One of the most exciting recent discoveries in ecology has been the realization that a wide variety of organisms change their developmental pathways and behaviors based on chemical cues (i.e. kairomones) released by potential predators. Studies by various groups of the aquatic crustacean Daphnia, a freshwater waterflea, have provided many of the most important and startling results. Based on these findings the taxonomy of this group has been revised, new perspectives on the evolution of life histories have opened, and questions on seasonal changes in morphology that have been baffled researchers for over 100 years have been answered. Although ecologists have pursued a variety of ecological and evolutionary questions related to this topic, progress has been slowed by the lack of interdisciplinary efforts to identify the chemical nature of the messengers. There has also been a lack of developmental analyses, especially at the molecular level of the organism. The goals of this project are to isolate and characterize the chemical messenger(s) that induce morphological changes in Daphnia, and to determine the molecular basis for the induced developmental pathways in this important aquatic prey.
Two main groups of kairomones have been reported to affect Daphnia. A kairomone released by larvae of the phantom midge fly Chaoborus is reported to be a low-molecular-weight (<500 Dalton), water-soluble non-protein molecule that is reasonably heat-stable. Its chemical structure apparently includes hydroxyl and carboxyl groups, which are necessary for activity. A possibly different kairomone released by fish is thought to be a non-olefinic anion of intermediate lipophilicity, also with a low molecular weight (<500 Dalton). Aside from these aspects, the chemical identity of both groups of chemical messengers has eluded researchers to date. It has also been proposed that the ultimate source of the kairomone(s) is the daphnia themselves, and that the active compound(s) is a result of digestion and perhaps bacterial action on Daphnia that have been ingested by the predators.
Students in my lab have worked over the past four summers to concentrate enough of the active material for chemical analysis. Initially we had hoped to derivatize the material and use the department’s GC/MS instrument, but quantities of active kairomone concentrated either from water in which Chaoborus larvae were kept or from hot-water extraction of whole Chaoborus failed to produce useful information. In the last year we have obtained access to a new LC-MS instrument at the University of Wisconsin – Oshkosh, and we are now turning our attention to that technique.
The collaborators in the biology department have done the bioassays necessary to guide our concentration efforts, and are also working to on the developmental changes in the Daphnia in the presence of the kairomone. Inducible clones of Daphnia develop classic “neckteeth” in response to the kairomone, a transformation of polyploidy cells located in the cephalic region of the head resulting in small, tooth-like projections. The molecular basis of this morphological induction is being explored by students working with Nancy Wall and Beth De Stasio. It is our hope that this combination of chemical, ecological and molecular approaches will permit a significant extension of our understanding of this important ecological and evolutionary phenomenon.
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