Evolutionary genomics of the cold-adapted diatom Fragilariopsis cylindrus

Abstract

The genome of the Southern Ocean phytoplankton Fragilariopsis cylindrus differs markedly from the genomes of its more temperate relatives, with divergent alleles being differentially expressed in environmentally specific conditions such as freezing and darkness. Diatoms are the main primary producers in the Southern Ocean, but how they have adapted to an environment with such extremes of light and temperature has remained unknown. Here Thomas Mock et al. report the genome sequence of a cold-adapted diatom from the Southern Ocean, Fragilariopsis cylindrus, and compare this 'psychrophile' with diatoms that evolved in temperate oceans. They find that its genome contains highly diverged alleles that are differentially expressed across environmental conditions. The Southern Ocean houses a diverse and productive community of organisms1,2. Unicellular eukaryotic diatoms are the main primary producers in this environment, where photosynthesis is limited by low concentrations of dissolved iron and large seasonal fluctuations in light, temperature and the extent of sea ice3,4,5,6,7. How diatoms have adapted to this extreme environment is largely unknown. Here we present insights into the genome evolution of a cold-adapted diatom from the Southern Ocean, Fragilariopsis cylindrus8,9, based on a comparison with temperate diatoms. We find that approximately 24.7 per cent of the diploid F. cylindrus genome consists of genetic loci with alleles that are highly divergent (15.1 megabases of the total genome size of 61.1 megabases). These divergent alleles were differentially expressed across environmental conditions, including darkness, low iron, freezing, elevated temperature and increased CO2. Alleles with the largest ratio of non-synonymous to synonymous nucleotide substitutions also show the most pronounced condition-dependent expression, suggesting a correlation between diversifying selection and allelic differentiation. Divergent alleles may be involved in adaptation to environmental fluctuations in the Southern Ocean.