Maximum likelihood phylogeny of RepB proteins. LG + I + G + F was used as model of amino acid substitution. Labels indicate the replicon and the GenBank accession numbers. Squares indicate proteins with genes found in symbiotic plasmids, circles indicate RepB of R. grahamii and R. mesoamericanum megaplasmids: filled circles specify proteins encoded by genes organized in a repABC operon and empty circles specify RepB proteins encoded in a repAB operon. Stars indicate proteins of R. grahamii and R. mesoamericanum encoded in symbiotic plasmids, together with RepB of pRetCFN42a. The arrow indicates the chromosomal RepB. Numbers close to tree nodes indicate branch support evaluated by the Shimodaira–Hasegawa-like approximate likelihood-ratio test (only values higher than 50% are shown). Scale bar, 0.2 amino acid substitutions per site.