Many roads to convergence

Plant genomes highlight complex mechanisms behind evolutionary convergence Many plants form specialized symbiotic root structures, called nodules, that harbor beneficial associations with nitrogen-fixing bacteria in the genera Rhizobium or Frankia (see the photo). How this nitrogen-fixing root nodule (NFN) symbiosis arose repeatedly during plant evolution is an age-old mystery: It shows signatures of convergence (the repeated emergence of similarity during evolution) yet builds on similar gene sets in phylogenetically distant plants. On page 144 of this issue, Griesmann et al. ( 1 ) sequenced the genomes of 10 plant species to reveal the genetic correlates of the origin and loss of NFN symbiosis. Their work reveals intricate gain and loss patterns of symbiosis-associated genes, calling for new models to explain convergent evolution.