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Algal genomes reveal evolutionary mosaicism and the fate of nucleomorphs

Published in:

Nature 492(7427) , 59-65 (Dec 6 2012)

Author(s):

Curtis, B. A., Tanifuji, G., Burki, F., Gruber, A., Irimia, M., Maruyama, S., Arias, M. C., Ball, S. G., Gile, G. H., Hirakawa, Y., Hopkins, J. F., Kuo, A., Rensing, S. A., Schmutz, J., Symeonidi, A., Elias, M., Eveleigh, R. J., Herman, E. K., Klute, M. J., Nakayama, T., Obornik, M., Reyes-Prieto, A., Armbrust, E. V., Aves, S. J., Beiko, R. G., Coutinho, P., Dacks, J. B., Durnford, D. G., Fast, N. M., Green, B. R., Grisdale, C. J., Hempel, F., Henrissat, B., Hoppner, M. P., Ishida, K., Kim, E., Koreny, L., Kroth, P. G., Liu, Y., Malik, S. B., Maier, U. G., McRose, D., Mock, T., Neilson, J. A., Onodera, N. T., Poole, A. M., Pritham, E. J., Richards, T. A., Rocap, G., Roy, S. W., Sarai, C., Schaack, S., Shirato, S., Slamovits, C. H., Spencer, D. F., Suzuki, S., Worden, A. Z., Zauner, S., Barry, K., Bell, C., Bharti, A. K., Crow, J. A., Grimwood, J., Kramer, R., Lindquist, E., Lucas, S., Salamov, A., McFadden, G. I., Lane, C. E., Keeling, P. J., Gray, M. W., Grigoriev, I. V., Archibald, J. M.

DOI:

10.1038/nature11681

Abstract:

Cryptophyte and chlorarachniophyte algae are transitional forms in the widespread secondary endosymbiotic acquisition of photosynthesis by engulfment of eukaryotic algae. Unlike most secondary plastid-bearing algae, miniaturized versions of the endosymbiont nuclei (nucleomorphs) persist in cryptophytes and chlorarachniophytes. To determine why, and to address other fundamental questions about eukaryote-eukaryote endosymbiosis, we sequenced the nuclear genomes of the cryptophyte Guillardia theta and the chlorarachniophyte Bigelowiella natans. Both genomes have >21,000 protein genes and are intron rich, and B. natans exhibits unprecedented alternative splicing for a single-celled organism. Phylogenomic analyses and subcellular targeting predictions reveal extensive genetic and biochemical mosaicism, with both host- and endosymbiont-derived genes servicing the mitochondrion, the host cell cytosol, the plastid and the remnant endosymbiont cytosol of both algae. Mitochondrion-to-nucleus gene transfer still occurs in both organisms but plastid-to-nucleus and nucleomorph-to-nucleus transfers do not, which explains why a small residue of essential genes remains locked in each nucleomorph.

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