Meeting Abstract
P1.93 Saturday, Jan. 4 15:30 A structural and functional comparison of the collar-flagellar systems in the choanoflagellate Monosiga brevicollis and freshwater sponge Spongilla lacustris MAH, J; CHRISTENSEN-DALSGAARD, KK; LEYS, SP*; University of Alberta; University of Alberta; University of Alberta sleys@ualberta.ca
The similarities between choanoflagellates and the choanocytes of sponges have been discussed for over a century, and yet few studies exist that allow a direct comparison between the two. We compared the structure and function of the collar and flagellum of the choanoflagellate Monosiga brevicollis and the sponge Spongilla lacustris. There were basic similarities in the pumping mechanism. The flagella maintained contact with the microvilli along most of the length of the collar, suggesting that the collars and flagella were integrated systems rather than independent units. In M. brevicollis the microvilli were joined by a ring-like structure allowing the opening of the collar to flare; in S. lacustris a mesh of glycocalyx joined the microvilli, forming a tube. Flagellar vanes composed of glycocalyx fibrils perpendicular to the flagellar axis spanned the base of both collars; vanes continued for the full length of the flagellum but were ragged beyond the collar in M. brevicollis. There were fundamental differences in the integration of the collar-flagellum system of the two cells, however. In S. lacustris the flagellum bent upon contact with the collar; the flagellar amplitude was fitted to the collar diameter. In M. brevicollis the flagellar amplitude was unaffected by the presence of the collar; the collar diameter may have been fitted to the flagellum instead. Our results suggest that though choanocytes and choanoflagellates may be homologous, this cannot be taken for granted. Similarities in the collar-, vane-and-flagellum pump system in such different organisms separated by 600 million years of evolution suggests that it is an important adaptation for optimising fluid flow through micro-scale filters.
