Biology
Dr. Samay Pande
Institute of Integrative Biology, ETH Zurich
More than 50 years after William Hamilton laid the mathematical foundations of kin-selection theory, how cooperation among genetically distinct individuals evolves and is maintained remains one of the most fundamental and fascinating themes of evolutionary research. Here we tried to understand this conundrum by testing the effects of natural diversity within natural fruiting bodies of the social bacterium Myxococcus xanthus on total group spore productivity.
Myxococcus xanthus is a predatory soil bacterium that exhibits vegetative growth and multicellular fruiting body development as two distinct life history stages. It was previously demonstrated that genetically heritable social variation within fruiting bodies is common. Given the prevalent competition among individuals in natural microbial communities, factors that affect the maintenance of such natural within group variation remain unknown.
We asked whether natural within-group diversity has positive, negative or negligible effects on both total-group and individual-strain spore productivity using eight representative isolates derived from a single natural fruiting body. Interestingly, total spore productivity of chimeric populations containing all eight isolates was higher than expected from monoculture controls. Further analyses suggest that chimerism does not stimulate total productivity by affecting just one or two isolates, but rather a majority of the eight isolates respond positively to being mixed with the other seven isolates. Finally, we show that positive effects of mixing several isolates on total group productivity are specific to isolate sets that derive from the same fruiting body group, whereas any significant effects of chimerism on total productivity among isolates from different fruiting bodies were negative, and increasingly so as a function of mean distance between the soil sites from which fruiting bodies were derived.
Our study demonstrates what we believe are the first examples of chimeric synergy - i.e. positive effects of social chimerism on total group productivity - among conspecific microbes derived from the same natural social group. Furthermore, the positive effects of chimerism observed here may sometimes enhance the absolute fitness (and hence aid in survival) of a majority of group members. These studies also show the importance of maintaining precise spatial structure while sampling natural microbial populations so that information on spatial locations can be incorporated into analyses of interactions among members of local populations.
