Bt Research 2025, Vol.16, No.2, 47-54 http://microbescipublisher.com/index.php/bt 50 4.2 Niche specialization: adaptation to different insect hosts and environments The diversity of plasmids directly promotes the adaptation of Bt strains to different ecological niches. Functional genes on different plasmids, such as toxin, stress resistance or metabolism-related genes, enable the strain to cope with multiple insect host and environmental stresses (De Maayer et al., 2012; Ciok et al., 2018). For instance, some plasmids are equipped with regulatory systems that can control spore formation and survival patterns, thereby facilitating better reproduction and diffusion of strains in insect corpses (Perchat et al., 2024). Furthermore, in extreme environments, soils and hosts, different plasmids carry specific adaptive genes, which enable the strains to undergo ecological differentiation and expand their niches (Davray et al., 2020; Koch et al., 2020; Margos et al., 2023). 4.3 Contribution to strain competition and persistence in natural ecosystems Plasmids can not only provide competitive advantages such as antibiotics, heavy metal resistance and bacteriocins, but also utilize mechanisms such as the toxin-antitoxin system to ensure their stable presence in the population and avoid being lost (Ciok et al., 2018; Perchat et al., 2024). In natural ecosystems, the diversity of plasmids enhances the competitiveness and long-term viability among strains, enabling them to maintain and expand in complex microbial communities (Heuer and Smalla, 2012; Finks et al., 2024). Furthermore, plasmids promote gene exchange and functional complementarity among strains, and also drive ecological synergy, thereby further enhancing the adaptability and ecological functions of the population (Smalla et al., 2015; Koch et al., 2020). 5 Impact of Plasmid Diversity on Field Performance of Bt 5.1 Variations in insecticidal activity among strains with different plasmid profiles The plasmid diversity of Bt strains will directly affect their insecticidal ability. Different plasmid spectra carry different δ -endotoxin genes, which determine the targeting and virulence of the strains against pests. Studies have shown that Bt strains of different plasmid types vary greatly in the coding and expression of insecticidal proteins, so their insecticidal effects on Diptera, Lepidoptera and Coleoptera are also different. Some strains have particularly good insecticidal effects on certain pests, while others exhibit new insecticidal spectra, showing potential for the development of new biological insecticides (Nair et al., 2018). Therefore, the diversity of plasmid types provides abundant resources for the selection and utilization of strains with specific insecticidal abilities (Das and Dangar, 2007). 5.2 Influence on strain stability and persistence in soil and plant environments In addition to carrying insecticidal genes, plasmids may also affect the adaptability and stability of Bt strains in the natural environment. Some studies have not found a direct relationship between plasmid type and phenotypic characteristics such as crystal form or salt tolerance. However, Bt strains isolated in different environments often have different plasmid spectra, which indicates that plasmid diversity helps them survive for a long time in complex environments (Das and Dangar, 2007). In addition, plasmids can also be transferred, which enables Bt strains to acquire new adaptive characteristics through gene exchange, thereby enhancing competitiveness and ecological stability in soil and plant environments (Nair et al., 2018). 5.3 Relevance to resistance management in target insect populations The management of plasmid diversity antagonism is also very important. Because insecticidal genes mainly exist on transferable plasmids, recombination between different strains will produce new protein combinations. This diversity slows down the rate at which pests develop resistance to a single toxin. Research indicates that the continuous screening and utilization of diverse Bt resources on a global scale can provide more options for resistance management and reduce the risk of the emergence of resistant pests (Nair et al., 2018). Meanwhile, plasmid diversity can also provide genetic resources for the development of multi-toxin compound Bt formulations, which helps to enhance the persistence and effectiveness of field control. 6 Case Study: Plasmid Diversity and Adaptation in Representative Bt Strains 6.1 Selection of representative strains and plasmid profiles When studying the diversity of Bt strains, the selection of representative strains is often based on the morphology
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