One of the obvious things that needed to be done if we were going to study Hydra biology at the molecular level was to clone genes from Hydra. Thus, my lab and Hans Bode’s lab set out to do this. Upon joining Hans’ lab as a postdoc in the summer of 1986, Doug Fisher set as his first task the cloning of an actin gene from Hydra. The reason for this was to obtain the promoter from a constitutively expressed gene that could be used to drive the expression of transgenes in Hydra. Because of the extraordinarily high sequence conservation of actin genes across diverse taxa, it was expected that an actin gene could be isolated from Hydra by screening of genomic and/or cDNA libraries from Hydra with a probe from another animal. Doug had been trained in molecular biology as a graduate student in Lee Hood’s lab, so he got off to a fast start, producing both phage cDNA and phage genomic libraries from Hydra attenuata (now the Zurich strain of Hydra vulgaris). His screen with an actin cDNA clone from chicken worked, making him the first person to clone a gene from Hydra (Fisher and Bode, 1989).

While Thomas had no training in molecular biology before arriving in Irvine, he was a remarkably fast learner. Since my lab was working on the src gene in Xenopus, we decided that Thomas should try to identify a src gene homologue in Hydra. Thomas screened the Hydra phage cDNA library made by Doug with oligonucleotide probes designed by Mike Bishop’s lab to search for protein-tyrosine kinase genes in yeast. I still recall the day when we got the sequence from one of Thomas’ clones; when translated, it clearly encoded a src family protein-tyrosine kinase gene! Doug’s paper on the Hydra actin gene and Thomas’ paper on the Hydra src gene (Bosch et al., 1989) were accepted for publication one day apart, on 25 June 1989 and 26 June 1989 respectively. The actin promoter did indeed eventually get used to make transgenic Hydra, a feat accomplished in the Bosch lab and published in PNAS in 2006 (Wittlieb et al., 2006), 20 years after Doug Fisher embarked on the cloning of the actin gene. I had the pleasure of writing a commentary that accompanied the publication of the paper (Steele, 2006).

When Doug examined the base composition of his actin gene, it became apparent that Hydra DNA was unusually A+T-rich. This was consistent with the 71% A+T base composition of Hydra DNA determined from unpublished buoyant density measurements made by Joe Gall. The exceptional A+T-richness of the Hydra genome had multiple consequences, it makes PCR amplification of non-coding DNA (e.g. promoter regions) more difficult, it caused significant loss of clones when phage genomic libraries from Hydra were amplified, and it made the production of fosmid and BAC libraries from Hydra virtually impossible. It also meant that we had to take into account codon bias when making heterologous transgenes (e.g. fluorescent protein genes) for expression in Hydra.

REFERENCES

Bosch, T.C., Unger, T.F., Fisher, D.A., and Steele, R.E. (1989). Structure and expression of STK, a src-related gene in the simple metazoan Hydra attenuata. Mol. Cell. Biol. 9, 4141-4151.

Fisher, D.A., and Bode, H.R. (1989). Nucleotide sequence of an actin-encoding gene from Hydra attenuata: structural characteristics and evolutionary implications. Gene 84, 55-64.

Steele, R.E. (2006). Trembley’s polyps go transgenic. Proc. Natl. Acad. Sci. U. S. A. 103, 6415-6416.

Wittlieb, J., Khalturin, K., Lohmann, J.U., Anton-Erxleben, F., and Bosch, T.C.G. (2006). Transgenic Hydraallow in vivo tracking of individual stem cells during morphogenesis. Proc. Natl. Acad. Sci. U. S. A. 103, 6208-6211.