3. The cellular slime mold Dictyostelium discoideum is a eukaryote that lives on the forest floor as independent motile cells, called amoebae, which feed on bacterial and yeast. When their food supply is exhausted the amoebae stop dividing and gather together to form tiny, multicellular structures that crawl about as slugs. How do individual amoebae know when to stop dividing and how do they find their way to a common aggregate. A set of classic experiments investigated this phenomenon more than half a century ago.
Amoebae aggregate when placed on a glass cover-slip under water, provided that simple salts are present. The center of the aggregate pattern can be removed with a pipette and placed in a field of fresh amoebae, which immediately start streaming toward it. So, it seems that the center is emitting some sort of attractive signal. Four experiments were designed to determine the nature of the signal. In each an existing center of aggregation was used as the source of the signal and previously unexposed amoebae served as the target cells.
In one experiment the actively signaling aggregation center was placed in the top, center of a glass slide and a layer of amoeba on the bottom of the slide. Over time a second actively signaling aggregation center randomly formed on the bottom of the slide.
In the second experiment the actively signaling aggregation center was placed in the top, edge of a glass slide and a layer of amoeba on the bottom of the slide. Over time the lower amoeba streamed around the edge.
In a third experiment the actively signaling aggregation center was placed in the top, center of a semipermeable membrane and a layer of amoeba on the bottom of the slide. Over time a second actively signaling aggregation center formed exactly the top center on the bottom of the slide
In a forth experiment the actively signaling aggregation center was placed in the in the center of a field of amoeba on the top of a slide and a gentle stream of medium was allowed to flow across the slide from left to right. Over time the amoeba downstream of center streamed to join it but those upstream ignored the center.
Do these results show that Dictyostelium discoideum aggregates through the action of a secreted chemical? Explain.
