Fig. 6

Dendrogram of mutation accumulation between Ef.PcfG from E. faecalis and the various Ll.LtrB introns identified from L. lactis. The root of the tree is depicted as the Ef.PcfG intron (pTEF-2-like plasmid), which likely disseminated throughout L. lactis following a single horizontal transfer event. The plasmids harboring the Ef.PcfG (pTEF-2-like plasmid) and Ll.LtrB (pRS01) introns discussed in this study are bolded. The eight point mutations (Mut #1 to Mut #8) that distinguish these two introns are shown in circles, with an asterisk adjacent to mutations conferring significant increases (upwards arrow) and decreases (downwards arrow) in mobility efficiency to the ltrB-HS (p < 0.05). Introns found in the chromosome are designated by the strain’s name, whereas introns found within plasmids are represented by the plasmid’s name. Strains or plasmids that are underlined belong to L. lactis subsp. lactis, whereas strains or plasmids that are not underlined belong to L. lactis subsp. cremoris. Thick branches of the dendrogram represent an insertion event into an HIHN-H relaxase motif, whereas thin branches represent an insertion event into an HLHN-H relaxase motif; the only exception being the intron present in the chromosome of SK11, which is likely a retrotransposition event into an ectopic site (cell surface protein). Numbers between parentheses denote the amount of additional mutations that distinguish a particular intron from Ef.PcfG in relationship to its position in the dendrogram. Three groups are present in the dendrogram which encompass a number of additional identical introns: Group (a) contains 6 additional introns (HP, TIFN5, TIFN6, FG2, B40, LMG6897), group (b) contains 3 additional introns (p3, SK110, AM2), and group (c) contains 4 additional introns (MG1363, pFI430, NZ9000, NCDO763) (Additional file 1: Table S3)