Genomic evidence of speciation and adaptation in diatoms

Abstract

Diatoms are one of the most ecologically important groups of organisms in the ocean. They are the youngest and most species-rich group of phytoplankton, having colonized both marine and terrestrial ecosystems. The ocean is constantly changing, and understanding the mechanisms of species-diversification and adaptation in diatoms is important to assessing their resilience to future environmental changes. Speciation and adaptation were investigated in three diatom genera using genomic and genetic signals. One mechanism of speciation was tested by bringing isolates of two populations of the planktonic diatom Ditylum brightwellii into the lab to measure genome size differences indicated by cell size differences in the field. Genome sizes differed by two-fold between individuals of each population, suggesting that the populations are in fact cryptic species, thus corroborating previous research indicating reproductive isolation between the populations. Growth rates of Ditylum isolates from within a species differed significantly depending on where they were collected, southern or northern Pacific Ocean, suggesting that they were differentially adapted to their local environments. Natural selection acts directly on phenotypes; positively selected genes control those phenotypes and their sequences vary among populations and between species. Positively selected genes were investigated in Pseudo-nitzschia, Ditylum and Thalassiosira, but the greatest number of selected genes was found within a single species, Thalassiosira pseudonana. All of the protein coding genes from seven strains of T. pseudonana were analyzed and 809 (7%) were found to be positively selected. These genes encode protein-binding proteins, transcriptional regulators, and proteins associated with cell signaling and the cell wall. One quarter of the positively selected genes was novel to T. pseudonana, thus differentiating this species while conferring a selective advantage to individuals. Genome duplications, such as occurred in D. brightwellii, provide an opportunity for increased genetic variability upon which selection may act under changing environments. In the absence of polyploidization, genetic variability is maintained through mutations accrued during each round of cell division. The positively selected genes presented here for T. pseudonana provide future opportunities to test hypotheses concentrated on linking genotypes of positively selected genes with the phenotypes that they control and their associated selective pressures.