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residues), not cleaved in algae, with no equiva- lent in conventional COXII (7). The conserved PxxxPxxY motif in this region of COXIIB alsoimplies a common origin for apicomplexan and chlorophyte cox2b genes (fig. S1). In phyloge-netic analyses of the available COXIIA andCOXIIB sequences and representative orthodox Soledad Funes,1 Edgar Davidson,2 Adria´n Reyes-Prieto,1 COXII sequences, apicomplexans group with Susana Magallo´n,3 Pascal Herion,4 Michael P. King,2 chlorophytes (Fig. 1). In contrast, apicomplexan sequences of mtDNA-encoded cytochrome bgroup independently from green algal sequences.
Apicomplexan parasites, including the hu- predicted that this alga would also have a nuclear The data strongly argue that apicomplexan cox2a man pathogens Toxoplasma and Plasmodi- cox2b (7) and have confirmed this (fig. S1) (9).
and cox2b derive from green algae, not from an um, contain a vestigial plastid, the apicoplast.
The frequency of cox2a and cox2b in Chloro- independently split apicomplexan cox2. T. gon- This chloroplast-derived organelle is the rem- phyceae suggests an ancestral chlorophyte cox2 dii cox2a has an intron that is conserved in nant of a secondary endosymbiosis between split in the mtDNA, before independent transfers location and phase with introns in cox2a of C. an ancestral apicomplexan and a photosyn- of cox2a and cox2b to the nucleus. S. obliquus reinhardtii and Polytomella sp. (figs. S1 and S2).
thetic organism whose origin is moot (1– 4 ).
appears as an intermediate, retaining cox2a in the This suggests a lateral transfer of cox2a and We have identified two distinctive apicom- mtDNA. The incompatibility between the S. cox2b from the green algal nucleus to the api- plexan nuclear genes that suggest a green obliquus mitochondrial and nuclear genetic codes (8) may have developed after the transfer Previous analyses have favored either a green of cox2b to the nucleus, preventing the subse- algal (2) or a red algal (1, 4) apicoplast precur- cytochrome oxidase, is usually encoded by a sor. Because the mtDNAs of a wide variety of mitochondrial cox2 gene. However, members red algae contain a single conventional cox2 (6), of at least three genetic lineages lack cox2 in cox2 is the Apicomplexa (10). We identified the nuclear cox2a and cox2b genes of the Api- cox2a and cox2b cDNAs from Toxoplasma complexa indicate a green algal endosymbiont guminous plants express full-length COXII gondii and cloned the corresponding nuclear (2) from the class Chlorophyceae. This is a re- from the nucleus (5). Members of the algal genes (fig. S1). We also identified nuclear markable example of lateral gene transfer, in thata single apicomplexan cytochrome oxidase sub-unit (COXII) has been replaced by two proteins(COXIIA and COXIIB) of green algal origin.
The use of functional split genes as atypicalmolecular markers has helped to clarify the com-plex evolution of the causative agents of malariaand toxoplasmosis.
References and Notes
1. R. J. Wilson et al., J Mol Biol. 261, 155 (1996).
2. S. Ko¨hler et al., Science 275, 1485 (1997).
3. G. I. McFadden, R. F. Waller, Bioessays 19, 1033 (1997).
4. N. M. Fast, J. C. Kissinger, D. S. Roos, P. J. Keeling, Mol. Biol. Evol. 18, 418 (2001).
5. K. L. Adams et al., Proc. Natl. Acad. Sci. U.S.A. 96,
6. M. W. Gray, Curr. Opin. Genet. Dev. 9, 678 (1999).
7. X. Pe´rez-Martı´nez et al., J. Biol. Chem. 276, 11302
8. A. M. Nedelcu, R. W. Lee, C. Lemieux, M. W. Gray, G.
Burger, Genome Res. 10, 819 (2000).
9. Supporting online material is available on Science 10. J. E. Feagin, Int. J. Parasitol. 30, 371 (2000).
Fig.1. Maximum likelihood (ML) analyses showing the position of apicomplexan parasites based on
11. Sequencing of T. parva and P. yoelii is carried on by cytochrome b (CYB) and COXIIA ϩ COXIIB sequences. COXIIA and COXIIB (excluding MTS and the Institute for Genomic Research. We thank J. M.
extensions) were fused in silico as a single polypeptide and aligned with orthodox mitochondrial Quinn and S. Merchant for the S. obliquus library and COXII sequences. Bootstrap support Ͼ50% is indicated above branches. See materials and methods A. Go´mez-Puyou, D. W. Krogmann, and D. A. Maslov in supporting online material (9).
for critical comments. Work supported by TW01176 (NIH), HL59646 (NIH), and IN207201 (UNAM).
class Chlorophyceae also lack cox2 in the cox2a and cox2b in the apicomplexans Plasmo- Supporting Online Material
mtDNA (6). Two such algae, Chlamydomonas dium falciparum, P. yoelii, Theileria annulata, www.sciencemag.org/cgi/content/full/298/5601/2155/DC1 reinhardtii and Polytomella sp., have two dis- T. parva, and Eimeria tenella.
tinct nuclear cox2 genes: cox2a encodes Several lines of evidence suggest a common COXIIA, corresponding to the NH -terminal ancestry for apicomplexan and green algal cox2 Instituto de Fisiologı´a Celular, Universidad Nacional genes. Both algal and apicomplexan cox2 genes Auto´noma de Me´xico (UNAM), 04510 D.F., Mexico.
cox2b encodes COXIIB, corresponding to the appear to have been split in identical locations Department of Biochemistry and Molecular Pharma- cology, Thomas Jefferson University, Philadelphia, PA COOH-terminal domain (7). COXIIA and (fig. S1). All predicted COXIIA proteins have a 19107, USA. 3Instituto de Biologı´a, UNAM, 04510 COXIIB are imported into the mitochondrion 125– to 170 –amino acid NH -terminal exten- D.F., Mexico. 4Instituto de Investigaciones Biome´dicas, (7). A truncated cox2 in the mtDNA of the sion that is a cleavable mitochondrial targeting chlorophyte alga Scenedesmus obliquus (8) has sequence in algal COXIIA (7). COXIIB proteins *To whom correspondence should be addressed. E- been recognized as a cox2a homolog. We have shorter NH -terminal extensions (43 to 60 www.sciencemag.org SCIENCE VOL 298 13 DECEMBER 2002

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