NEW YORK (GenomeWeb News) - Scientists led by the Marine Biological Laboratory in Woods Hole, Mass., have published the genome sequence of Giardia lamblia, a water-borne parasite that causes intestinal illness in humans.
Giardia is “the most prevalent parasitic protist in the United States,” and is responsible for more than 20,000 reported infections a year in the US, according to the authors of the study, which is featured in the latest issue of Science.
The scientists said the “enigmatic” parasite has two forms: in water it lives as a highly infectious cyst, but once a cyst encounters the acidic juices in the gut of a human or other animal, it changes into a swimming and feeding form called a trophozoite.
Unlike most other parasites, trophozoites attach to cells rather than invade them. The parasite is able to evade the immune system of the intestine by changing its surface proteins.
The researchers said the genome of this organism has fewer genetic components than most other eukaryotes, which may offer information about the development of eukaryotes. One theory for the parasite’s simplicity is that it lost genes over time, while another posits that it split from other eukaryotic organisms more than a billion years ago, before modern eukaryotes developed.
According to the MBL researchers, the draft genome supports the latter theory. “Careful analysis of the genome reveals that Giardia's molecular machinery — even for the most basic processes usually shared by other eukaryotes — is simple by comparison. This suggests that it has always been so,” the said in a statement.
The study, which examined around 10 percent of the genome, found 149 proteins in the Giardia genome that the authors see as “promising” targets for vaccines or drugs that might prevent or treat infections from the parasite.
The Giardia genome is around 11.7 megabases in size, and is distributed over five chromosomes. The edited draft genome sequence contains 306 contigs on 92 scaffolds, and is “compact,” the study found.
“Giardia is an excellent functional and genomic model for other intestinal protozoan parasites whose complete life cycles cannot be replicated in the laboratory,” the authors suggest. Furthermore, they note, the expected release of a draft genome of the related organism Spironucleus vortens should “reveal which features of the Giardia genome result from its obligate parasitic lifestyle and which reflect its basal evolutionary position.”