The ability to participate in nitrogen-fixing root nodule symbiosis shows an interesting phylogenetic pattern, with some families showing a large number of nodulating species interspersed with non-nodulating ones, and some families showing only a few nodulating species. A current model suggests that in some families, there is a pre-disposition towards nodulation, which lowers the bar for the full acquisition of this trait. To further test this model, Griesmann et al. have assembled genome sequences for many additional nodulating and non-nodulating species. Interestingly, their data point to a different model, in which the nodulation trait has been selectively lost, specifically through loss-of-function of one of two genes, NIN and RPG. Critically, their study suggests a stronger-than expected negative selection against symbiosis, which needs to be considered in efforts to engineer nitrogen-fixing symbiosis into crop plants. (Summary by Mary Williams) Science 10.1126/science.aat1743
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