Even bread mold sometimes follows a protectionist strategy

Published on
February 4, 2021

It is known that colonies of some ant species can merge with each other to become one big super-colony. In fact, even individuals of some animals and plants such as corals and trees, can merge to become one large individual. Fusion between organisms can provide potential benefits for both partners, but it is also risky, as one partner may benefit at the expense of the other. Although the consequences of fusion may thus be uneven, the fusion process itself appears symmetrical: “you fuse with me, which means I necessarily fuse with you”. But is this really the case?

In a paper appearing today in Nature Communications, researchers from Wageningen University & Research show that this is not necessarily the case. In an evolution experiment with the bread mold Neurospora crassa, millions of fungal individuals were let to merge and reproduce unhindered for more than 30 generations. At the end of this experiment, the researchers observed that among the selected individuals there were consistently mutants, who behaved like "cheaters".

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Asexual spore production of a Neurospora colony on a Petri dish, captured by microphotographer Wim van Egmond. The timelapse of this process has been accelerated about 9000 times and shows a process that normally takes three days.

Cheating' pays off in the short term...

These mutants did not actively participate in the fusion process, but were, as it were, taken possession of by neighbours who did want to fuse. Surprisingly, after the fusion the 'cheaters' emerged as the dominant party by claiming a larger share of the spores produced by the newly merged individual. However, this increased competitiveness came at the expense of the total spore production of the new individual.

... but is also limited

By comparing the complete DNA sequence of these cheaters with that of their ancestors, the researchers surprisingly found that it is precisely the loss of the ability to fuse that causes the fungus to cheat. If the proportion of cheaters in a population is not too high, they are mainly surrounded by the 'social' types that fuse with the cheaters, whereby cheaters gain access to the entire colony for exploitation by claiming a larger fraction of spores.

This works as long as the proportion of cheaters remains reasonably low, so that they are surrounded by sufficient numbers of social types. As soon as cheaters become overrepresented, there are too few social types left to exploit causing cheaters to lose away. Such dynamics explains why these cheaters did not go to fixation but were stuck at roughly 30% frequency in the population during experimental evolution.

Resemblance to trade strategies between countries

Those new results lead to a fundamentally different view on fusion between fungal individuals. Previously, fusion was seen as a symmetrical process, while this work not only shows that it may not be, but also that such asymmetry can lead to unequal distribution of benefits.

This is reminiscent of possible trading strategies between countries: although countries mutually benefit from open trade treaties, it is tempting for an individual country to adopt a protectionist strategy. Such a country would benefit from free-trading neighbors, and favor its own industry. However, if all countries adopt this strategy, the trade will collapse under pressure of restrictions, leading to the tragedy of the commons, a well known game theory. The researchers intend to conduct follow-up research to further characterize such “trade flows” within fungal networks and between merging individuals.