Allopatric divergence of cooperators confers cheating resistance and limits the effects of a defector mutation

Abstract

Social and genomic context may constrain the fates of mutations in cooperation genes. While some mechanisms limiting cheaters evolve in the presence of cheating, here we ask whether cheater resistance can evolve latently even in environments where cooperation is not expressed and cheaters are absent. The bacterium Myxococcus xanthus undergoes cooperative multicellular development upon starvation, but developmentally defective cheaters can outcompete cooperators within mixed groups. Using natural isolates and an obligate cheater disrupted at the developmental-signaling gene csgA, we show that cheating range is narrow among natural strains due to antagonisms that do not specifically target cheaters. Further, we mixed the cheater with closely related cooperators that diverged from it allopatrically in nutrient-rich environments in which cooperative development does not occur, showing that even slight divergence under these conditions can eliminate cheating phenotypes. Our results suggest that such cooperation- and cheater-blind divergence can generate a geographic mosaic of local cheater-cooperator compatibility patches that limit cheater spread. We also ask whether genomic divergence can shape the fitness effects of disrupting a cooperation gene. Construction of the same csgA mutation in several natural-isolate cooperators generated a wide range of pure-culture sporulation phenotypes, from a complete defect to no defect. Thus, we find that epistatic interactions limit the range of genomes within which a mutation creates a cooperation defect. Moreover, these results reveal Developmental System Drift in a microbial system because sporulation proficiency is conserved across the natural isolates despite divergence in the role of csgA.Significance statementSelection on cooperators exploited by obligate cheaters can induce evolution of resistance to cheating. Here we show that cooperators can also rapidly evolve immunity to cheating simply as a byproduct of evolutionary divergence in environments in which cooperation and cheating at the focal trait do not occur because the trait is not expressed. We also find that differences in the genomic context in which a cooperation-gene mutation arises can profoundly alter its phenotypic effect and determine whether the mutation generates a social defect at all - a pre-requisite for obligate cheating. These findings suggest that general divergence of social populations under a broad range of environmental conditions can restrict both the set of mutations that might generate social defectors in the first place and the host range of such defectors once they arise.

Antagonistic, synergistic, and social pleiotropy in microbial cheaters

“…Mutations creating obligate social cheaters provide a fitness advantage in social environments that allow effective exploitation of cooperator genotypes, but decrease fitness in social environments in which exploitation is either not possible (i.e. pure groups of cheaters or groups with non-compatible cooperators (Schaal et al 2022)) or is insufficient to generate a relative fitness advantage (e.g. in groups with low cooperator frequencies (Velicer et al 2000, Velicer andVos 2009)).…”

“…We used as our cooperative wild-type strain GJV2 (Fiegna et al 2006), a variant of the lab type strain GJV1 (Velicer et al 2006) (aka strains R andS, respectively, in Velicer et al (1998)) with a spontaneous mutation in rpoB that confers resistance to rifampicin (Zee et al 2014). Our mutant strains were DK4324 (Kuspa et al 1986, Mayo andKaiser 1989) and DK5208 (Shimkets and Asher 1988, Kashefi and Hartzell 1995, Velicer et al 2000, Kruse et al 2001, Manhes and Velicer 2011, Schaal et al 2022, which have loss-of-function mutations in the A-signal gene asgB and the C-signal gene csgA, respectively. Mutants defective at these loci were previously shown to produce very few spores in monoculture relative to wild-type, while in mixes with the wild-type the mutants produce more spores than the wild-type, i.e.…”

“…For example, a stress tolerance value of -2.0 indicates that the number of viable spores surviving in a non-standard environment was 1% of the number surviving standard conditions. We tested for relative fitness differences between the cheaters and GJV2 in mixed competitions with the previously-published parameter W ij (Vos & Velicer 2009, Schaal et al 2022):…”

Why Aggregate? On the Evolution of Aggregative Multicellularity

“…Forces other than direct selection have been shown to drive evolution of a range of traits, including extreme cheating phenotypes (Velicer, Kroos and Lenski 2000), social fitness inequalities between specific genotypes (referred to as cheating in Dictyostelium literature (e.g. Khare et al 2009)), facultative social exploitation during development (Nair 2018), kin-discrimation phenotypes , colony-level morphology (Rendueles et al 2020), quality as phage host (Freund et al 2020), and even susceptibility to cheating (Schaal et al 2021). Disentangling which features of aggregative systems evolved as adaptations and which did not remains a major challenge for future research.…”

Hidden paths to endless forms most wonderful: Ecology latently shapes evolution of multicellular development in predatory bacteria

“…The relative origins and long-term evolutionary integration of the genetic systems enabling the motility, predation, aggregative development and germination components of myxobacterial multicellular life cycles remain to be thoroughly characterized 91,92 . However, the pervasiveness of LPE emergent from MyxoEE evolution experiments 29,30,56,57,59,60,74,84,86,93 and of pleiotropy across M. xanthus behaviors from mutations engineered or induced in mechanistic molecular studies 58,94 together indicate that many loci contribute to more than one of these behaviors. Increasingly systematic and extensive investigations of pleiotropy across M. xanthus behaviors under standardized conditions will provide greater insight into the shared vs. modular components of the genetic systems underlying these behaviors and their evolution 55 .…”