DIFFERENTIATION OF ULVA MUTABILIS (CHLOROPHYTA) GAMETANGIA AND GAMETE RELEASE ARE CONTROLLED BY EXTRACELLULAR INHIBITORS1

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Abstract
Blade cells of Ulva mutabilis Føyn (Chlorophyta) excrete regulatory factors into their cell walls and into the environment. These factors are essential for the maintenance of the vegetative state. “Sporulation inhibitor‐1a” (SI‐1a) is a glycoprotein that was isolated from the culture medium of axenic Ulva growing as an undifferentiated callus. This protein was unusually stable to denaturing treatments and showed an extremely high apparent molecular mass (Mr) of 1–4 × 107 daltons estimated by size exclusion chromatography. The glycosylation was not essential for activity. SI‐1a suppressed gametogenesis completely at concentrations lower than 10−14 M. When Ulva developed normally in the presence of their symbiotic bacteria, smaller forms of SI‐1 accumulated in the medium (104–106 daltons). Sporulation inhibitors of the same size spectrum and with similar properties were also extracted from crude cell walls of nonaxenic Ulva. A class of different nonprotein sporulation inhibitors (SI‐2) of very low Mr and yet unknown structure was isolated from the inner space between the two blade cell layers. Excretion of all SI‐1 forms decreased with maturation of the thallus, whereas the overall concentration of SI‐2 in the thallus stayed constant throughout the life cycle. The SI‐2 affected different Ulva species whereas SI‐1 was species‐specific. Gametogenesis was induced upon removal of both Sporulation inhibitors from small single‐layered fragments of mature blades.After a “determination phase” of 23–46 h, dependent on the time of induction within the cell cycle, the cells became irreversibly committed to differentiation and were no longer susceptible to SI‐1 or SI‐2. Subsequently, during a 28‐h “differentiation phase,” 16 progametes were formed by synchronous genome doublings and cell divisions and differentiated into mature gametes. These became motile and were released from the gametangia when the concentration of a “swarming inhibitor” of low Mr, excreted mainly during the “determination phase,” declined below a threshold concentration. The biochemical properties of these regulatory factors and their effects on gametogenesis and gamete release are described.