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Forming syncytia by vegetative cell fusions in the coralline red alga Neopolyporolithon reclinatum (Rhodophyta, Hapalidiales)

ABSTRACT Light microscopy and transmission electron microscopy were used to examine the developmentally important but little studied process of red algal vegetative cell fusion. Cell fusions were pervasive in the coralline alga Neopolyporolithon reclinatum; aggregates composed of as many as 15 laterally fused perithallial cells were reconstructed from paradermal sections. Fusions did not occur in a consistent pattern or orientation. Oblique fusions between cells of different perithallial tiers integrated the thallus both horizontally and vertically. Ultrastructural study showed that the fusion process was initiated by the formation of a blunt, truncated projection of the cell surface. A thin layer of new wall was deposited on the sides of the projections, but no other distinctive structural cellular features that might be associated with localized decalcification of the wall between cells were detected. Microfibrillar wall components were removed during growth of the fusion protuberance. After cell fusion, a reorganized cytoskeleton formed a connected network bridging the fusion zone and cell lumens. A dominant cytoplasmic strand, if present, spanned the longest axis of the cell, which was longitudinal in unfused cells but was oblique or transverse in laterally fused cells. The frequency of cells involved in fusions, the number of conjoined cells within a fused assemblage, and the integration of the merged cells by a shared cytoskeletal network demonstrated that the fusion process in this alga remodelled the thallus, converting filaments of uninucleate cells into a highly integrated anatomy composed also of vertically and horizontally connected, multinucleate syncytia.