The mutualistic association between the squid Euprymna scolopes and the bacterium Vibrio fischeri is an emerging experimental system for the study of the influence of bacteria on animal development. Taking advantage of the ability

Studies of animal developmental processes have typically focused on defining the differing yet complementary roles of cell-cell interactions and intrinsic cell factors in processes of determination and differentiation. With few exceptions, it has been assumed that all the information needed to complete the formation of a complex multicellular organism is contained within the genome of that organism and the structure of its egg. However, the developmental programs of most, if not all animals, include specific interactions with benign, essential microorganisms during embryonic and/or postembryonic periods of morphogenesis (Hungate, 1966; Schwemmler and Gassner, 1989; Saffo, 1992). Technical difficulties, however, have precluded the analysis of the interactions between the partners that lead to the establishment and maintenance of proper stable associations. In many instances, such as the cow rumen and mammalian hindgut, the microbial complement exists as a consortium, where it is difficult to discern the influence on the host of any one microorganism independent of the others. Or, if the association does involve the more experimentally tractable situation of one host and one microbial partner, often the host and symbiont are not viable outside of the symbiosis and, thus, cannot be raised separately under laboratory conditions. While animal experimental systems have been lacking for the study of animal/bacterial interactions in development, our knowledge of the influence of bacteria on plant development is comparatively sophisticated. The legume/nitrogen-fixing bacteria symbiosis has been studied under laboratory conditions for over 100 years, providing a wealth of knowledge that has described the cellular, biochemical and molecular language between the plant and microbe that characterizes the development of a stable association. Presently dozens of genes in each partner have been identified as essential to a complex ‘conversation’ between these two disparate genomes that results in a series of developmental changes which lead to a mature association (Long, 1989; Nap and Bisseling, 1990; Hirsch, 1992). The descriptions of tissues of animals raised in microbe-free environments have revealed vastly different morphologies from the normal condition (Kenworthy, 1967; Bartizal et al., 1984; Pabst et al., 1988), suggesting that in animals we should expect a no less complex dialogue between the animal and microbial complements of an association. 1719 Development 120, 1719-1729 (1994) Printed in Great Britain © The Company of Biologists Limited 1994