ENCYSTMENT STAGES OF DICTYOSTELIUM

¹ Department of Bacteriology, University of Wisconsin, Madison, Wisconsin

1. Two encystment stages of cellular slime molds belonging to the genus Dictyostelium are described:

The first of these, termed microcysts, are unicellular and represent a transient resting stage in the vegetative phase of these simple slime molds. If returned to a favorable environment, microcysts germinated by excystment to re-initiate vegetative growth.

The second encystment stage, termed macrocysts, are multicellular and arise through a morphogenetic process possibly alternative to normal sorocarp formation. Myxamoebae aggregate to form typical but generally diminutive pseudoplasmodia which, instead of forming normal sorocarps, subdivide into rounded cell masses that become encased in relatively heavy cellulose walls. Concurrent with this development, the myxamoebae that comprise the nascent macrocyst undergo limited differentiation and appear as polyhedral cells with highly refractive membranes. After a period of 10 to 14 days these so-called endocytes generally disappear whereupon the content of the macrocyst assumes an acellular, homogeneous appearance. With further aging the protoplasmic content shrinks away from the heavy cellulose wall and in this contracted stage retains its viability for protracted periods. Under favorable conditions of temperature and substrates, macrocysts of different ages germinate to release amoeboid cells which re-initiate the vegetative stage. The sequence of cytological changes underlying this behavior has not been elucidated, and this propagative function may or may not represent the full measure of their significance in the life-cycle of those slime molds which produce them.

2. The ability to produce microcysts is apparently inherent in all members of the Acrasieae, including the genus Dictyostelium. In contrast, the capacity to produce macrocysts is more restricted, having been observed only in occasional isolates of D. mucoroides and in many strains of D. minutum. Various environmental factors influence their production, and from one strain of D. mucoroides temperature-dependent "cyst-forming" and "spore-forming" clones have been isolated.