The Effects of Elevated Osmotic Concentration on Control of Germination in the Gemmules of Freshwater SpongesEunapius fragilisandAnheteromeyania ryderi

Abstract

Freshwater sponges produce gemmules during the fall as an adaptation to survive cold winters. Most gemmules are produced in a state of diapause and must undergo a vernalization period before diapause is broken and they enter the quiescence state. Quiescent gemmules will germinate if placed at room temperature. We examined the mechanism of germination in two species of freshwater sponges, Eunapius fragilis and Anheteromeyania ryderi. Germination, cell division, and oxygen consumption are all inhibited when the osmotic concentration of the gemmules of either species is maintained at or above 50 mOsm by placing them in a solution of impermeable osmolytes. The internal osmotic concentration of cells of quiescent gemmules is maintained above 100 mOsm by the presence of sorbitol (in E. fragilis) and myoinositol (in A. ryderi). During the early stages of germination, levels of sorbitol and myoinositol decline to less than 50 mM by 20 h after the initiation of germination. The onset of cell division and beginning of germination correlate with the drop in osmolyte levels below 50 mOsm. Thus, an early trigger initiating germination is most likely the catabolism of sorbitol or myoinositol leading to a drop in the osmotic concentration of the cells.