Why Some Cells Die So Others Can Live

Episode summaryWhen 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 coveredThe starvation-induced social cycle of DictyosteliumPulsed cAMP signaling and cellular aggregationDivision of labor between stalk and spore cellsWhy aggregative multicellularity creates opportunities for cheatingLeo Buss’s stalkless mutant and somatic cell parasitismKin selection as a defense against cheatersDeer-dung sampling at Mountain Lake Biological StationMicrosatellite estimates of relatedness within wild fruiting bodiesThe biochemical and social effects of the fbxA− mutationThe tragedy of the commons and the evolution of multicellularityHuman institutions as possible defenses against low-relatedness cheatingKey timestamps00:11 — The escape-tower thought experiment02:28 — The evolutionary problem of cooperation and cheating04:45 — Prokaryotes, eukaryotes, and convergent social behavior07:03 — Pulsed cAMP signaling and aggregation09:16 — Slug migration, culmination, and cellular sacrifice11:35 — Why fruiting bodies provide an evolutionary advantage13:53 — Aggregative multicellularity and genetic conflict16:11 — Buss’s stalkless mutant as a somatic parasite18:27 — Maynard Smith’s kin-selection solution and need for testing20:53 — Testing clonal structure in the wild23:19 — Microsatellite genotyping and relatedness estimates25:39 — Why near-clonality blocks cheating28:02 — How the fbxA− mutant cheats30:20 — Experimental collapse and the relatedness threshold32:41 — Testing the mutant on deer dung and in wild samples34:48 — Cooperation among unrelated humansWorks discussedLeo W. Buss’s 1982 research on somatic cell parasitism in cellular slime moldsJohn Maynard Smith and Eörs Szathmáry’s 1997 book, The Major Transitions in EvolutionGilbert and colleagues’ 2007 field and experimental research on clonal structure, relatedness, and cheating in Dictyostelium