Conjugation of Paramecium tetraurelia cells: selective wheat germ agglutinin binding, reversible local trichocyst detachment and secretory function repair*

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Abstract
Conjugation of ciliated protozoa involves precisely defined stages. In Paramecium there occur: (1) the formation of ciliary contacts; (2) local loss of cilia and formation of narrow contacts between anterioventral somatic cell membranes; followed by (3) formation of small cytoplasmic bridges (which allow for the exchange of molecular components only); before (4) their enlargement enables the cells to exchange micronuclei. This work concentrates on stages (2) and (3) in P. tetraurelia cells. We analysed lectin binding during conjugation, using mainly secretory mutants nd6 and tl (occasionally nd9-28°C and 7S (wild-type) cells); (tl is a trichocyst-free mutant, nd strains are non-discharge mutations). Our most essential findings are: already early contact sites (stage (2)) can be labelled with wheat germ agglutinin-fluorescein isothiocyanate; only early stages (2) can be reversed with an excess of WGA; no selective labelling was found with concanavalin A-fluorescein isothiocyanate (commonly observed with other ciliate species); WGA binding sites most probably involve GlcNAc residues. As known before, trichocysts disappear from contact sites in stages (2)/(3). We then analysed this process in more detail. Trichocysts are locally detached from the cell membrane (without transfer to the other conjugant) and re-inserted after conjugation. From our data obtained with nd6/tl conjugants one can conclude that trichocysts (in particular secretory lectins or glycoproteins contained in them) cannot play a regulatory role in the conjugation process, since conjugation precedes secretory function repair. We also present evidence for a secretory function repair in nd mutations by tl cells (though tl cells are devoid of any recognizable ‘exocytotic apparatus’). nd6 cells are more difficult to ‘cure’ than nd9-28°C. The ‘curing effect’ seems to emanate from the conjugation zone. This might indicate the occurrence of diffusible ‘curing factors’ exchanged mainly in stage (3). Finally, we show that secretory function repair in nd cells induces the assembly of ‘fusion rosettes’, i.e. the formation of the wild-type ultrastructure of the trichocyst release sites, to about the same extent as nd cells are functionally repaired (visualized by the release of trichocysts in response to two different trigger agents). Note: *This paper is dedicated to Professor Hellmuth Sitte on the occasion of his 60th birthday