Bt Research 2024, Vol.15, No.1, 20-29 http://microbescipublisher.com/index.php/bt 23 a highly variable genome, with a core set of genes essential for basic cellular functions and a pan-genome that includes strain-specific genes contributing to their adaptability and pathogenicity. For instance, the genome of Bt strain BM-Bt15426 contains 5 409 predicted genes, highlighting the extensive genetic repertoire of Bt strains (Liu et al., 2017). Additionally, the comparative genomic analysis of nearly 900 Bt strains has led to the proposal of two distinct genomovars, B. thuringiensis gv. thuringiensis and B. thuringiensis gv. cytolyticus, based on core gene sequences (Baek et al., 2019). 4.2 Gene content and functional categories Bt strains are characterized by a diverse array of genes encoding for various functional categories, including virulence factors, insecticidal proteins, and metabolic enzymes. For example, the genome of Bt strain GR007 contains multiple pesticidal protein genes, including 10 Cry genes, two Vip genes, and several virulence factors (Pacheco et al., 2021). Similarly, Bt strain HD521 encodes eight types of virulence protein factors and several insertion sequences and prophage sequences, indicating a complex genomic architecture (Sun et al., 2021). The presence of genes related to antibiotic resistance and heavy metal resistance in Bt strain HM-311 further underscores the functional versatility of Bt genomes (Zuo et al., 2020). 4.3 Mobile genetic elements Mobile genetic elements, such as plasmids, transposons, and prophages, play a crucial role in the genomic plasticity and horizontal gene transfer in Bt strains. The genome of Bt strain GR007, for instance, includes three megaplasmids that harbor multiple pesticidal protein genes and virulence factors (Pacheco et al., 2021). Bt strain HS18-1 contains nine circular plasmids, encoding various virulence factors and insertion sequences (Sun et al., 2021). The presence of numerous plasmids and transposons in Bt genomes facilitates the acquisition and dissemination of beneficial traits, such as insecticidal activity and antibiotic resistance, thereby enhancing their adaptability to diverse environments (Wang et al., 2018; Reyaz et al., 2019). 5 Evolutionary Mechanisms in Bt Strains 5.1 Horizontal gene transfer Horizontal gene transfer (HGT) plays a significant role in the evolution of Bacillus thuringiensis (Bt) strains. The presence of mobile genetic elements such as bacteriophages, insertion sequences (IS elements), and transposases in Bt genomes suggests that these elements facilitate the transfer of genes between different strains and species. For instance, prophages have been identified as likely candidates for the mobilization of chromosomally encoded cry-toxins in Bt strains, which are crucial for their virulence and adaptability to various ecological niches (Cao et al., 2018). Additionally, the genome of Bt strain BM-Bt15426 contains multiple virulence factors and antibiotic resistance genes, indicating the acquisition of these traits through HGT (Schäfer et al., 2023). 5.2 Gene duplication and diversification Gene duplication and subsequent diversification are key mechanisms driving the evolution of Bt strains. The presence of multiple copies of Cry-toxin genes on plasmids and chromosomes in different Bt strains highlights the importance of gene duplication in expanding the functional repertoire of these bacteria. For example, the high virulence of Bt strain MYBt18679 was associated with elevated copy numbers of plasmids containing nematicidal toxin genes, which were favored during pathogen-host coevolution. Furthermore, comparative genomic analyses have revealed the irregular distribution of pesticidal genes among Bt strains, suggesting that gene duplication and diversification contribute to the genetic diversity observed within this species (Zhou et al., 2024). 5.3 Adaptive evolution and natural selection Adaptive evolution and natural selection are crucial in shaping the genetic landscape of Bt strains. Host-parasite coevolution experiments have demonstrated that high virulence is specifically favored during the coevolution of nematicidal Bt strains with their host organism, Caenorhabditis elegans. This adaptive process involves real-time genetic changes, including the fixation of specific cry-toxin genes in response to selective pressures. Additionally, the identification of genes involved in the biosynthesis of antimicrobial compounds, such as zwittermicin and
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