Extreme Lewontin's paradox in ubiquitous marine phytoplankton species.

Larger populations are expected to have larger genetic diversity. However, as pointed out by Lewontin in 1974, the range of population sizes exceeds the range of genetic diversity by many orders of magnitude (aka "Lewontin's paradox", LP). The reasons for LP remain obscure. Here I report an extreme case of LP in astronomically large populations of the ubiquitous unicellular marine phytoplankton species Emiliania huxleyi (Haptophyta) - the species that accounts for 10 to 20% of primary productivity in the oceans and its blooms are so extensive that they are visible from space. I demonstrate that despite the wide distribution and enormous population size, the world-wide sample of E. huxleyi strains with sequenced genomes represents a single cohesive species and contains surprisingly limited genetic diversity (π ∼ 0.006 per silent site). The patterns of polymorphism reveal even larger populations in the past, and frequent recombination (ρ ∼ 0.006) throughout the genome, ruling out demographic history and asexual reproduction as possible causes of low polymorphism in E. huxleyi. Natural selection wiping out genetic diversity at linked sites (aka 'genetic draft') must be strong and frequent to account for low polymorphism in E. huxleyi. This study sheds the first light on poorly understood evolutionary genetic processes in astronomically large populations of marine microplankton.