effective in inducing the formation of trophocysts. Since carotinoids and flavins absorb light in this portion of the visible spectrum, an effort was made to determine which of these classes of compounds was involved by the use of inhibitors. Diphenylamine, which inhibits the formation of certain carotinoids, had no consistent effect on trophocyst formation although pigment formation was suppressed. L-lyxoflavin, which has been reported as a competitive inhibitor of riboflavin, strongly inhibited trophocyst formation. This inhibition of trophocyst formation by lyxoflavin was overcome in part by increasing the duration of exposure of the mycelium to light and was almost completely reversed by addition of riboflavin. The results of trials with inhibitors suggested that it is a flavin rather than a carotinoid in the mycelium of Pilobolus which absorbs light to initiate trophocyst formation. Investigation of the factors affecting sporangiophore initiation by trophocysts showed that (a) few sporangia were produced by trophocysts allowed to develop either in darkness or in continuous light, and (b) when cultures were exposed to light twice, the number of sporangia was proportional to the duration of the second exposure. These results were taken to indicate that light stimulates the initiation of sporangiophores by trophocysts and that the exposure of trophocysts to light is most effective if preceded by a period of darkness.