Comparison of Independent Evolutionary Origins Reveals Both Convergence and Divergence in the Metabolic Mechanisms of Symbiosis

Abstract

Highlights d Paramecium-Chlorella symbiosis has evolved independently multiple times d Independent origins use a convergent nutrient exchange, enabling partner switching d Chlorella genotypes have evolved divergent mechanisms to cope with excess light d Consequent phenotypic mismatches could mediate hostsymbiont specificity

Rapid compensatory evolution can rescue low fitness symbioses following partner-switching

“…To investigate the potential metabolic mechanisms underlying the observed intergenomic epistasis we performed untargeted global metabolomics with ESI-ToF-MS independently for the host and symbiont metabolite fractions for each host-symbiont pairing across the light gradient (22). Light irradiance was the primary driver of differential metabolism for both host and symbiont, however, host-dependent differences in the metabolism of symbiont-genotypes could be detected.…”

“…This symbiosis is geographically widespread and genetically diverse, in part due to its multiple independent evolutionary origins (20, 21). The primary nutrient exchange is convergent among these origins, facilitating partner-switching, with concurrent divergence in other metabolic traits, causing phenotypic mismatches in partner-switched host-symbiont pairings (22). Here, using experimental partner-switches, we examined the pattern and mechanisms of intergenomic epistasis for three diverse host-symbiont strains, observing significant G H × G S × E interactions for host-symbiont growth rate and symbiont load, together with corresponding differences in metabolism.…”

Evolution: Convergent Pathways to Symbiosis

“…Importantly, knowledge of the factors that promote and facilitate the evolution of beneficial symbiotic interactions could provide valuable insight into how species are evolving today, and critically, how species may evolve in the future. A new study by Sorensen et al [3] in this issue of Current Biology provides novel insight into the establishment of beneficial symbioses by testing for convergence in the nutrient exchange of different Paramecium bursaria-Chlorella symbioses.…”

“…Are beneficial symbionts interchangeable [11,12]? In their new study, Sorensen et al [3] ask if independent beneficial interactions between P. bursaria and Chlorella species share evolutionarily conserved mechanisms of nutrient exchange. Then, they go one step further by testing whether the hosts and symbionts continue to function after switching symbiont partners between the hosts.…”

“…Host fitness is generally greater when colonized by the symbiont and tends to increase with greater light intensity [14]. Sorensen et al [3] use the HA1 and 186b host and symbiont pairs to test for convergent evolution of nutrient exchange in the P. bursaria-Chlorella symbioses. Importantly, the HA1 P. bursaria-Chlorella symbiosis and 186b P. bursaria-Chlorella symbiosis evolved independently.…”