With some parasites it's just “take...take...take” and this can include the host's genes! However, other unwelcome guests actually make generous DNA donations to their hosts.

Independent studies of Trypanosoma cruzi — the protozoan that causes Chagas disease — and the Rafflesiaceae — a family of plants that rely entirely on their hosts for nutrition — show that there have been horizontal gene transfers between these parasites and their respective vertebrate and plant hosts.

Following up on the hypothesis that frequent integration of T. cruzi DNA into the host genome might underlie Chagas disease, Nadjar Nitz and colleagues extracted genomic DNA from 13 patients with Chagas disease. A probe derived from components of the T. cruzi mitochondrial DNA (kinetoplast minicircles; kDNA) was then Southern hybridized to these extracts. This hybridization showed that there were smaller fragments of kDNA than would be expected if it were present in its native form, which indicated that kDNA had integrated into the host genome.

Using 5′-RACE (rapid amplification of cDNA ends), the authors isolated the genomic integration sites in each patient that they studied, identifying integration sites in a total of 5 loci. But could such integration events be observed in an experimental system?

To address this question, the authors examined rabbits that had been experimentally infected with T. cruzi for up to 3 years. These rabbits had de novo kDNA integrations, which indicated that horizontal transfer of parasite DNA to the host could be a normal part of the infection process. The rabbit and human data also indicated that β-globin loci and long interspersed nuclear elements (LINE-1) are frequent targets for kDNA integrations.

Interestingly, the authors went on to show that kDNA was also integrated in the genomes of the offspring of chronically infected rabbits and in chickens hatched from T. cruzi-innoculated eggs. Importantly, they also found kDNA integrations in the germline of F2 chickens without persistent infections, conclusively showing vertical transfer to infection-free progeny and so quashing any chance that their results could be artefactual.

Charles Davis and Kenneth Wurdack took an entirely different, but no less convincing, approach to their analysis of horizontal gene transfer between plants. Their phylogenetic analyses showed that, although Rafflesiaceae are close relatives of members of the order Malpighiales, the rafflesian mitochondrial nad1B-C gene is more closely related to the homologue in its obligate host, Tetrastigma (Vitaceae). Clearly, this is strong evidence that a section of the host mitochondrial genome was transferred to the parasite.

The huge significance of horizontal gene transfer for the evolution of prokaryotes has been known for a long time, as has the large contribution that intracellular endosymbiont ancestors of mitochondria and chloroplasts have made to eukaryotic genomes (see further reading). Studies such as these show that ongoing horizontal gene transfer from a range of parasites and endosymbionts might be more important for eukaryotic evolution than we previously realized — just how important remains to be seen.