Why Some Cells Die So Others Can Live

Episode summary

When starvation strikes, hundreds of thousands of solitary cellular-slime-mold cells ("social amoebae") assemble into a temporary multicellular body. Roughly one-quarter sacrifice reproduction to construct a dead stalk, allowing the remaining cells to become spores and disperse. That extraordinary cooperation creates an equally extraordinary opportunity for cheating.

This episode follows the problem from Leo Buss’s 1982 description of a naturally occurring stalkless parasite, through John Maynard Smith and Eörs Szathmáry’s suggestion of the importance of kin selection and the need for a genetic study of  natural populations, to Gilbert and colleagues’ 2007 field and laboratory tests. The central finding is that wild Dictyostelium fruiting bodies are overwhelmingly clonal. Their exceptionally high relatedness prevents obligate cheaters such as the fbxA− mutant from finding unrelated cooperators to exploit.

The episode closes by asking whether human laws, institutions, and cultural norms are necessary because cooperation among unrelated individuals would otherwise be evolutionarily unstable.

Topics covered

• The starvation-induced social cycle of Dictyostelium
• Pulsed cAMP signaling and cellular aggregation
• Division of labor between stalk and spore cells
• Why aggregative multicellularity creates opportunities for cheating
• Leo Buss’s stalkless mutant and somatic cell parasitism
• Kin selection as a defense against cheaters
• Deer-dung sampling at Mountain Lake Biological Station
• Microsatellite estimates of relatedness within wild fruiting bodies
• The biochemical and social effects of the fbxA− mutation
• The tragedy of the commons and the evolution of multicellularity
• Human institutions as possible defenses against low-relatedness cheating

Key timestamps

00:11 — The escape-tower thought experiment
02:28 — The evolutionary problem of cooperation and cheating
04:45 — Prokaryotes, eukaryotes, and convergent social behavior
07:03 — Pulsed cAMP signaling and aggregation
09:16 — Slug migration, culmination, and cellular sacrifice
11:35 — Why fruiting bodies provide an evolutionary advantage
13:53 — Aggregative multicellularity and genetic conflict
16:11 — Buss’s stalkless mutant as a somatic parasite
18:27 — Maynard Smith’s kin-selection solution and need for testing
20:53 — Testing clonal structure in the wild
23:19 — Microsatellite genotyping and relatedness estimates
25:39 — Why near-clonality blocks cheating
28:02 — How the fbxA− mutant cheats
30:20 — Experimental collapse and the relatedness threshold
32:41 — Testing the mutant on deer dung and in wild samples
34:48 — Cooperation among unrelated humans

Works discussed

• Leo W. Buss’s 1982 research on somatic cell parasitism in cellular slime molds
• John Maynard Smith and Eörs Szathmáry’s 1997 book, The Major Transitions in Evolution
• Gilbert and colleagues’ 2007 field and experimental research on clonal structure, relatedness, and cheating in Dictyostelium