Biologists have re-shaped the body design of snails. Exposure to platinum results in the formation of an internal shell instead of the normal external shell. During embryogenesis of the freshwater snail, Marisa cornuarietis, a time slot of just one or two days determines whether the animals form an outer shell or not. Reprogramming of the direction of growth of the molluscs´ shell-generating tissue during this sensitive phase prevents the development of the usual convoluted shell. In lieu thereof a small hollow cone grows inside the body -- similar to whats happens in squids.
This new program has implications also for other organs: the gill is not located in a mantle cavity over the head, as usual, but rather no mantle cavity is formed at all and the gill remains at the posterior end of the visceral sac and floats freely in the water. These results which recently have been published in Evolution & Development by the group of Prof. Heinz Köhler and Prof. Rita Triebskorn from the Institute for Evolution and Ecology of Tübingen University, Germany, support the theory of 'macromutation'-based radical developmental shifts caused by modifications of signal transduction pathways which may have led to sudden body plan alterations during evolution.
Initially, the phenomenon of snail-slug conversion was discovered by Raphaela Osterauer, PhD candidate supervised by Heinz Köhler, while studying the toxicity of metal ions. Some years ago, the group had established a comparatively sensitive embryo toxicity test on the basis of developing Marisa eggs. When testing the noble metal platinum, which is released into the environment by abrasion from automobile catalytic converters, she found the embryos lacking a shell when being exposed to high concentrations of bivalent platinum ions. Further experiments revealed crucial relevance of exactly that small time slot in which the direction of growth of the shell gland´s tissue is defined. During one or two days only, this tissue is either programmed to evaginate and to form a shell-secreting mantle covering the dorsal part of the snail´s body or, alternatively in the presence of platinum, to invaginate into the gastropod´s body.
It is thus possible to spatially re-direct the shell-forming tissue with all irreversible consequences for the formation of the mantle and the shell and the position of the gill simply by a temporary exposure to platinum ions. After removal of platinum, the artificial slug embryos proceed with their development according to the new developmental program, hatch from their eggs, feed as usual, and do not change their new body plan anymore. They reach an age of more than half a year. During his time, an internal calcareous shell in the shape of a slightly bend, hollow cone grows inside of the body of these animals which remains after their death. Since also extant slugs, sea-slugs, and cephalopods bear small internal shells, the artificial Marisa slugs may serve as a model organisms to investigate the evolution of shell internalization. In recent studies, the Köhler and Triebskorn group was able to raise artificial slugs also in two only distantly related lung snail species.
The molluscs are not genetically modified by platinum -- they are no mutants. However, the scientists expect gene activity regulation to be modified. Similar, mutation-based modifications of gene regulation may have contributed to the evolution of different mollusc body plans and, consequently, the Tübingen research group now plans to concentrate on the analysis of platinum-sensitive gene activity during the early embryogenesis of gastropods.
The biologists assume that the artificial slugs do not occur in the field, since platinum pollution in the environment does not yet reach the concentrations necessary for snail-slug conversion.
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