Cell–cell fusion is central for sexual reproduction, and generally involves gametes of different shapes and sizes. In walled fission yeast Schizosaccharomyces pombe, the fusion of h+ and h− isogametes requires the fusion focus, an actin structure that concentrates glucanase-containing vesicles for cell wall digestion. Here, we present a quantitative correlative light and electron microscopy (CLEM) tomographic dataset of the fusion site, which reveals the fusion focus ultrastructure. Unexpectedly, gametes show marked asymmetries: a taut, convex plasma membrane of h− cells progressively protrudes into a more slack, wavy plasma membrane of h+ cells. Asymmetries are relaxed upon fusion, with observations of ramified fusion pores. h+ cells have a higher exo-/endocytosis ratio than h− cells, and local reduction in exocytosis strongly diminishes membrane waviness. Reciprocally, turgor pressure reduction specifically in h− cells impedes their protrusions into h+ cells and delays cell fusion. We hypothesize that asymmetric membrane conformations, due to differential turgor pressure and exocytosis/endocytosis ratios between mating types, favor cell–cell fusion.

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