Comparison of metamorphic induction in hydroids

Studies of hydroid metamorphosis have been primarily limited to 3 species: the athecate Hydractinia echinata and 2 thecates, Phialidium gregarium and Mitrocomella polydiademata. To determine if findings from studies on those species can be applied generally, the effects of various agents implicated in hydroid metamorphosis were examined in 2 additional athecates, Halocordyle disticha and Hydractinia symbiolongicarpus. As in all species studied to date, H. disticha and H. symbiolongicarpus can be induced with activators of protein kinase C (PKC) and cannot be induced by the Ca2+ ionophore A23187 (which was actually inhibitory in H. symbiolongicarpus). Unlike H. echinata and H. symbiolongicarpus, H. disticha can be induced by the permeabilizing agents saponin and dimethylsulfoxide. In addition, H. disticha and H. echinata are induced by ammonium chloride, but H. symbiolongicarpus is not. Whereas the thecates respond to PKC-activating phorbol esters in the absence of exogenous Ca2+ and in the presence of Ca2+ channel blockers, H. symbiolongicarpus does not respond to the PKC activator dioctanoylglycerol under these conditions. Further, planulae of thecates induced with CsCl in the presence of Ca2+ channel blockers metamorphose upon release from treatment, whereas those of H. symbiolongicarpus do not. These studies support a central role for PKC in control of hydroid metamorphosis, but suggest that interspecific differences do exist. Additional key words: metamorphosis, diacylglycerol, calcium, Hydractinia, Halocordyle, Hydrozoa The hydrozoan Hydractinia echinata provides an excellent system for examining cellular signaling involved in metamorphosis and has been studied extensively (reviewed by Thomas & Edwards, in press). The motile planula larva, given the proper stimulus, attaches to the substrate and undergoes transformation into a sessile polyp. In 1969, Muller discovered that the natural inducer for metamorphosis in H. echinata is a bacterial product. That product was later shown to be lipophilic (Muller 1973; Wittmann 1977; Leitz & Wagner 1993). The bacterium Alteromonas espejiana produces such an inducing agent (Leitz & Wagner 1993). a Author for correspondence. E-mail: mbthomas @ email.uncc.edu b Present address: Department of Biology, Pennsylvania State University, 208 Mueller Lab, University Park, PA 16802, USA In 1989, Buss and Yund determined that hydroids from North America are not conspecific with the European species Hydractinia echinata; thus, despite the title of the article by Edwards et al. (1987), the species used was H. symbiolongicarpus. A variety of chemicals can cause hydroids to metamorphose. Muller & Buchal (1973) found that a number of monovalent cations could substitute for the natural inducer in H. echinata, and that Cs+ was the most effective of these. Subsequently, Cs+ was shown to induce metamorphosis in several other species, including Phialidium gregarium (Freeman 1981), Halocordyle disticha (Edwards et al. 1987), Mitrocomella polydiademata, and Eutonina indicans (Freeman & Ridgway 1987). Phorbol esters and diacylglycerol (DAG), which in other systems activate protein kinase C (PKC), trigger metamorphosis in H. echinata (Muller 1985; Leitz & Muller 1987; Leitz & Klingmann 1990). Freeman & Ridgway (1990) showed that phorbol esters are effective inducers in M. polydiademata. These results suggest some commonality among hydroid species in the inductive process. Studies by Edwards et al. (1987), however, demonstrated interspecific differences. Planulae of H. disticha are induced to metamorphose by exposure to certain catecholamines, while these agents have no inductive effect on planulae of H. symbiolongicarpus' (Edwards et al. 1987), P. gregarium, or M. polydiademata (Freeman & Ridgway 1990). Later studies suggested that the sigThis content downloaded from 157.55.39.159 on Sun, 18 Sep 2016 06:12:55 UTC All use subject to http://about.jstor.org/terms Thomas, Edwards, Ball, & McCauley nal is mediated in H. disticha by a receptor similar to the mammalian D-1 receptor (Thomas et al. 1989). The purpose of this study is twofold. First, because a difference exists among athecate hydroids in response to catecholamines, we tested agents that trigger metamorphosis in the well-studied H. echinata on both H. symbiolongicarpus and H. disticha to look for other possible interspecific (H. echinata vs. H. symbiolongicarpus) and intergeneric (Hydractinia vs. Halocordyle) differences within the athecates. Second, because requirements for Ca2+ during induction have been examined only in thecate species (Freeman & Ridgway 1987, 1990), we also studied Ca2+ requirements in an athecate, H. symbiolongicarpus. We examined the results in light of 3 signal transduction mechanisms that have been proposed for hydroids: the phosphatidylinositol phosphate system, Ca2+-facilitation, and transmethylation.