Professor Thomas Mock, East Anglia
The evolution and significance of diverged alleles in diatoms
Natural selection is the hallmark of evolution. However, inference of selection in algal genomes has been a challenge but is key to provide unequivocal evidence of adaptive evolution under given environmental conditions. Here we explore the role of selection for the evolution of diatom genomes under variable environmental conditions. Diatoms often outcompete other phytoplankton species in variable environments, which indicates a significant level of adaptation to frequently changing environmental conditions. However, how selection has acted on the evolution of diatom genomes with respect to ecosystem variability has not been extensively studied yet but is important to understand why diatoms have become a dominant force in variable marine environments, which are considered the most productive marine ecosystems on Earth. To address that question, we sequenced a diatom genome from the Southern Ocean, which is considered to be a highly variable ocean due to strong seasonality in light, temperature and nutrients. Our study reveals that the genome of Fragilariopsis cylindrus contains highly diverged alleles that are differentially expressed depending on the environmental conditions and stresses imposed. Alleles with largest ratio of replacement over silent substitutions (largest dN/dS ratio) show the most pronounced condition-dependent expression. This suggests that environmentally-induced diversifying selection drives the allelic differentiation. The highly diverged alleles with nucleotide divergence of up to 6% show nevertheless a signature of genetic recombination. Many of the diverged alleles encode proteins from conserved core and lineage-specific metabolism indicating the requirement to fundamentally adjust metabolism to cope with an extreme and variable environment. Homologs of diverged alleles account for 60% of all F. cylindrus-specific transcripts in natural communities, including the most highly abundant transcripts. Diverged alleles adapted to particular conditions are maintained in a vast gene pool and enable the population to respond to the highly variable environment of the surface Southern Ocean.