Abstract
Cooperative interactions between species, termed mutualisms, play a key role in shaping natural ecosystems, economically important agricultural systems, and in influencing human health. Across different mutualisms, there is significant variation in the benefit that hosts receive from their symbionts. Empirical data suggest that transmission mode can help explain this variation: vertical transmission, where symbionts infect their host's offspring, leads to symbionts that provide greater benefits to their hosts than horizontal transmission, where symbionts leave their host and infect other hosts in the population. However, two different theoretical explanations have been given for this pattern: firstly, vertical transmission aligns the fitness interests of hosts and their symbionts; secondly, vertical transmission leads to increased relatedness between symbionts sharing a host, favouring cooperation between symbionts. We used a combination of analytical models and dynamic simulations to tease these factors apart, in order to compare their separate influences and see how they interact. We found that relatedness between symbionts sharing a host, rather than transmission mode per se, was the most important factor driving symbiont cooperation. Transmission mode mattered mainly because it determined relatedness. We also found evolutionary branching throughout much of our simulation, suggesting that a combination of transmission mode and multiplicity of infections could lead to the stable coexistence of different symbiont strategies.
Highlights
There is considerable variation in the benefit that hosts gain from their symbionts
While transmission mode was correlated with the level of symbiont cooperation, this was mainly through its influence on relatedness (Figure 3)
Transmission mode can be a less useful predictor of the level of cooperation, because it is just one of a number of factors that determine relatedness—other fac‐ tors include the degree of bottlenecking that occurs when symbi‐ onts infect new hosts
Summary
There is considerable variation in the benefit that hosts gain from their symbionts. In some cases, hosts are completely dependent upon their symbionts. As relatedness between symbionts goes down, this can favour symbionts who avoided the cost of helping their hosts, but could still benefit from the benefits provided to the hosts by other symbi‐ onts In this “relatedness” scenario, transmission mode matters, but it does so through its influence on relatedness—vertical transmission reduces conflict between symbionts. Models that examine the influence of variable re‐ latedness do not usually explicitly model horizontal and vertical transmission (Frank, 1994, 2010) In nature, both mechanisms are likely to occur, and we have a poor understanding of the conse‐ quences. We first build an analytical model of a specified symbiont life cycle in which we can tease apart the sepa‐ rate causal influences of relatedness and transmission mode This allows us to test which mechanism plays the larger causal role in the evolution of cooperation. Our simulation allows us to investigate whether evolutionary branching can occur, as has been observed in the early stages of experimentally evolved mutualisms (Harcombe et al, 2018)
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