Microcysts: The Third Developmental Pathway of Social Amoebozoans

Abstract

Microcyst formation and germination, the least well studied of the three alternative developmental pathways open to the social amoebozoans, is also the simplest since it involves single cells. While classical studies profiled the morphological and biochemical changes that characterize microcyst differentiation, recent studies have focused on the molecular and evolutionary aspects of this system. Microcyst differentiation (encystment) is primarily triggered by high osmolarity and to a lesser extent by starvation. The transformation of an amoeba into a microcyst involves the formation of a cellulose cell wall as well as specific enzymatic and biochemical changes leading to dormancy. The spherical microcyst remains dormant as long as high osmotic conditions persist. When the osmolarity decreases, germination begins during which the cell wall is digested as the microcyst swells followed by the emergence of an amoeba. This rapid event typically takes 3-6 hours. The relationship between encystment and sporulation, which has evolutionary significance, is also reviewed with particular emphasis on the role and regulation of actin tyrosine phosphorylation. In light of the recent sequencing of the genomes of microcyst-forming cellular slime mold species, which will generate more research on microcysts, this critical review covers past and recent research to set the stage for future work on this alternative developmental pathway.