Evolution of specialization in a plant‐microbial mutualism is explained by the oscillation theory of speciation

Abstract

Specialization in mutualisms is thought to be a major driver of diversification, but few studies have explored how novel specialization evolves, or its relation to the evolution of other niche axes. A fundamental question is whether generalist interactions evolve to become more specialized (i.e., oscillation hypothesis) or if partner switches evolve without any change in niche breadth (i.e., musical chairs hypothesis). We examined alternative models for the evolution of specialization by estimating the mutualistic, climatic, and edaphic niche breadths of sister plant species, combining phylogenetic, environmental, and experimental data on Acmispon strigosus and Acmispon wrangelianus genotypes across their overlapping ranges in California. We found that specialization along all three niche axes was asymmetric across species, such that the species with broader climatic and edaphic niches, Acmispon strigosus, was also able to gain benefit from and invest in associating with a broader set of microbial mutualists. Our data are consistent with the oscillation model of specialization, and a parallel narrowing of the edaphic, climatic, and mutualistic dimensions of the host species niche. Our findings provide novel evidence that the evolution of specialization in mutualism is accompanied by specialization in other niche dimensions.

Pangenome Evolution Reconciles Robustness and Instability of Rhizobial Symbiosis

“…In Mesorhizobium , entire symICEs originating from inoculum strains were predicted to be transferred into indigenous nonsymbiotic rhizobia, or among strains already naturalized under monoculture crops, a scenario that imposes intense selection for host crop compatibility ( 14 , 16 , 17 , 19 ). Conversely, our findings are based on investigation of phenotypically variable strains of Bradyrhizobium isolated from diverse native plant communities, where multiple legume species overlap and select for differential subsets of rhizobia ( 35 – 37 ). Our study suggested that symICE transfer has recurrently promoted novel host acquisition, and that loss of effectiveness on one host is associated with gains of other hosts, processes that likely require a diverse array of potential hosts.…”

Phylogenetic reconstruction of ancestral ecological networks through time for pierid butterflies and their host plants

“…Future avenues of research should explore the extent to which host shifts, host‐range expansions, and host recolonisations characterise the evolution of other networks of intimate interactions. Given the support for the oscillation hypothesis from a variety of systems such as polyphagous moths (Wang et al, 2017), parasites (D'Bastiani et al, 2020; Hoberg & Brooks, 2008), and even plant‐microbial mutualism (Torres‐Martfnez et al, 2021), we expect similar dynamics to be found in many systems, ranging from parasitisms to mutualisms.…”

Phylogenetic reconstruction of ancestral ecological networks through time for pierid butterflies and their host plants

“…Future avenues of research should explore the extent to which host shifts, host-range expansions, and host recolonizations characterize the evolution of other networks of intimate interactions. Given the support for the oscillation hypothesis from a variety of systems such as polyphagous moths (Wang et al 2017), parasites (Hoberg and Brooks 2008; D’Bastiani et al 2020), and even plant-microbial mutualism (Torres-Martínez et al 2021), we expect similar dynamics to be found in many systems, ranging from parasitisms to mutualisms.…”