Eukaryote-to-eukaryote gene transfer gives rise to genome mosaicism in euglenids
1 Department of Biological Sciences, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
2 The Canadian Institute for Advanced Research, Program in Integrated Microbial Biodiversity, Department of Biochemistry and Molecular Biology, Dalhousie University, Sir Charles Tupper Medical Building, 5850 College Street, Halifax, Nova Scotia, B3H 1X5, Canada
3 Department of Biology, Graduate School of Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
4 Institute for Plant Biochemistry, Heinrich-Heine-University, Geb. 26.03.01, Universitätsstrasse 1, D-40225 Düsseldorf, Germany
BMC Evolutionary Biology 2011, 11:105 doi:10.1186/1471-2148-11-105Published: 18 April 2011
Euglenophytes are a group of photosynthetic flagellates possessing a plastid derived from a green algal endosymbiont, which was incorporated into an ancestral host cell via secondary endosymbiosis. However, the impact of endosymbiosis on the euglenophyte nuclear genome is not fully understood due to its complex nature as a 'hybrid' of a non-photosynthetic host cell and a secondary endosymbiont.
We analyzed an EST dataset of the model euglenophyte Euglena gracilis using a gene mining program designed to detect laterally transferred genes. We found E. gracilis genes showing affinity not only with green algae, from which the secondary plastid in euglenophytes evolved, but also red algae and/or secondary algae containing red algal-derived plastids. Phylogenetic analyses of these 'red lineage' genes suggest that E. gracilis acquired at least 14 genes via eukaryote-to-eukaryote lateral gene transfer from algal sources other than the green algal endosymbiont that gave rise to its current plastid. We constructed an EST library of the aplastidic euglenid Peranema trichophorum, which is a eukaryovorous relative of euglenophytes, and also identified 'red lineage' genes in its genome.
Our data show genome mosaicism in E. gracilis and P. trichophorum. One possible explanation for the presence of these genes in these organisms is that some or all of them were independently acquired by lateral gene transfer and contributed to the successful integration and functioning of the green algal endosymbiont as a secondary plastid. Alternative hypotheses include the presence of a phagocytosed alga as the single source of those genes, or a cryptic tertiary endosymbiont harboring secondary plastid of red algal origin, which the eukaryovorous ancestor of euglenophytes had acquired prior to the secondary endosymbiosis of a green alga.