Bt Research 2025, Vol.16, No.2, 63-69 http://microbescipublisher.com/index.php/bt 64 2.2 Structural features of plasmids relevant to mobility Many plasmids carrying the cry gene are large in size and often contain repetitive sequences, transposase genes and conjugation-related elements. These structures can promote the transfer and recombination of plasmids between strains (Whiteley and Schnepf, 1986; Wang et al., 2020). cry genes are often surrounded by multiple copies of repetitive DNA elements, which carry transposases and are helpful for the transfer of genes between different plasmids or between plasmids and chromosomes. Wang et al. ’s research in 2020 found that some conjugation-related genes on plasmids can enable them to achieve self-transfer through conjugation. 2.3 Comparative genomics insights into plasmid diversity in Bt populations Comparative genomics studies have found that the plasmid diversity among Bt strains is very high. Bt strains from different regions and ecological environments often carry different types and quantities of plasmids. For instance, among the 700 Bt isolates in the Qatar region, there were as many as 7 types of plasmid profiles, and they were closely related to the types of insecticidal crystalline proteins (Nair et al., 2018). In some cry gene-positive strains, there was only one 15 kb plasmid, while in others, there were multiple plasmids of different sizes (Apaydin et al., 2005). Whole-genome sequencing also revealed that cry9Aand vip3Agenes coexist on some plasmids, indicating that plasmid recombination and gene insertion events are common and promoting the formation of insecticidal protein diversity (Wang et al., 2020). These findings all indicate that plasmids play a core role in the genetic diversity and functional innovation of Bt strains. 3 Mechanisms of Plasmid-Mediated Horizontal Gene Transfer 3.1 Conjugation as the primary route of plasmid transfer In Bt strains, conjugation is the main way for plasmid-mediated horizontal gene transfer (HGT). Conjugated plasmids encode a specialized transfer system that enables direct contact between donor and recipient cells, efficiently transferring plasmid DNA to the new host. This process not only accelerates the spread of functional genes such as resistance and virulence, but also enables bacteria to adapt to environmental changes more quickly and promotes evolution (Sun, 2018; Tokuda and Shintani, 2024). Both experimental and genomic studies have found that conjugate plasmids are common in many bacterial populations and are the main driving force of HGT (Figure 1) (Harrison and Brockhurst, 2012; Che et al., 2021; Tokuda and Shintani, 2024). 3.2 Role of transposons, integrons, and mobile elements in gene mobilization Mobile genetic elements (MGEs) such as transposons, integrons and insertion sequences can insert resistance or virulence genes into different positions of plasmids or chromosomes, promoting gene recombination and diversification. Studies have found that most resistance genes on plasmids are closely related to insertion sequences and integrons, forming complex gene transfer networks and greatly expanding the range and speed of gene transmission (Sun, 2018). Che et al. (2021) as well as Tokuda and Shintani (2024) all hold that the combined effect of these MGEs makes plasmids the most active "gene transporters" in the bacterial gene library. 3.3 Conditions that promote plasmid transfer within and betweenBt strains The biofilm environment is regarded as a "hotspot" for HGT because cells are densely aggregated here and the microenvironment is more stable, which is beneficial for conjugation and the activities of MGEs. Environmental stresses such as antibiotics selectively promote the diffusion of plasmids carrying beneficial genes (Figure 2) (Harrison and Brockhurst, 2012; Tokuda and Shintani, 2024). Ecological interactions, nutritional conditions and population density among strains can also affect the efficiency of plasmid transfer (Sun, 2018). These factors jointly determine the frequency and range of plasmid-mediated HGT, which are important regulatory forces for the genetic diversity and adaptive evolution of Bt strains. 4 Functional Contributions of HGT via Plasmids 4.1 Acquisition and dissemination of insecticidal toxin genes (Cry, Cyt, Vip) Plasmid-mediated horizontal gene transfer (HGT) is the main way for Bt strains to acquire and disseminate insecticide toxin genes (such as Cry, Cyt, Vip). These genes are usually on mobile plasmids. They can be efficiently transferred between different strains through conjugation or natural transformation. In this way, the Bt
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