THE ACRASIEAE IN NATURE. I. ISOLATION

The clonal isolation of Acrasieae from suspensions of soils or organic substrates may be accomplished by employing a dilute hay-infusion agar medium to the surface of which pregrown bacteria have been added as a source of nutrient for the myxamoebae. The method described permits quantitative sampling of cellular slime mold populations, and it has been used by the authors in a study of the occurrence and distribution of these organisms in North and Central American forests.     

Induction of fruiting in two aggregateless mutants of Dictyostelium discoideum

Five general groups of morphogenetically aberrant mutants of Dictyostelium discoideum were isolated. Each group of mutants was characterized either by the absence of any fruiting structures or by the formation of abnormal fructifications. Among these developmental mutants were two aggregateless isolates, Agg⁻-1 and Agg⁻-2, that could be induced to form normal sorocarps under certain conditions. Sorocarps of the normal D. discoideum type were formed when growing myxamoebae from either of these mutants were allowed to come in contact with myxamoebae of the other mutants, wild-type D. discoideum, D. purpureum, or D. mucoroides. No sorocarps were formed when myxamoebae of Agg⁻-1 and Agg⁻-2 were paired. These two aggregateless mutants, while incapable of aggregating or fruiting when cultivated singly with Escherichia coli B/r on a glucose-salts medium, formed normal fruiting structures after being freed of what appeared to be a product of bacterial growth. The spores produced by Agg⁻-1 and Agg⁻-2 myxamoebae again gave rise to aggregateless clones of the original parental types.     

MACROCYSTS IN THE LIFE CYCLE OF THE DICTYOSTELIACEAE. II. GERMINATION OF THE MACROCYSTS

Macrocyst germination was demonstrated in the five species of the Dictyosteliaceae known to produce these structures. The morphological changes that occurred during germination appeared to be identical in all of the strains examined, showing the following stages: (1) swelling of the dark, contracted content of the dormant cysts, (2) gradual loss of color and reappearance of cells within what previously appeared as a homogeneous protoplasmic mass, and (3) rupture of the heavy cellulosic cyst wall to liberate the myxamoebae. The age of the macrocyst appeared to be the most critical factor in determining whether or not germination would occur, since the cysts in many of the strains needed to age for several weeks or months before germination could be demonstrated. In Dictyostelium mucoroides strain DM-7, upon which the current study was centered, light was necessary to stimulate germination of young macrocysts—a requirement that gradually diminished as the cysts aged. The rate of germination and the temperature permitting germination were also age dependent: older macrocysts germinated more rapidly and at considerably higher temperatures than did young cysts. Although light was not essential for germination in every strain, the results obtained with strain DM-7 seem to be generally applicable to the germination process.