biostudies-literaturede Faria SMSwiss National Science FoundationConselho Nacional de Desenvolvimento Científico e TecnológicoFundação de Amparo à Pesquisa do Estado da BahiaForeign, Commonwealth and Development OfficeBill and Melinda Gates Foundation2365-2377https://www.ebi.ac.uk/biostudies/studies/S-EPMC9541511biostudies-literatureUnknown235(6)Nitrogen-fixing symbiosis is globally important in ecosystem functioning and agriculture, yet the evolutionary history of nodulation remains the focus of considerable debate. Recent evidence suggesting a single origin of nodulation followed by massive parallel evolutionary losses raises questions about why a few lineages in the N₂ -fixing clade retained nodulation and diversified as stable nodulators, while most did not. Within legumes, nodulation is restricted to the two most diverse subfamilies, Papilionoideae and Caesalpinioideae, which show stable retention of nodulation across their core clades. We characterize two nodule anatomy types across 128 species in 56 of the 152 genera of the legume subfamily Caesalpinioideae: fixation thread nodules (FTs), where nitrogen-fixing baThe New phytologistThe innovation of the symbiosome has enhanced the evolutionary stability of nitrogen fixation in legumes.PMC9541511OPP11772165310003A_156140APP0037/2016312125/2020‐815614031003A_182453/1Hughes CETak NSongwattana Pde Faria SMde Lima HCRingelberg JJCardoso DTeaumroong NArdley JGehlot HSAmetsitsi GKDZartman CEKoenen EJMMaluk MJames EKWright KMAkomatey JPrin YGross ESprent JIfalseThe innovation of the symbiosome has enhanced the evolutionary stability of nitrogen fixation in legumes.Nitrogen-fixing symbiosis is globally important in ecosystem functioning and agriculture, yet the evolutionary history of nodulation remains the focus of considerable debate. Recent evidence suggesting a single origin of nodulation followed by massive parallel evolutionary losses raises questions about why a few lineages in the N₂ -fixing clade retained nodulation and diversified as stable nodulators, while most did not. Within legumes, nodulation is restricted to the two most diverse subfamilies, Papilionoideae and Caesalpinioideae, which show stable retention of nodulation across their core clades. We characterize two nodule anatomy types across 128 species in 56 of the 152 genera of the legume subfamily Caesalpinioideae: fixation thread nodules (FTs), where nitrogen-fixing ba2022-01-01T00:00:00Z2022 Sep2025-04-04T20:03:41.948Z2025-04-04T20:03:41.948ZS-EPMC95415113590126410.1111/nph.18321