Regulation of size and pattern in the cellular slime molds

Abstract

The multicellular stages of cellular slime mold development exhibit important characteristics of embryonic development on a very simple scale. Only two cell types, i.e., stalk cells and spores, differentiate from an originally homogeneous population of amebae. The proportion of these cell types is regulated, and the cells become arranged in a simple pattern. Interactions between the cells lead to coordinated cell movement that allows the cellular agglomerate to assume a variety of specific shapes. A small group of cells, referred to as the tip, determines the direction of morphogenetic movement and the polarity of the pattern of the two cell types. The tip functions as the classical organizer, a feature which is generally thought to act in embryonic development and tissue regeneration. Over the past 20 years, both stage-specific and cell-type-specific gene expression have been extensively studied in the cellular slime mold species, Dictyostelium discoideum. Morphogenetic factors have been recognized and characterized, and a number of regulatory mechanisms that may control cell differentiation and pattern formation have been proposed. In the present study, most of these data are summarized and are compared with the known developmental characteristics of other cellular slime mold species in order to achieve a conceptual framework from which the formation of shape and pattern in the cellular slime molds can be understood. The available data indicate that oscillatory cAMP signaling is the main regulatory mechanism of cellular slime mold development. cAMP signals induce stage- and cell-type-specific gene expression and control the generation of shape by coordinating the movement of individual cells. Feedback between cAMP signals and the cAMP-hydrolysis product, adenosine, determines the size of the organism and stabilizes the prestalk/prespore pattern. Additional regulatory functions as well as the induction of terminal spore- and stalk-cell differentiation are probably controlled by ammonia and the stalk-inducing factor, DIF, or other factors which have yet to be characterized.