In this study, primer set Lap5640f/Lap5650r yielded one to three amplicons for a given HLB samples. A total of five amplicons with different sizes were identified. They are related by insertion/deletion events, demonstrating the mosaicism in the population genome of 'Ca. L. asiaticus'. In another word, at the locus of CLIBASIA_05640-CLIBASIA_05650, 'Ca. L. asiaticus' possesses alleles composed of sequences identical in some parts but polymorphic in other parts. DNA mosaicism described in this study is largely from size variation of different PCR amplicons and confirmed by sequencing with limited strains. Deng et al.  showed the co-amplification of different amplicons from primer sets targeting the rrn locus in the chromosome of 'Ca. L. asiaticus'. However, further sequencing investigation was not reported.
As shown in Figure 2, the mosaicism of E-types B, D, E, G and H is represented by multiple DNA bands from the same PCR primer set, raising a question if a HLB sample has single or multiple clones (or clonal strains) of 'Ca. L. asiaticus'. This is of particular interest, since 'Ca. L. asiaticus' DNA obtained was not from a clonal pure culture. Further complicated the issue is the variation of amplicon intensity, suggesting different concentration of PCR templates. If a single clone scenario is considered, the bacterium should have multiple Lap5640f/Lap5650r loci, either in chromosome or/and in the form of a phage. Lytic phage possessing this genomic locus has recently been reported . Alternatively, the HLB samples may contain multiple clones of 'Ca. L. asiaticus'. More evidence is, however, needed. A third scenario could be the combination of both of the above.
Since the sequenced Florida strain Psy62 belongs to E-type C (Table 2, Figure 2), it is interesting that the frequency of E-type C is low in Florida (4.1%), as well as in China (5.9%). This could mean strain Psy62 may not be the most representative strain. We noted that Psy62 originated from a psyllid and all the 'Ca. L. asiaticus' samples in this study were from citrus. Could it be possible that bacterial population was difference between psyllids and plant hosts? Zhang et al.  recently reported that phages behaved differently between plants and psyllids in Florida. Phage SC1 and SC2 were lytic in dodder plant but remained lysogenic in psyllids.
Among the six E-types in China, five were found in Yunnan and two were in Guangdong (Table 1). The higher E-types number suggests that 'Ca. L. asiaticus' population in Yunnan could be more diverse than that in Guangdong. The uniqueness of P3 (E-type D and E) to Yunnan samples further substantiates the speculation. It should be noted that Yunnan is one of the world origins of citrus species . It remains to be tested if a long history of the presence of citrus species is associated with more diversity of 'Ca. L. asiaticus' population. Information about the population diversity of 'Ca. L. asiaticus' in Yunnan is currently very limited.
The challenge of in vitro culture of 'Ca. L. asiaticus' has been a critical factor limiting our capacity to study the bacterial biology. DNA sequencing and in silico analyses provide a different venue to collect information of unculturable bacteria. Regarding to CLIBASIA_05650, the P1/P3/P4 alleles which encode 18 hexapeptides predominately occurred in 'Ca. L. asiaticus' populations in China, whereas the P2/P5 alleles which have 22 hexapeptides distributed mostly in Florida populations. Hexapeptide variation has been reported in other bacteria . This type of genetic heterogeneity may be associated with phenotypic variation for environment adaptation [17, 28].