Species recognition in social amoebae
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Aggregative multicellularity requires the ability of cells to recognise conspecifics. Social amoebae are among the best studied of such organisms, but the mechanism and evolutionary background of species recognition remained to be investigated. Here we show that heterologous expression of a single Dictyostelium purpureum gene is sufficient for D. discoideum cells to efficiently make chimaeric fruiting bodies with D. purpureum cells. This gene forms a bidirectional pair with another gene on the D. purpureum genome, and they are both highly polymorphic among independent wild isolates of the same mating group that do not form chimaeric fruiting bodies with each other. These paired genes are both structurally similar to D. discoideum tgrB1/C1 pair, which is responsible for clonal discrimination within that species, suggesting that these tgr genes constitute the species recognition system that has attained a level of precision capable of discriminating between clones within a species. Analysis of the available genome sequences of social amoebae revealed that such gene pairs exist only within the clade composed of species that produce precursors of sterile stalk cells (prestalk cells), suggesting concurrent evolution of a precise allorecognition system and a new ‘worker’ cell-type dedicated to transporting and supporting the reproductive cells.
KeywordsChimaera Dictyostelium purpureum evolution interspecific recognition social amoeba tgr genes
We would like to thank T. Oyama and M. Ichikawa for their continuous help and encouragement during the course of the study. We also thank P. Schaap and H. Kuwayama for genome sequence data, T. Hayakawa for advice on de novo assembly of NGS data, S. Ito for discussions and help in quantitative PCR, T. Shimada for the use of a qPCR cycler, S. Nonaka, Y. Kamei and Olympus Corporation for use of confocal microscopes in Department of Physics, Y. Yamaguchi for help in interspecies mixing experiments of non-group 4 species, M. M. Hayakawa for insightful comments and suggestions, and V. Nanjundiah for discussions and invaluable comments on the manuscript. Some of the wild isolates were obtained from National BioResource Project Nenkin (http://nenkin.nbrp.jp). This work was supported by Grant-in-Aid for JSPS Fellows 12J01422 to ISH. The DNA sequences for Dp-csaA and Dp-tgrs have been deposited at DDBJ/ENA/GenBank under accession numbers MK002463 to MK002469, for rDNA-ITS of the D. purpureum strains, MK002470 to MK002481, and assembled contigs of the genome of D. purpureum strain P1Ba, RBCH00000000. The strains used in this study can be obtained from National BioResource Project Nenkin (http://nenkin.nbrp.jp).
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