Nutrient Scarcity as an Evolutionary Cause of Haploidy

Abstract

Diploidy is characteristic of complex organisms, although it also appears in numerous groups of simple organisms. Many groups of simple organisms are haploid or haplo-diploid. Diploidy is widely considered to have certain categorical advantages that account for its appearance in the more complex members of all evolutionary lines. Any categorical advantage of diploidy begs an explanation of the widespread retention of haploidy among simple organisms. The retention of haploidy can be explained on a nutritional basis. There is an essential connection of unknown cause between the amount of DNA and cell size; more DNA maintains more cytoplasm. Factors favoring small size will favor haploidy (minimal DNA) in a single-celled organism. Multicellular organisms are not similarly affected because they can reduce the number of cells, rather than reducing cell size in response to size-reducing selection pressure. The most general of size-reducing selection pressures is nutrient scarcity. For unicellular organisms, this selection pressure is probably most pronounced in taxa that obtain their energy and their nutrients by processes subject to temporal separation (autotrophs) and that live in environments where nutrients are frequently exhausted (plankton environments). Unicells that obtain energy and nutrients simultaneously (e.g., Protozoa) are not subject to the same selection pressures and are expected to be diploid. The nutrient-sparing hypothesis is generally consistent with the sporadic distribution of haploidy among unicellular organisms, and appears to explain why haploidy is retained in some groups and not others.