THE HYDRA PHYLOGENY


THE HYDRA LIBRARY


HYDRA ESTS


WINGED_HELIX PROTEINS


POMONA COLLEGE BIOLOGY DEPARTMENT

 

 

 

 

 

 

 

 

 

 

Daniel E. Martínez
Assistant Professor of Biology
dmartinez@pomona.edu

 

Comparison of medusa and polyp morphology in Podocoryna carnea

In the basal metazoan phylum Cnidaria, the life cycle of many species includes both a sessile polyp and a swimming medusa stage. These life cycle stages show striking differences in the position of structures along the oral-aboral body axis and in overall body proportions. It is of considerable interest to determine which developmental features are common to both cnidarian life cycle stages, since such features are particularly relevant to early metazoan evolution. In addition, comparison of the development of the two stages should provide insight into how major differences in body organization arose within the phylum.

The polyp stage consists of an elongated tube with a mouth and an adjacent ring of tentacles at one end. The medusa consists of a hemispherical bell, with tentacles around the bell rim and the mouth on the underside of the bell, far from the tentacle-bearing region. In many species, both polyps and medusae can arise through an asexual budding process with similar initial stages. Formation of polyps through budding has been well characterized in members of the genus Hydra, which lack a medusa stage. We have worked with the species Podocoryna carnea to test hypotheses about parallels and differences between polyp and medusa formation.

To address the question of whether the mouth and tentacle zone are specified using some of the same molecular mechanisms in the polyp and the medusa, we examined expression in medusae of genes whose homologues have been implicated in the formation of the polyp mouth and tentacle zone. Two such genes are orthologues of mouse FoxA2 and Drosophila empty spiracles (ems). Mouse FoxA2 and the Xenopus orthologue pintallavis are expressed in the organizer regions of embryos and in tissue derived from them. In polyps of the cnidarian genus Hydra, issue adjacent to the mouth shares with vertebrate organizer regions the ability to induce formation of a second body axis. The Hydra vulgaris FoxA2 orthologue budhead is expressed in endoderm of the prospective mouth and tentacle region beginning early in budding. Ems is expressed during the formation of head structures in Drosophila. Its mouse homologues are involved in brain development. In polyps of the cnidarian Hydractinia symbiolongicarpus, ems orthologue Cn-ems is expressed in the endoderm of the mouth region.

We have isolated the full-length coding region of a P. carnea budhead orthologue and a P. carnea ems orthologue. We have characterized the expression of both genes in polyps and medusae using whole mount RNA in situ hybridization.

While a mouth must be specified at one end in both polyp and medusa, the medusa does not possess a morphologically well-defined aboral end as does the polyp. To address the question of whether the oral-aboral axis of the medusa has been truncated relative to that of the polyp by elimination of molecular processes specifying aboral tissue identity, we are examining expression of a P. carnea homologue of the NK-2 gene Hydra vulgaris CnNK-2. In H. vulgaris, CnNK-2 expression is correlated with aboral tissue identity in both normal animals and those in which tissue fate has been experimentally altered. We have isolated the full-length coding region of a P. carnea CnNK-2 orthologue and are in the process of characterizing expression in polyps and medusae.

This work was done in collaboration with Dr. Diane Bridge (Elizabethtown College, PA) with the suport of NSF Grants to both Diane Bridge and Daniel Martínez. During a period of four years the Bridge and the Martínez labs merged for ten weeks every summer at Pomona College. Students and PIs benefited tremendously by the opportunity to interact directly for a significant period of time.  As a consequence, student training was greatly enhanced. Students also experienced working in a relatively large research group, but one unusually focused on training undergraduates. A total of eleven undergraduate students (five from Pomona College and six from Elizabethtown College) participated in this project since the beginning of the NSF funding period.

Some of the results of this project have been already published (Bridge et al. 2004). Additonal results will be included in a manuscript currently in preparation.

References

Bridge D. M., C. T. Ha*, A. Nemir*, A. Renden*, M. M. Rorick*, A. Shaffer*, D. M. Underkoffler*, A. E. Wills*, and D. E. Martínez, 2004.Variations on a theme? Polyp and medusa development in Podocoryna carnea.  Hydrobiologia, 530-531: 299-307

* Undergraduate student