Continuous gene flow contributes to low global species abundance and distribution of a marine model diatom

Abstract

Unlike terrestrial ecosystems where geographical isolation often leads to a restricted gene flow between species, genetic admixing in aquatic micro-eukaryotes is likely to be frequent. Diatoms inhabit marine ecosystems since the Mesozoic period and presently constitute one of the major primary producers in the world ocean. They are a highly diversified group of eukaryotic phytoplankton with estimates of up to 200,000 species. Since decades, Phaeodactylum tricornutum is used as a model diatom species to characterize the functional pathways, physiology and evolution of diatoms in general. In the current study, using whole genome sequencing of ten P. tricornutum strains, sampled at broad geospatial and temporal scales, we show a continuous dispersal and genetic admixing between geographically isolated strains. We also describe a very high level of heterozygosity and propose it to be a consequence of frequent ancestral admixture. Our finding that P. tricornutum sequences are plausibly detectable at low but broadly distributed levels in the world ocean further suggests that high admixing between geographically isolated strains may create a significant bottleneck, thus influencing their global abundance and distribution in nature. Finally, in an attempt to understand the functional implications of genetic diversity between different P. tricornutum ecotypes, we show the effects of domestication in inducing changes in the selection pressure on many genes and metabolic pathways. We propose these findings to have significant implications for understanding the genetic structure of diatom populations in nature and provide a framework to assess the genomic underpinnings of their ecological success.