Conservation of the retinal determination gene cascade in the jellyfish "Cladonema radiatum"

The species Cladonema radiatum belongs to the Cnidaria, a basal animal phylum which represents the closest sister group to bilateria. Despite their low position in the metazoan phylogenetic tree, Cnidaria are the only non-bilaterian animals with a defined body axis, a nervous system, sensory organs of great complexity such as photoreceptors and statocysts, and a remarkable regeneration capacity. Therefore cnidarians, for their basal position and with their surprising level of complexity, have become in the last years the organism of choice for evolutionary developmental studies, representing the appropriate outgroup necessary to understand the ancestral bilaterian condition. The jellyfish of C. radiatum bears eight to twelve lens eyes at the bell margin, on the tentacle bulb. Each eye displays a cornea, a lens, pigmented cells and a retina. This species appears therefore suitable for studying the conservation of an important gene network, the Retinal Determination Gene Cascade (RDGC), that has been demonstrated to be responsible for the eye development in species as diverse as Drosophila and mice. This network is made up of four gene families: Pax, Eya, Six and Dac. The full length sequence of a Pax gene from C. radiatum (CrPaxA) was already known. During my Ph.D. studies, I was able to isolate, by means of degenerate PCR, two more members of the Pax family (CrPaxB and CrPaxE) and one member of the Eya family (CrEya), described for the first time in Cnidaria. I then characterized the expression patterns of these genes by in situ hybridization, and analyzed by Real Time PCR their expression in the different tissues during the development of the jellyfish and at the different stages of the life cycle. CrPaxA is expressed in the retina and in nematocytes precursor cells in the tentacle bulb, whereas both CrPaxB and CrPaxE are expressed in the manubrium, the feeding and reproductive organ of the jellyfish where the gonads develop. In particular it was possible to detect the signal for CrPaxB in the maturing oocytes. CrEya is expressed at the same time in the retina and in the manubrium where it shows the same pattern at the level of the oocytes as CrPaxB. Taking advantage of the capability of the jellyfish to regenerate the eye once it has been micro-surgically removed, I was able to investigate the involvement of these genes in the development of the eye. Surprisingly none of them seems to be clearly up-regulated during the eye regeneration. This could indicate that CrPaxA and CrEya are involved in the
maintenance of the adult eye.
To gain further insights on the role of the isolated genes in the eye determination we used
targeted gene expression in Drosophila. Taking advantage of the UAS/GAL4 system, we misexpressed
the jellyfish genes in the imaginal discs of the fly and analysed the adults for
ectopic eyes induction. At the same time we examined the capability of these genes to rescue
Drosophila mutant phenotypes. Indeed UAS-CrPaxA was able to induce ectopic eyes, and
both UAS-CrPaxA and UAS-CrPaxB were able to rescue the Drosophila Pax2 mutant
sparkling.
The expression of CrPaxA and CrEya in the retina taken together with the functional assays
carried out in Drosophila argue for a conserved role of this gene network in the jellyfish eye.
This result is also supported by data from a previous report, showing the expression of two
members of the Six genes family in the eye of Cladonema. These results overall indicate a
high structural conservation of the members of the RDGC between Cnidaria and Bilateria,
and are in agreement with the theory of the monophyletic origin of the eye. The evidence for
conservation is further strengthened by the expression of CrPaxB, CrEya and a third Six gene
CrSix4/5 in the oocytes, suggesting a possible preservation of the interactions among the
members of the network and its redeployment to a different context. Changes in the temporal
and spatial pattern of genes expression are one of the main mechanisms by which the
phenotypic diversity arises, the redeployment of the RDGC in Cladonema radiatum might
offer an example of this process.