In‐planta sporulation phenotype: a major life history trait to understand the evolution of A lnus ‐infective F rankia strains

Summary Two major types of F rankia strains are usually recognized, based on the ability to sporulate in‐planta : spore‐positive ( Sp +) and spore‐negative ( Sp −). We carried out a study of Sp + and Sp − Frankia strains based on nodules collected on A lnus glutinosa , A lnus incana and A lnus viridis . The nodules were phenotyped using improved histology methods, and endophytic F rankia strain genotype was determined using a multilocus sequence analysis approach. An additional sampling was done to assess the relation between Sp + phenotype frequency and genetic diversity of Frankia strains at the alder stand scale. Our results revealed that (i) Sp + and Sp − Alnus ‐infective Frankia strains are genetically different even when sampled from the same alder stand and the same host–plant species; (ii) there are at least two distinct phylogenetic lineages of Sp + F rankia that cluster according to the host–plant species and without regard of geographic distance and (iii) genetic diversity of Sp + strains is very low at the alder stand scale compared with Sp − strains. Difference in evolutionary history and genetic diversity between Sp + and Sp − Frankia allows us to discuss the possible ecological role of in‐planta sporulation.