Cooperation has played a grand role in the evolution of diversity and complexity of life on our planet. Some of the most spectacular and most visible examples of cooperation are reproductive cooperation among social insects such as honeybees, division of labor in the colony of leaf-cutting ants and among cells of multicellular organisms. In all of these examples individuals within a group are highly related. Such high relatedness might be favored because it prevents conflict that might disrupt the functioning of the cooperative group. However, interactome within and between social groups of organisms that have retained reproductive autonomy and exhibit within group variations remain unknown.
Using natural social groups of Myxococcus xanthus that are internally diverse, we show that within-social-group diversity in the natural populations has positive effects on overall group productivity. This positive effect of mixing was only limited to interactions between isolates from the same social group and might be the result of overwhelmingly common bidirectional positive interaction effects between pairs of isolates. Strikingly though, mixing effects between isolates derived from different fruiting body were strongly negative. Our findings therefore provide an intuitive ecological explanation for the ease, with which possible shift in the level of selection might occur in the populations that coexist together over an evolutionary timescale.
Next, using two natural isolates of M. xanthus sampled from the same fruiting body on soil, we demonstrate that M. xanthus spore germination is a cooperative process involving production of public good molecule. Moreover, inability to germinate defect exhibited by one strain under saline stress in pure culture is socially complemented by the presence of another strain that is fully proficient at germination. Complementation confers a cheating advantage to the defective strain and is mediated by secretion of glycine betaine, an osmo-protectant utilized in all domains of life that functions as a public-good molecule necessary for germination in salty environments. These findings suggest that spore germination may be a socially multi-faceted process in other microbes as well.
Bacteria use dormancy as a bet-hedging strategy to overcome unfavourable environmental conditions. The dormant individuals such as spores or persister-cells with low metabolic activity can determine community dynamics in future generations. Although importance of social interactions has been documented among actively growing microbes, role of social interactions between individuals in diverse communities during transition to, and away from dormancy, remained unclear. Our results demonstrate importance of synergistic interactions within natural social groups of M. xanthus during conversion of actively growing cells to dormant spores and also during resuscitation of spores to actively growing cells.