Julie Koester

Post-doctoral Researcher

The focus of my research is to find connections between genes, phenotypes and the environmental forces of natural selection. Diatoms are my test-organisms of choice.

Two populations of the planktonic diatom Ditylum brightwellii co-occur in Puget Sound, but are differentiated by genome size (Koester et al. 2010); they are hypothesized to be cryptic species. Species Dbr2 has a 2-folder larger genome size than Dbr1, and likely arose from Dbr1 via whole genome duplication because its distribution is restriction to the northeastern Pacific Ocean, whereas Dbr1 has a circum-global distribution. Duplicated genes tend to accumulate mutations that are masked by working copies. These mutations may be selected for and sweep through a population providing adaptive benefit.

Seven percent of the protein coding genes in Thalassiosira pseudonana were determined to be positively selected (i.e. adaptively beneficial) by comparing homologous genes from seven strains of T. pseudonana. These genes disproportionately code for transcriptional regulators and protein-binding proteins. In addition, five positively selected genes that are putatively associated with the cell wall are up-regulated under iron and silicate limitation (Koester et al. in prep).


Left to right 1-3: 1) Two gametic valves and one large valve from a sexual clone of Ditylum brightwellii from the Gulf of Maine, 2-3) Ditylum brightwellii: a large clone from Puget Sound and small clone from New Zealand.



Ph.D. University of Washington, 2012

M.S., University of Maine, 2005

B.Sc. Honours, University of British Columbia, 1991



  • Koester, J.A., Swanson, W.J., Armbrust, V.E. (submitted) Positive selection within a diatom species acts on putative protein interactions and transcriptional regulation
  • Koester, J.A., Brawley, S.H.B., Karp-Boss, L. and Mann, D. 2007. Sexual reproduction in the marine centric diatom Ditylum brightwellii (Bacillariophyta). European Journal of Phycology. 42(4):351-366
  • Koester, J.A., Swalwell, J.E., von Dassow, P., Armbrust, V.E. 2010. Genome size differentiates co-occurring poplations of the planktonic diatom Ditylum brightwellii (Bacillariophyta).  BMC Evolutionary Biology. 10:1 doi:10.1186/1471-2148-10-1