Evolution on the bright side of life: microorganisms and the evolution of mutualism

Abstract

Mutualistic interactions, where two interacting species have a net beneficial effect on each other's fitness, play a crucial role in the survival and evolution of many species. Despite substantial empirical and theoretical work in past decades, the impact of these interactions on natural selection is not fully understood. In addition, mutualisms between microorganisms have been largely ignored, even though they are ecologically important and can be used as tools to bridge the gap between theory and empirical work. Here, I describe two problems with our current understanding of natural selection in mutualism and highlight the properties of microbial mutualisms that could help solve them. One problem is that bias and methodological problems have limited our understanding of the variety of mechanisms by which species may adapt to mutualism. Another problem is that it is rare for experiments testing coevolution in mutualism to address whether each species has adapted to evolutionary changes in its partner. These problems can be addressed with genome resequencing and time-shift experiments, techniques that are easier to perform in microorganisms. In addition, microbial mutualisms may inspire novel insights and hypotheses about natural selection in mutualism.

Metabolic excretion associated with nutrient-growth dysregulation promotes the rapid evolution of an overt metabolic defect

“…Chemical release by cells ("niche construction") mediates diverse microbial interactions [20][21][22][23] . Indeed, auxotrophs are predicted to be widespread 19,24 and have been found in natural isolates [25][26][27][28][29][30] .…”

Synergistic epistasis enhances cooperativity of mutualistic interspecies interactions

“…Not surprisingly, frequent fluctuations between SR 15 and syntrophy was demonstrated to be energetically unsustainable for a coculture of Desulfovibrio 16 vulgaris Hildenborough (Dv) and Methanococcus maripaludis S2 (Mm) causing the syntrophic 17 community to collapse with as few as 4 transitions . By contrast, prolonged 18 laboratory evolution of the same community under obligate syntrophy conditions resulted in 19 significantly improved growth and stability within 300 generations, but at the expense of loss of 20 independence through the erosion of SR (Hillesland andStahl, 2010, Hillesland et al, 2014). 21 While SR eroded across nearly all evolution lines, other processes such as regulation and signal 22 transduction also accumulated mutations suggesting that modulation of multiple pathways could 23 have also contributed to improved syntrophic growth.…”

“…Many differences 21 between populations in mutation order could have occurred due to the chance occurrence of 22 mutations at different times in different populations (Lenski et al, 1991). However, we cannot rule 23 out the possibility that epistasis and evolutionary history caused some of the differences between 24 populations (Blount et al, 2018, Elena et al, 1996. For example, it is possible that a mutation 25 unique to HS3 precluded or significantly delayed erosion of SR in this coculture.…”

“…This relationship between fitness and the possible combinations of genetic 32 variants is called an adaptive landscape, and has been the subject of intense research (Orr, 2005, In the present work, we investigated evolutionary trajectories of not just one but two species that 2 rely on one another for survival. One might expect the interaction to amplify the effects of chance 3 if the adaptive landscape is affected by genetic changes in the partner population (coevolution; 4 (Thompson, 1989, Hillesland, 2018), and those partner genetic changes depend on chance. In 5 an extreme case, this situation could send each population down completely different trajectories, 6 with very little parallelism.…”

“…Efficient transfer of electrons 22 through hydrogen is in the best interests of both species(Stolyar et al, 2007). However, evolution 23 could hypothetically change this situation by altering mechanisms of electron transfer, or through 24 the evolution of new dependencies that are costly(Harcombe et al, 2018, Hillesland, 2018. One 25 high G-score mutation in Mm (MMP1511) could reflect the evolution of a new costly dependency 26.…”