Cell-Cell Interactions
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CHAPTER 1.8 l l eC- l l eC
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Cell-Cell Interactions DALE KAISER
Discovery and Distribution Roland Thaxter published a time bomb in December 1892. He reported that Chondromyces crocatus, before then considered an imperfect fungus because of the morphological complexity of its fruiting body, was actually a bacterium. Thaxter had discovered the unicellular vegetative stage of C. crocatus; the cells he found were relatively short and they divided by binary fission, unlike the mycelium of a fungus. C. crocatus was, he concluded, a “communal bacterium.” Thaxter described the locomotion, swarming, aggregation and process of fruiting body formation of C. crocatus and its relatives, which are collectively called myxobacteria, with an accuracy that has survived 100 years of scrutiny. He described the behavior of myxobacteria in fructification in terms of a “course of development” because it was “a definitely recurring aggregation of individuals capable of concerted action toward a definite end” (Thaxter, 1892). These qualities of an integrated multicellular unit clearly implied positive interaction between the cells. Existence of favorable cell-cell interactions is one hallmark of multicellular life and the theme of this essay. Thaxter’s discovery called attention to the transition from single, apparently independent cells to an integrated multicellular unit. There is general agreement that this step has been taken many times in the course of the organic evolution of plants and animals. For example, the sponges are thought to have arisen from solitary cells separately from all other animals; moreover the seed plants, the fungi, and the algae are all believed to have gained their multicellular condition independently (Whittaker, 1969). Given the apparently strong natural selection for multicellular life, it would be surprising if prokaryotes also had not adopted the multicellular condition many times, and perhaps even earlier in time. One current estimate suggests that more than 99% of the bacteria on earth live as cell masses (Costerton et al., 1995), a condition conducive to cell interactions. As the broader significance of
multicellular microbial life has been recognized, the cell interactions that facilitate multicellularity have been revealed.
Evolution of Cell Interactions To imagine how two partners to a cell interaction could coordinately evolve high, yet complementary, specificity seems difficult. To start with a weak and poorly specific signal would seem to have little, if any, selective advantage. But what other way is there to start? The relative structural simplicity of bacteria belies their metabolic versatility and their sophistication in responding to the environment by their sensory systems. Given current knowledge of bacteria and given some advantage for an interaction, at least two evolutionary scenarios are plausible. 1) The progressive evolution of a cell-cell interaction between dissimilar cells could start with cells of one type feeding on the metabolic products of cells of another typ
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