The Diatom Sex Clock and Its Evolutionary Significance

Abstract

Diatoms, which possess distinctive skeletal valves of opaline silica, show steady incremental reduction of size with successive vegetative divisions. Size reduction is explained by consistent formation of slightly smaller new valves inside the old ones. Diatoms cannot be induced to become sexual until a certain critical minimum size has been achieved through this steady reduction of size. Sex leads to regeneration of the largest size. There is strong support for the hypothesis that the primary evolutionary purpose of the vegetative size reduction is to serve as a sex clock, thus allowing sex to be spaced optimally even over very long intervals in genetic lines of these unicellular organisms whose lifespans are too short to support the function of long-term physiological clocks. This control over sex, which is unique to diatoms among the unicellular algae, may contribute substantially to the high diversity and worldwide abundance of diatoms.