Cell division and filament splitting in Methanothrix concilii

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Abstract
Cells of Methanothrix concilii do not possess rigid, shape-forming cell walls; they obtain their cellular form from the portions of the sheath and spacer plugs which enclose them. Electron microscopy has shown that cell division proceeds by the ingrowth of spacer plugs, like the closing of a camera iris, from select regions of the sheath; this process forces the cells to split in two. Therefore, each spacer plug which traverses the filament of cells is a completed division annulus. Spacer plugs are two-layered structures; one layer is an assembly of fine concentric rings and is the first to be laid down during the division ingrowth. The second layer consists of larger, raised, concentric ribbons which progressively follow the advance of the first layer during division. Although cells within the filament are typically 2.5 .mu.m long, new daughter cells are ca. 1.0 .mu.m and can grow to ca. 4.0 .mu.m before division begins. Frequently, a developing spacer plug partitions a cell so that one daughter is a small, nonreplicating unit of protoplasm sandwiched between two completed plugs; eventually, this protoplasmic unit dies leaving a void in the chain of cells. The filament is susceptible to breakage at this juncture. In this instance, each of the two "void" plugs becomes a new terminal plug for the new ends of the split filament. This type of replication requires both cell division and filament splitting, and the series of structural events which are involved present a new form of prokaryotic division.