Bt Research 2024, Vol.15, No.3, 141-153 http://microbescipublisher.com/index.php/bt 148 The study of genetic divergence and relationships can inform the development of more effective Bt-based biopesticides by identifying strains with unique and potent insecticidal properties. The evolutionary insights gained from phylogenetic and genomic analyses also help in predicting the emergence of resistance in target pests and developing strategies to mitigate this issue. Understanding the genetic basis of host specialization and adaptation in Bt can lead to the discovery of novel biocontrol agents with broader or more specific target ranges, enhancing the sustainability of pest management programs. Overall, the genetic study of Bt strains provides valuable knowledge that drives the evolution of more efficient and adaptable biocontrol solutions. 7 Horizontal Gene Transfer and Recombination 7.1 Role of horizontal gene transfer horizontal gene transfer (hgt) plays a pivotal role in the evolution and diversification of Bacillus thuringiensis (Bt). It facilitates the rapid acquisition of new genes, including those encoding insecticidal toxins, which enhance the bacterium's ability to adapt to different environmental niches and hosts. HGT occurs through several mechanisms, including conjugation, transformation, and transduction. For instance, the large conjugative plasmid pXO16 from Bt serovar israelensis demonstrates the ability to transfer genes efficiently between different Bt strains and even other Bacillus species. This plasmid can mobilize and retro-mobilize non-conjugative plasmids, enhancing genetic diversity and adaptability (Makart et al., 2017). The transfer of plasmids carrying mosquitocidal toxin genes, such as the 144-kb plasmid pTAND672-2, underscores the importance of HGT in spreading beneficial traits among bacterial populations (Geng et al., 2023). These plasmids often carry genes for toxins like Cry and Cyt proteins, which are crucial for Bt's insecticidal properties, thereby significantly contributing to the bacterium's ecological success. 7.2 Evidence of recombination events Recombination events are integral to the genetic evolution of Bt, allowing for the reshuffling of genetic material and the creation of novel gene combinations. Evidence of recombination is often observed in the genetic structure of Bt strains, where segments of DNA are exchanged between different plasmids or between plasmids and the chromosome. For example, the plasmid pXO16 has been shown to transfer chromosomal markers at significant frequencies without the need for integration into the chromosome, indicating a unique mobilization mechanism (Makart et al., 2017). Studies have demonstrated the integration and circularization of plasmids like pTAND672-2 into the chromosome of recipient bacteria, highlighting the role of site-specific recombination in gene transfer (Geng et al., 2023). Another study by Wang et al. (2016) described the use of the Mob/oriT recombination system for markerless genetic manipulation in Bt, demonstrating the practical applications of recombination events in genetic engineering. These recombination events contribute to the genetic plasticity of Bt, enabling it to acquire and maintain a diverse array of genes that enhance its survival and ecological versatility (Figure 3). 7.3 Impact on genetic diversity The impact of horizontal gene transfer and recombination on genetic diversity in Bt is profound. These processes introduce new genetic material into bacterial populations, increasing genetic variation and enabling rapid adaptation to changing environments. The exchange of plasmids carrying toxin genes, for example, enhances the insecticidal capabilities of Bt strains, allowing them to target a wider range of insect pests. This genetic diversity is crucial for the long-term effectiveness of Bt as a biocontrol agent, as it helps prevent the development of resistance in target insect populations. The study by Hinnekens et al. (2019) on the extended host spectrum of pXO16 demonstrates how plasmid transfer can broaden the ecological niches that Bt can occupy. The genetic manipulation techniques developed by Wang et al. (2016) and others leverage recombination to create genetically enhanced Bt strains with improved biocontrol properties. The continuous influx of new genes through HGT and recombination ensures that Bt remains a dynamic and adaptable organism, capable of evolving in response to environmental pressures and maintaining its role as a key player in biological pest control.
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