Bt Research 2024, Vol.15, No.3, 154-163 http://microbescipublisher.com/index.php/bt 159 replicon, indicating a complex structure resulting from recombination events (Murawska et al., 2014). Additionally, the complete genome sequence of Bt HER1410 revealed two megaplasmids, pLUSID1 and pLUSID2, which are involved in virulence and potentially sporulation, respectively (Lechuga et al., 2020a; Lechuga et al., 2020b). The presence of a cryptic rolling-circle replicating plasmid, pGI2, further underscores the diversity of plasmid types in Bt, with its modular organization and mobilization capabilities6. Comparative genomics studies have also shown active plasmid exchange among Bt strains, contributing to genetic diversity and adaptability (Bolotin et al., 2017). 6.2 Transposons Transposons are another significant component of the mobile genetic elements in Bt. The insertion sequence IS231, identified in Bt strain berliner 1715, exemplifies the structural organization typical of transposons, with inverted repeats and a long open reading frame encoding a putative transposase. The presence of transposons like Tn4430 in plasmids such as pGI2 highlights their role in genetic rearrangements and horizontal gene transfer. Additionally, the identification of IS240-like and IS150-like elements in Bt ssp. fukuokaensis further illustrates the diversity of transposable elements and their potential impact on gene regulation and mobility. The dynamic nature of transposons is evident in the highly mobile genetic environment of plasmids like pH3-180, which carry multiple novel insertion sequences and class II transposable elements (Fayad et al., 2020). 6.3 Integrons and insertion sequences Integrons and insertion sequences (IS) are integral to the genetic plasticity of Bt. The IS231 element, the first identified IS element in Bt, shares structural homology with the Escherichia coli IS4 element, suggesting a conserved mechanism of transposition. The presence of IS elements like IS240 and IS150 in Bt plasmids indicates their role in gene capture and dissemination. The complete sequence of plasmids such as pFR55 from Bt INTA-FR7-4 reveals a near-complete conjugation apparatus and extensive homology with other Bt plasmids, highlighting the role of IS elements in plasmid evolution and function (Amadio et al., 2009). The identification of mobile genetic elements in plasmids like pAW63, which shares homology with the virulence plasmid pXO2 of Bacillus anthracis, further underscores the evolutionary significance of integrons and IS elements in shaping the genetic landscape of Bt (Auwera et al., 2005). 7 Genomic Variability and Evolution 7.1 Strain-specific genetic variation Bacillus thuringiensis (Bt) exhibits significant strain-specific genetic variation, which is crucial for its adaptability and effectiveness as a biocontrol agent. Studies have shown that different Bt strains possess unique genetic elements that contribute to their specific ecological niches and host interactions. For instance, the genomes of Bt strains MYBT18246, MYBT18247, and MYBT18679 contain approximately 15% to 20% genetic material encoding elements related to genome plasticity, such as virulence factors and mobile genetic elements like bacteriophages and transposases (Hollensteiner, 2017). Additionally, the RAPD technique has revealed distinct DNA patterns among various Bt isolates, indicating substantial genetic diversity even within local populations (Qasem et al., 2015). 7.2 Mechanisms of genetic diversity The genetic diversity in Bt is driven by several mechanisms, including horizontal gene transfer, mobile genetic elements, and bacteriophages. Prophages, for example, play a significant role in the mobilization of chromosomally encoded cry-toxins, which are critical for Bt's pathogenicity (Hollensteiner, 2017). Comparative genomic analyses of phages and prophages from Bt strains have shown that these elements contribute to the genetic variability and evolutionary dynamics of the species. For instance, the phiCM3 phage and proCM3 prophage from Bt strain YM-03 exhibit high genomic similarity to other Bacillus phages, suggesting a common evolutionary origin and subsequent genome rearrangements (Yuan et al., 2014). Furthermore, multi-REP-PCR fingerprinting has demonstrated that Bt strains cluster into distinct groups based on their genomic profiles, reflecting their evolutionary relationships and genetic diversity (Cherif et al., 2007).
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