Evolution of Mutation Rate in Astronomically Large Phytoplankton Populations

Area of Science:

• Evolutionary Biology
• Marine Biology
• Genetics

Background:

• Genetic diversity is typically proportional to population size.
• Lewontin's paradox highlights an unexplained lack of genetic diversity in large populations.
• Lower mutation rates in larger populations are hypothesized to explain this paradox.

Purpose of the Study:

• To test if lower mutation rates explain Lewontin's paradox.
• To measure the spontaneous mutation rate in the marine phytoplankton Emiliania huxleyi.
• To investigate the relationship between population size and genetic diversity in this species.

Main Methods:

• Genome sequencing of Emiliania huxleyi mutation accumulation lines.
• Analysis of mutation spectrum and rate estimation.
• Calculation of effective population size based on mutation rate.

Main Results:

• 455 mutations were identified with a GC-biased spectrum.
• The per site mutation rate (µ) was estimated at 5.55×10⁻¹⁰.
• An effective population size (Ne) of approximately 2.7×10⁶ was calculated, which is modest for this abundant species.

Conclusions:

• The measured mutation rate in Emiliania huxleyi is not unusually low.
• Exceptionally large populations do not evolve mutation rates significantly below ~10⁻¹⁰ per nucleotide per cell division.
• The paradox of modest genetic diversity despite enormous population size in E. huxleyi is not explained by a low mutation rate.