Social behaviour in genetically heterogeneous groups of Dictyostelium giganteum
The Dictyostelid or cellular slime moulds (CSMs) are soil amoebae with an asexual life cycle involving social behaviour and division of labour. The most obvious distinction is between ‘germ line’ or pre-spore cells, which survive, and ‘somatic’ or pre-stalk cells, which eventually die. A plausible hypothesis to explain the apparent altruism of pre-stalk cells is that it is directed at clonal relatives. We have tested this hypothesis by comparing indices of altruistic behaviour between clonal and chimeric (genetically heterogeneous) social groups. The groups were generated by mixing amoebae belonging to distinguishable strains of Dictyostelium giganteum. The amoebae of one strain do not aggregate at all when mixed with any of three other strains and aggregate poorly with a fourth. Among the latter, co-aggregation occurs but is followed by varying extents of sorting out. At times, two strains form separate fruiting bodies; in other cases, they remain together but are clustered in clonal groups within a single chimeric structure. Our expectation was that the allocation of cells to the stalk pathway would be higher, and to the spore pathway lower, in clonal social groups than in chimeras. The expectation was not always fulfilled. In addition, three strains could be arrayed in a linear rank order in terms of the relative efficiencies of spore-formation in binary mixtures; but when all three were mixed, they were equally efficient. More than overall genetic similarity, cell fate in a chimera seems to result from complex non-linear interactions based on epigenetic differences.
KeywordsSocial behaviour Dictyostelium Kin selection Altruism
We are grateful to J.C. Cavender for help in identifying the strains, and thank J.T. Bonner, R. Gadagkar and K. Inouye for several helpful comments on an earlier version of the manuscript. The text has benefited substantially from the detailed criticisms of anonymous referees. B.K. acknowledges the award of a Senior Research Fellowship from the Council of Scientific and Industrial Research.
- Atzmony D, Zahavi A, Nanjundiah V (1997) Altruistic behaviour in Dictyostelium discoideum explained on the basis of individual selection. Curr Sci 72:142–145Google Scholar
- Bonner JT (1967) The cellular slime molds, 2nd edn. Princeton University Press, PrincetonGoogle Scholar
- Bonner JT, Koontz PG Jr, Paton D (1953) Size in relation to the rate of migration in the slime mold Dictyostelium discoideum. Mycologia 45:235–240Google Scholar
- Huang SC, Tsai CC, Sheu CS (2000) Genetic analysis of Chrysanthemum hybrids based on RAPD molecular markers. Bot Bull Acad Sin (Shanghai) 41:257–262Google Scholar
- Kaushik S (2002) Genetic heterogeneity and social behaviour in cellular slime molds. Ph.D. thesis, Indian Institute of Science, Bangalore, IndiaGoogle Scholar
- Kaushik S, Nanjundiah V (2003) Evolutionary questions raised by cellular slime mould development. Proc Indian Natl Sci Acad B 69:825–852Google Scholar
- Legendre P, Legendre L (1998) Numerical ecology, 2nd English edn. Elsevier, Amsterdam, 853 ppGoogle Scholar
- Raper KB (1940) Pseudoplasmodium formation and organization in Dictyostelium discoideum. J Elisha Mitchell Sci Soc 56:241–282Google Scholar
- Raper KB (1984) The dictyostelids. Princeton University Press, Princeton, NJGoogle Scholar
- Sambrook J, Fritsch EF, Maniatis T (1989) Molecular cloning: a laboratory manual. Cold Spring Harbor Laboratory, Cold Spring HarborGoogle Scholar