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Aggregation, Segregation and Graded Discrimination among Social Amoeba

Amoeba are Eukaryotic micro-organisms. They are alive but they are neither animals nor plants. They are not fungi either, but they are mould-like. They are single-celled but it is a complex cell and some amoeba - social amoeba - aggregate with one another to form multi-cellular bodies consisting of a stalk and reproductive spores. Some amoeba "altruistically" sacrifice their lives to form the stalk; others exploit that "altruism" to disperse their spores, from which other amoeba are asexually produced.

Dictyostelium discoideum is a type of social amoeba. When single cells become starved they merge with others into the aforementioned multi-cellular bodies. These bodies can also be multi-genotypic (or chimeric), which means that they are formed by members of the species with significant genetic differences, but it has been observed that this variety of social amoeba discriminates socially in accordance with degree of genetic relatedness.

Amoeba are unwilling to form the sacrificial stalk unless it provides the base for the spores of others who are genetically near-identical, although they are unable to distinguish close kin from distant relatives until they have aggregated, which means that they seek to segregate once the chimeric body is forming, and may try to "cheat" by exploiting stalk-forming "altruist" outlanders. The less identical the others are, they more they seek to segregate, possibly because cells of different strains have adapted to different environments and therefore struggle to adhere to each other. Their discrimination, though dose-responsive, is nevertheless imperfect, because chimeric aggregates always contain spore-dispersing representatives of both contributing strains, even if in grossly uneven quantities.

Discrimination is also observed in bacteria and fungi, but their discrimination is not graded; they either accept or reject.

The research cited above was carried out by an aggregate of contributors from Rice University and Baylor College, but the latter contributed only 30% of the authors.