Pangenome Evolution Reconciles Robustness and Instability of Rhizobial Symbiosis

Abstract

Genetic variation is fundamental to evolution yet is paradoxical in symbiosis. Symbionts exhibit extensive variation in the magnitude of services they provide despite hosts having mechanisms to select and increase the abundance of beneficial genotypes.

Dynamic Interactions Between Mega Symbiosis ICEs and Bacterial Chromosomes Maintain Genome Architecture

“…Four strains in clade 2 have one of three monopartite symICEs, though most strains analyzed here lack a symICE. Strains of clade 3 are predicted to have a nif/fix island with some predicted to have remnants of symICEs ( Weisberg et al 2022 ).…”

“…When integrated in a chromosome, elements of a polypartite symICE are located in different regions and mediate large-scale rearrangements during recombination ( Haskett et al 2016 ). In Bradyrhizobium , the most prevalent type is a bipartite tRNA-Val symICE ( Weisberg et al 2022 ). The large element of the bipartite tRNA-Val symICE (∼0.50 Mb) is integrated near a tRNA-Val gene and the small element (∼0.05 Mb) is integrated near ybgC .…”

“…The Bradyrhizobium genus has a large and diverse accessory genome ( Weisberg et al 2022 ). The symICEs of these bacteria have clusters of genes that confer host specificity and nitrogen fixation, respectively ( Poole et al 2018 ).…”

“…The symICEs of these bacteria have clusters of genes that confer host specificity and nitrogen fixation, respectively ( Poole et al 2018 ). These symICEs are mega sized (typically between half to one megabase in size) because they include extensive variable regions with genes not known to be necessary for symbiosis ( Weisberg et al 2022 ). SymICEs integrate at different tRNA attachment ( att ) sites and vary in their structural configurations as monopartite or polypartite elements.…”

“…Acquisition of a symICE is expected to disrupt chromosome architecture. Polypartite symICEs are perplexing because they cause large inversions to chromosomes during recombination and can cause lethal deletions if the excision of elements is not executed in the proper order ( Haskett et al 2018 ; Weisberg et al 2022 ). Although costs for polypartite symICEs have been uncovered, benefits are less clear.…”