These 10 alleles were examined for evidence of IHR by comparing them to the previously described non-IHR X. fastidiosa subsp. multiplex alleles and to the known X. fastidiosa subsp. fastidiosa and sandyi alleles . Of these 10, 4 alleles were found to be derived in their entirety from X. fastidiosa subsp. fastidiosa, and 3 were found to be chimeric for X. fastidiosa subsp. multiplex and fastidiosa sequences, with significant evidence of one or more recombination breakpoints. These 7 alleles encompassed 4 loci: leuA, cysG, holC, and pilU. The locus most strongly implicated in IHR wascysG, since all of the 9 recombinant-group STs were characterized at this locus by 1 of 3 cysG alleles unique to the group. The involvement of IHR in the genesis of all 3 of these alleles is illustrated by their close genetic relationship to X. fastidiosa subsp.fastidiosa and sandyi alleles . Allele 12, apart from being found in the recombinant group, is an X. fastidiosa subsp. fastidiosa allele . The other two alleles were found to be chimeric: allele 18 contains a single recombinant region at the 3= end of 342 bp, while allele 6 has two short recombinant regions, one at the 5= end of at least 23 bp and another toward the 5= end of at least 35 bp . The DNA sequence variation defining these patterns is shown in Table 2. The patterns seen in the DNA sequences of the 3 cysGalleles are consistent with the hypothesis of a single IHR that introgressed donor allele 12 into X. fastidiosa subsp. multiplex, followed by subsequent intrasubspecific recombination reintroducing X. fastidiosa subsp. multiplex sequence to create alleles 6 and 18 . There are no inconsistent sites, container size for blueberries provided the 5= recombination breakpoint in allele 18 starts after position 71. Introgression of X. fastidiosa subsp. fastidiosa sequence into X. fastidiosa subsp. multiplexwas found in alleles at 3 other loci .

In the case of pilU, 7 of the 9 recombinant STs carried either an allele identical to a known X. fastidiosa subsp. fastidiosa allele or 1 bp different from it . Allele 1 is an allele that characterizes most U.S. isolates as well as several STs found in Costa Rica, while allele 9 is unique to the recombinant group. The leuA locus has a single statistically significant recombinant allele, allele 4 . It differed by 2 bp from the X. fastidiosa subsp. fastidiosa allele 9 but by 8 bp from the most similar nonIHR X. fastidiosa subsp. multiplex allele. X. fastidiosa subsp. fastidiosa allele 9 could be the donor for its entirety , although if the recombination region started after site 10 but before position 520 , then only one site would be unexplained. That remaining site carries a base unique to this allele and is probably a novel mutation. If the recombination breakpoint was 3= of position 295 then X. fastidiosa subsp. fastidiosa allele 1 provides as good a match as allele 9 . Similarly, holC allele 7 was also 8 bp different from the most similar non-IHR X. fastidiosa subsp. multiplex allele, providing clear evidence that the 5= end was derived from X. fastidiosa subsp. fastidiosa . The pattern can be explained if X. fastidiosasubsp. fastidiosa allele 19 is the donor of the 5= region ending somewhere between positions 183 and 286, since it leaves no inconsistent bases . The loci leuA and holC each had an additional allele that were unique to the recombinant group, as was an allele at another locus, nuoL4. Although these last 3 alleles did not show statistically significant evidence of introgression , they all showed a grouping of 2 or 3 nucleotide changes that were not found in non-IHR X. fastidiosa subsp. multiplex but were present in X. fastidiosa subsp. fastidiosa. Of these 3, the strongest case for IHR is holC9, where a region of possible IHR can be seen at the 3= end of the sequence .Analysis of the recombinant group ofX. fastidiosa subsp. multiplex showed three important results. First, intersubspecific recombination was shown to have occurred in 50% of 8 loci scattered throughout the genome that were chosen independently of the data . Second, it was shown that the donor of the introgressed sequence was X. fastidiosa subsp. fastidiosa, a subspecies introduced from Central America into the United States as a single strain .

However, the introgressed sequence at two of the loci did not come from any of the X. fastidiosa subsp. fastidiosa genotypes that have been found in the United States. This result suggests that another introduction of X. fastidiosa subsp. fastidiosa must have occurred, an introduction that resulted in successful IHR, after which the donor genotype seems to have disappeared. This involvement of an unexpected X. fastidiosa subsp. fastidiosa strain supports the hypothesis that the members of the recombinant group share a single ancestral IHR event. Third, the hypothesis that IHR has facilitated a shift to new hosts is strongly supported by the example of blueberry, where 10 isolates have been typed and potentially supported by the example of blackberry .Of course, the direct causation of this link can never be proved without knowledge of the genetic changes driving this shift. It can always be argued that the link is fortuitous and that one or more point mutations in the nonrecombined X. fastidiosa subsp. multiplex genome are causal in the host shift. Arguing against this possibility are 2 additional pieces of information. First, both blueberry and blackberry are native to the United States, so if only a simple genetic change was required to infect these species, why did the native non-recombinant X. fastidiosa subsp. multiplex apparently never acquire these changes? Second, a similar but even more extensive mixing of the genomes of X. fastidiosa subspp. fastidiosa and multiplex is found in the only form of X. fastidiosa that infects another U.S. native plant, mulberry . Furthermore, in other bacterial species, it has been demonstrated that recombination can drive rapid evolution, both in the laboratory and, in the case of Helicobacter pylori, in mice . Similarly, McCarthy et al. concluded that lineages of Campylobacter jejuni in chickens versus cattle and sheep were able to shift host type, because rapid adaptation was facilitated by recombination with the resident host population.

In the study by Nunney et al. , it was shown that the recombinant genotypes formed a well-defined group , demonstrating that intersubspecific homologous recombination was not randomly distributed across the X. fastidiosa subsp. multiplex isolates. This work was based on a survey of 143X. fastidiosa subsp. multiplex isolates using just 8 loci. There were 33 isolates that showed some evidence of IHR in at least 1 locus: all but 2 showed statistically significant evidence in at least 2 loci, while the remaining 110 showed no such evidence . The generality of this discrete group of recombinant forms was supported by our analysis presented here of the sequence data from 9 more loci sequenced by Parker et al. . These loci divided isolates into 2 groups that appeared to correspond to the recombinant and non-IHR groups, respectively ,raspberry grow in pots even though Parker et al. found no evidence of IHR. Upon reanalysis, we found statistically significant IHR in 6 of the 9 loci in the clade A data but no evidence of IHR in the clade B data. Clade A included 6 isolates that we had typed in the present study, and each of these showed evidence of IHR in 4 or 5 of the additional 9 loci. Thus, in two independent samplings that together examined 17 loci, there was clear evidence of substantial genomewide IHR in the recombinant group isolates, amounting to 50% of the genes showing IHR across the MLST locis plus the pilU locus . The average was higher when based on the loci sequenced by Parker et al. ; however, this was probably biased upwards by the manner in which the loci were chosen . None of the IHR events in 6 of the 9 loci identified using the targeted introgression test, or in the case of complete introgression, a chi-square test, were detected by Parker et al. using PHI and the 9 tests implemented in RDP . This failure of the standard tests of recombination to detect IHR was previously noted by Nunney et al. , motivating the development of their introgression test. We examined the hypothesis that the recombinant group STs were derived from a single IHR event involving a X. fastidiosa subsp. multiplex recipient and an X. fastidiosa subsp. fastidiosa donor. The distribution of allelic differences among the recombinant STs was consistent with them all being derived from a single initial event, but a small number of other intersubspecific and intrasubspecific recombination events would also be needed . More importantly, the genotypes seen in the recombinant group can be accounted for entirely, or very nearly so, based on a single X. fastidiosa subsp. fastidiosa donor genotype. For example, the substantial variation in cysGcan all be accounted for by an ancestral introgression of X. fastidiosa subsp. fastidiosa allele 12 followed by subsequent intrasubspecific recombination of X. fastidiosa subsp. multiplex sequence to form the other two alleles . In contrast, variation at pilU could be accounted for by a second donor contributing the X. fastidiosa subsp. fastidiosa pilU9 allele, but it could also have arisen by a single mutation in pilU1 unique to the recombinant group. A possible single X. fastidiosa subsp. multiplex recipient genotype was also identified . This genotype is consistent with a known ST: setting cysG to allele 3 makes the recipient identical to ST45, which was sampled from the states of California, Kentucky, and Texas .

Elsewhere, we consider a slightly different hypothesis regarding the origin of the recombinant group in which the donor and recipient subspecies are reversed—i.e., that it was derived from a single IHR event, but involving an X. fastidiosa subsp. multiplex donor and an X. fastidiosa subsp. fastidiosa recipient; however, apart from the role reversal, the conclusions are unaltered . The ancestral reconstruction allows us to consider the second question posed earlier: is the donor consistent with the X. fastidiosa subsp. fastidiosa genotypes found in the United States? The answer is very clearly “no.” The original donor carried cysG12 and holC19 . These alleles are both found in isolates from Central America, but no X. fastidiosa subsp. fastidiosa isolate found in the United States comes close to matching this criterion: the most similar U.S. ST has a 12-bp mismatch. There has been extensive sampling of X. fastidiosa subsp. fastidiosa within the United States, based on 85 isolates sampled across the United States from 15 different host plants . There is very little variation within X. fastidiosa subsp. fastidiosa isolates from the United States, consistent with the hypothesis that all X. fastidiosa subsp. fastidiosa isolates currently found in the United States are derived from a single strain introduced from Central America . Based on these data, we conclude that the X. fastidiosa subsp. fastidiosa donor was introduced into the United States from Central America and recombined with a native X. fastidiosa subsp. multiplex genotype similar to ST45; however, this donor lineage of X. fastidiosa subsp. fastidiosa was ultimately unsuccessful and died out. We can never conclusively prove the absence of this genotype from North America. However, X. fastidiosa has been extensively sampled from many plant species throughout the United States, and no isolates of X. fastidiosa subsp. fastidiosa have been found that carry alleles similar to the inferred donor alleles cysG12 and holC19 ; indeed all X. fastidiosa subsp. fastidiosa isolates so far found in the United States are consistent with the introduction into the United States of just a single genotype . The transient presence of the donor genotype is consistent with a single large-scale introgression event founding the recombinant group. This raises the possibility that conjugation might have been involved; however, if this was the case, the genomic DNA was broken into pieces prior to homologous recombination, since the data show short regions of recombination. The data from the MLST loci plus pilU show 7 significant recombination events , and 3 of them included at least one recombination break point.