# Genetic Adaptation of Mesorhizobium Symbionts Associated With Caragana in Northern China Deserts

## TL;DR

This study explores how Mesorhizobium bacteria, which help Caragana plants in Chinese deserts, adapt genetically to harsh environments through specific genes.

## Contribution

The study reveals that recombination, not mutation, drives adaptation in Mesorhizobium through key genes like cysW and uvrC.

## Key findings

- Mesorhizobium strains show strong genetic cohesion within lineages despite geographic separation.
- Purifying selection and recombination drive adaptation in membrane transporters and nucleotide repair genes.
- Harsh desert environments weaken the distance-decay relationship among bacterial populations.

## Abstract

Caragana, a keystone leguminous species dominating arid semi‐fixed deserts in northern China, forms specialized symbiotic nitrogen‐fixing partnerships with Mesorhizobium, which are indispensable for sustaining ecosystem function globally. However, the roles of membrane transporters and nucleotide repair genes in conferring survival advantages to desert‐dwelling Mesorhizobium across desert environments remained poorly elucidated. Therefore, a total of 68 representative Mesorhizobium strains associated with Caragana, isolated from five geographically distant areas (A to E) in the desert belt of northern China, were investigated to elucidate the pivotal roles of three membrane transporters (cysW, exoY, idhA) and two nucleotide repair genes (mutS, uvrC) in microbial adaptation to environmental stress. Phylogenetic analysis results revealed that strains assigned to the same genospecies primarily clustered by genetic lineage rather than geographic origin, with stronger intralineage sequence cohesion observed relative to interregional divergence. Notably, phylogenetic trees of membrane transporter genes, nucleotide repair genes, and core genes showed high topological congruence, underscoring their concerted evolutionary dynamics and shared selective pressures. Furthermore, consistent nucleotide diversity (π), low πN/πS ratios (<< 1.0) and genetic distance (Dxy) across populations indicated that purifying selection predominated in membrane transporters and nucleotide repair genes. Elevated recombination impact (r/m) and frequency (ρ/θ) revealed that homogenizing gene flow, rather than mutation, was the primary driver of population differentiation enabling rapid adaptation to desert environments.

This study investigates the genetic adaptation of Caragana‐associated Mesorhizobiumin northwest China's semi‐fixed deserts, focusing on membrane transporters and nucleotide repair genes. Despite geographic isolation, populations exhibited striking genetic congruence, driven by homogeneously harsh environments that weakened the distance‐decay relationship. Recombination (rather than mutation) dominated genetic divergence, with the horizontal transfer of adaptive loci (e.g., cysW, uvrC) enabling rapid trait dissemination, underscoring these genes' roles in desert symbiotic adaptation.