RmInt1 is a mobile group II intron from that is exceptionally abundant in this bacterial species. the acquisition of ISand subsequent retrohoming of RmInt1 to this homing site. These results highlight the role of intron homing sites (ISs) in facilitating intron dispersal and the dynamic and ongoing nature of the spread of the group II intron RmInt1 in strains,22 the group,23 Wolbachia bacterial endosymbionts24 and and related rhizobiales.25 Studies on these introns have reported considerable variability in intron copy number between strains. In particular, RmInt1 is usually very abundant in strains, which may contain up to 11 copies. It is widespread and has been detected in 90% of the strains tested.26,27 Within the genome of strains.25,26,28 Other copies of the intron have been found in genes such as have been identified (ISand IShas been detected in several isolates (but only in one or two copies), whereas ISwas initially found as a single copy in one isolate from Uruguayan soils.9 Interestingly, ISwas found to be more abundant, with several copies per genome, in 11 of 36 field isolates from an Italian soil collection. This IS element was originally detected in the intergenic region between and genome occurs principally by retrohoming into the ISelement.30 Other Rhizobium and Sinorhizobium species were recently shown to have acquired the RmInt1 intron by vertical inheritance and independent Lornoxicam (Xefo) horizontal transfer events.25 It has been suggested that RmInt1 location, together with the inefficiency of the splicing, is consistent with a role for SDC4 this Lornoxicam (Xefo) intron in preventing the spread of other potentially harmful mobile elements in these bacteria.31 However, the dynamics of bacterial group II introns in natural conditions and the factors influencing their gain or loss from some strains remain to be elucidated.32 In this study, we investigated the presence and distribution of RmInt1 and its IS homing sites in two genomic clusters from a Lornoxicam (Xefo) collection of Italian field isolates of isolates from an Italian collection of alfalfa-nodulating field isolates (Table 1),33 by AFLP (amplified fragment Lornoxicam (Xefo) length polymorphism) analysis.34 The dendogram obtained identified two main clusters (I and II), with a Pearson correlation index value in the range of 75 to 95% (Fig. 1). These AFLP differences are significant because the isolates can be differentiated from the reference strain 1021 and other strains from different sources. Cluster I comprises the isolates of types B4, C1, B11, A1 and B12, and the more distantly related A2 (correlation index of only 62%). Cluster II comprises isolates of types B1, C10, B9, C4 and B10, and the more distantly related C3 (correlation index of only 60%). This clustering pattern was further supported by IS/intron fingerprint data (Fig. 2). Clusters I and II were clearly distinguished by the ISfingerprint. Thus, all the strains of cluster I shared at least five hybridizing bands and differed in terms of the number of additional copies (copies 7C11; Fig. 2B and C). The cluster II fingerprint was also defined by five common bands, but these bands differed in size from those of cluster I. The number of additional copies (9C12 bands in total) differentiated between the isolates within this cluster. This genetic variation was even more pronounced in C4, which had six additional bands absent from the other isolates of cluster II (Fig. 2C). By contrast, ISfingerprinting showed that only five isolates, all belonging to cluster II, harbored this IS element (Fig. 2C and Table 1). C3 from cluster II was devoid of this IS element; four isolates contained four identical copies (B1, B9, B10 and C10), and C4 harbored seven copies with only the highest molecular weight band in common (Fig. 2C). Thus, cluster II isolates account for the high relative abundance of the ISelement previously reported for this Italian collection (32% of the isolates).29 These findings further support the hypothesis that ISis ancestral in the evolution of is probably a recent acquisition. Figure 1 AFLP analysis of the isolates. AFLP patterns were normalized and transformed to horizontal.