The tiny green tentacles of Hydra polyps are a familiar sight for most zoology students. Matthias Habetha and Thomas Bosch at Kiel University now have something to add to the textbooks: while scrutinizing the genes that underlie Hydra's relationship with an algal symbiont, they discovered that Hydra viridis expresses a plant gene (p. 2157)!

Habetha and Bosch are intrigued by the genetic basis of the relationship between Hydra viridis and Chlorella, the algal symbiont that lives in Hydra's epithelial cells and gives the creature its familiar green colour. Since all eukaryotic cells are products of symbiosis between once free-living bacteria, `studying this ancient inter-kingdom communication process can provide us with interesting evolutionary insights,' Bosch says. Habetha and Bosch want to know which genes are activated when Hydra harbours its symbiont, to find out what regulates this intimate coexistence.

To screen for symbiosis-specific genes, Habetha and Bosch examined differences in gene expression between symbiotic Hydra and aposymbiotic Hydra - those that had been artificially induced to give up their symbionts. They identified six genes that are only expressed in symbiotic Hydra, and concluded that these genes are specific to symbiosis. But they were in for a surprise; when they searched animal genomes for homologues of the six genes, they found that one of the newly discovered genes bears no resemblance to any animal genes. Instead, it is closely related to plant peroxidase genes; they had found a plant gene in an animal genome! `We were so surprised that at first we thought that our samples had been contaminated with Chlorella DNA,' Bosch recalls. But when they checked Chlorella's genome, they couldn't find the gene - it really was in the Hydra genome.

So how did it get there? Habetha and Bosch suggest that, at some point in evolution, a symbiont transferred some of its genetic material into Hydra's genome. But they knew that Chlorella couldn't be the donor, because they hadn't found the peroxidase gene in its genome. `So the gene probably came from an earlier plant symbiont, which was later replaced by Chlorella,' Bosch concludes. They had another clue: Hydra's peroxidase gene lacks introns (segments of DNA that don't code for proteins) but all the modern plant homologues of this gene have introns. Since introns are spliced out when DNA is transcribed into mRNA, `the lack of introns suggests that the gene transfer occurred via mRNA,' Bosch explains.

Habetha and Bosch also discovered that the peroxidase gene is only expressed in Hydra undergoing oogenesis. Bosch isn't surprised that a symbiont-related gene is expressed at this point in the life cycle. `Chlorella is transmitted by oogenesis. The algae ensure that they end up in new polyps by crawling into developing oocytes,' Bosch says. He speculates that horizontally transferred genes like the one just identified `might be involved in oocyte formation to ensure the survival of the symbiont.' Bosch expects to find more horizontally transferred genes once Hydra's genome sequencing project, which is underway in the US, is completed.