https://phys.org/news/2018-05-symbiosesfragile-partnerships.html

The ability to form root nodules in response to the presence of nitrogen-fixing bacteria is currently restricted to four orders of flowering plants (Fabales, Fagales, Cucurbitales and Rosales), to which many agriculturally significant species belong, including beans, peas and soya. The authors of the new study therefore assumed that the genetic basis for this type of symbiosis initially evolved in a common ancestor of these groups.

"However, only 10 of the 28 plant families that make up these four orders are now able to form nitrogen-fixing root nodules. Moreover, most of the genera in nine of these families do not adopt the symbiotic lifestyle," says LMU geneticist Professor Martin Parniske, who initiated the international collaboration. In order to identify the genetic grounds for the relatively low incidence of root nodule symbioses among the plant families in which it occurs, the consortium collected DNA samples and the BGI in China sequenced the entire genome of 10 species that form distinct types of nodules with diverse bacterial symbionts, which employ different infection strategies. In close cooperation with LMU and the HMGU, the genomic sequence data for these species were then included in a genome-wide comparison of 37 plant species representing the four orders mentioned above. Much to everyone's surprise, the analyses revealed that many members of these groups—including well known cultivars such as strawberries, blackberries and apples—had once possessed, but subsequently lost the capacity to form symbioses.