Reproduction (asexual)

The diatom life history is unique among all organisms, and is one consequence of possession of a rigid, silica cell wall. Diatom cell walls are composed of two parts, or valves, the epitheca and hypotheca. When a cell divides by mitosis, each parental valve becomes an epitheca of each of the two daughter cells. The new, and smaller, hypothecal valves are formed from within the parent cell to fit inside each existing parent valve. Because the silica valves are inflexible, mitosis results in daughter cells of unequal length; one cell is the same size as the parent, and one cell is slightly smaller.

Over time, descendants of a single clone decrease in cell length, and in most diatoms, other cell dimensions are altered. This process is well-known as the MacDonald-Pfitzer rule, which was independently described more than 140 years ago. The MacDonald-Pfitzer rule predicts that, within a population, mitotic division will (i) decrease the mean apical length and (ii) increase the variance in length.

Citations

Length Vs Frequency — Conceptual schematic of the MacDonald-Pfitzer hypothesis and estimated output parameters from finite mixture models. As predicted by the MacDonald-Pfitzer hypothesis, a diatom population (represented here as a histogram of cellular apical length) will through time reduce in mean apical length (l) and increase in length variability (r). The third parameter, k, represents the number of cells at t = 1 over the number of cells in all three unimodal distributions.