Developmental genetics of hydra 
My main research interest is the origin and evolution of metazoan development. Evidence is rapidly accumulating regarding the conservation of function of genes involved in developmental processes between protostomes and deuterostomes. This suggests that the function of these genes arose before these two clades diverged. Thus, in order to understand how early a regulatory gene or a developmental mechanism arose, it will be necessary to investigate animals that diverged before the protostome-deuterostome split.
Studies on Cnidaria, one of the earliest diverging metazoan phyla, are providing important information in this regard. For example, work at the laboratory of Hans Bode (UC Irvine) demonstrates the structural and functional conservation of genes involved in the specification of cell fate in cells in hydra and drosophila. In hydra, CnASH, a hydra relative of the Drosophila achaete-scute class of basic-helix-loop-helix genes, is expressed in nematocyte precursor cells. Nematocytes are highly specialized cells involved in the capture of prey, defense, and locomotion. Nematocytes arise from multipotent stem cells of the interstitial cell lineage, which differentiate into neurons, secretory cells and gametes. In Drosophila, achaete-scute genes are required for the specification of cell precursors of sensory neurons and associated cells. Interestingly, CnASH can replace the Drosophila genes during specification of these neural cell fates. CnASH under the control of the Drosophila hsp70 promoter induces the formation of ectopic bristles and sensory organs as do the achaete and scute genes in the same situation. Furthermore, CnASH is as effective as the Drosophila genes in rescuing achaete and scute double mutant flies. These results suggest that CnASH and the achaete-scute genes have not changed much structurally or functionally since they last shared a common ancestor at the time of the split of cnidarians from the rest of the metazoans more than 600 million years ago. The results also suggest that the common ancestor of the acheate-scute genes might have been involved in the specification of cell fate.
The work on my laboratory focus on the study of transcription factors involved in developmental process in hydra. Presently, we (two students and I) are studying two genes of the forkhead family: budhead (bdh) and HyFkh2. I began the characterization of budhead while a postdoctoral fellow at Hans Bode's lab. The expression pattern in adult hydra suggests that budhead is involved in head formation and budding. A phylogenetic analysis indicates that budhead belongs to a subset of closely related forkhead genes that are associated with "organizer" regions of embryos.
The organizer is a region of an embryo capable of inducing the formation of a second embryonic axis when transplanted into an area which normally would have a different fate. The dorsal blastopore lip in frogs, Hensen's node in birds, and the node in mammals are well studied organizing regions in vertebrate embryos. Hydra tissue has "organizing" capacity: when a piece of tissue is transplanted to an appropriate site it can generate a secondary body axis and a head. This organizing property has been experimentally shown to be maximal in the hypostome (the apex of the head) and to decline down the body column. During head regeneration, the tissue of the regenerating tip reaches high levels of head organizing capacity a few hours after decapitation. Also, the tip of a developing bud exhibits high levels of head organizing capacity. Budhead is expressed in the area of maximum head organizing capacity, the hypostome, as well as in the apical tip during head regeneration, and in the tip of a developing bud, suggesting that the gene is associated with hydra's organizer.
Our phylogenetic analysis indicates that budhead is structurally related to members of the forkhead family that are expressed in organizing regions of embryos. The pattern of transcription of budhead suggests that the gene is associated with hydra's head organizer. It is possible that this correlation between the expression of a related set of genes and a related set of developmental processes is simply a coincidence. However, it is also quite plausible that this correlation indicates that the molecular circuitry underlying an "organizer" arose early in metazoan evolution, and has been substantially conserved ever since. If so, several components of the organizing pathway might have been also conserved. For example, the secreted protein Sonic Hedgehog can induce the expression of HNFbeta and axial in the floor plate of the neural tube of developing mouse and zebrafish. Plausibly, a sonic hedgehog homolog may exist in hydra and induce budhead. We plan to look for such a homolog by PCR of a cDNA library and genomic DNA.
My plan for the immediate future is to continue to study hydra genes during pattern formation, asexual development (budding) and regeneration, and to extend the work to embryos. Budhead is clearly involved in head formation during budding and regeneration. Is bdh also involved in head formation during embryogenesis? During hydra embryogenesis the head is formed by the cells located at the sperm entry point. Is bdh expressed in those cells? I plan to determine the temporal and spatial expression pattern of budhead in hydra embryos and compare it to the expression during budding. This comparison should tell us much about differences and similarities between embryonic and asexual development. It would be important to determine, for example, whether or not hydra embryos have an organizer. And if so, to compare it to the organizer of vertebrate embryos. Whether or not the "organizer" is a region (or a tissue property) that has been conserved throughout evolution cannot be answered with the information available today. However, our results are provocative and suggest that further analysis of budhead may provide important clues regarding the conservation of very important developmental processes.
Our knowledge of classical models of animal development has reached high levels of sophistication. It is time to expand the scope of our search to include other metazoan systems. My personal preference is to study animals with lower levels of complexity. Among them, hydra is arguably the developmental model system most useful for inferring presence of genes and developmental processes in a common metazoan ancestor.
Publications:
Bode H., D. E. Martínez, M. A. Shenk, K. Smith, R. Steele & U. Technau. Evolution of Head Development. Biological Bulletin, in press.
Martínez D. E., D. Bridge, L. Masuda-Nakagawa & P. Cartwright, 1998. Cnidarian homeoboxes and the zootype. Nature 393:748-749. [see http: //pages.pomona.edu/~dmartinez/zootype.htm]
Martínez D.E., M-L. Dirksen, P. M. Bode, M. Jamrich, R. E. Steele
&H. R. Bode, 1997. Budhead, a fork head/HNF-3 homolog, is expressed
during axis formation and head specification in hydra. Developmental
Biology 192: 523-536.