Evolution of development in the cellular slime molds.

, the amoebae first feed as separateisolated cells that engulf bacteria in the soil. Once they havecleared an area of food, they aggregate to form a multicellu-lar cell mass that migrates in the form of a slug to a suitablespot and then differentiates into a small fruiting body con-sisting of a delicate cellulose stalk that encloses vacuolatedead amoebae and an apical spore mass in which eachamoeba has become encapsulated in a resistant cellulosespore case (Fig. 1).GENERAL SCHEME FOR THE EVOLUTIONOF THE CELLULAR SLIME MOLDSHow might the successive stages in the life cycle of D. dis-coideum have evolved? It has been estimated that cellularslime molds must have come into being something in the or-der of a billion years ago, and during this vast span of timehave evolved into the complex forms we find today. First, Icompare the various stages of the D. discoideum life cyclewith other forms that exist today, whose mature state corre-sponds to one of the earlier stages of the development of D.discoideum. This is a component of my larger argument thateach stage of D. discoideum development is adaptive. Thosesimpler forms would not exist today if they were not compet-ing successfully in their natural environment. Finally, I dis-cuss the possible evolutionary sequence of the steps in thedevelopment of D. discoideum.The basic assumption is that every aspect of slime molddevelopment is governed by the advantage of dispersing itsspores effectively. They feed primarily on bacteria that willbe found in a patchy distribution in the soil where a decayingbit of vegetable matter, or dung, or a small dead invertebratemight be found. Once the food is depleted they have to reachanother spot rich in bacteria to continue to propagate. Themeans of spreading spores or cysts are limited: It could be byflooding in the rain, or it could be by sticking to or beingeaten by some passing soil invertebrate. There is no evidencethat cellular slime molds are dispersed by the wind. One ofthe especially interesting phenomenon is the elaborate physio-logical mechanisms that have evolved to guide the cell massesfrom their feeding site to the best spot for spore dispersal.SOLITARY CYSTS IN THE SOILNumerous species of soil amoebae form cysts as individualamoebae and are spread about throughout the ground. It isworthy of note that some higher slime molds not only havecompound fruiting bodies, but also, under some conditions,will produce microcysts where again individual amoebae en-capsulate as separate isolated cells. In other words, this kindof nonsocial spore formation must be, under some circum-stances, a viable method of dispersal. We do know that an in-crease in the concentration of ammonia will induce micro-cyst formation in Polysphondylium pallidum (Lonski 1976);perhaps, as been suggested, it is a means of quickly forminga resistant stage if adverse conditions are descending rapidly(for a review, see Raper 1984, pp. 101–103).AGGREGATION OF CYSTSThere is a species of Hartmanella that shows an aggregationof cysts (Ray and Hay es 1954). The formation of these cystshas not been studied, but one might imagine that these clus-ters form by amoebae becoming adhesive as part of the pro-cess of encystment and the random motion of the starved andsticky amoebae produce the accumulation of cysts in onespot.This cyst cluster might be an aid to dispersal under specialcircumstances. No evidence has been gathered to addressthis point, but one could make some wild speculations sim-ply to show that there is a possibility that such an aggrega-tion provides an advantage for dispersal. For instance, it isknown that nematodes are voracious predators of soil amoe-bae (Kessin et al. 1996). They are also known for their abilityto sense chemical gradients in the soil (Dusenbery 1983).Could it be that the amoebae aggregate to concentrate a