Evidence for organelle origin

Genetic evidence points to an evolutionary link between hydrogenosomes – organelles found in some ciliates, trichomonads, and fungi that generate hydrogen and ATP – and mitochondria,¹ according to Dutch and German researchers led by Johannes Hackstein of Radboud University Nijmegen in the Netherlands.

The team performed long-range PCR on hydrogenosome DNA from Nyctotherus ovalis, a ciliate found in the hindgut of American cockroaches. It yielded a 12-kilobase fragment of the organellar genome that encodes four genes of a mitochondrial complex I (nad2, nad4L, nad5, and nad7), two genes encoding mitochondrial ribosomal proteins (rpl2 and rpl14), and a tRNA tyrosine gene.

Investigating N. ovalis, the researchers identified three additional mitochondrial I complex genes in its nucleus, for a total of seven of the 14 core genes needed for that complex. They also found genes for half of the proteins of mitochondrial complex II in the nucleus. Phylogenetic analysis revealed that both hydrogenosomal and nuclear genes appear well conserved and that they cluster with their homologs from the mitochondrial genomes of aerobic ciliates.

Hackstein plans to use Southern blotting to further analyze the complete hydrogenosomal genome, estimated at 60 kilobases. "It would be very interesting to see how this organelle produces ATP and what happens with electron flow given the apparent presence of complex I and II and the absence of III and IV," says Patricia Johnson of the University of California, Los Angeles.