Bt Research 2024, Vol.15, No.2, 76-86 http://microbescipublisher.com/index.php/bt 82 Figure 3 Fine-scale genomics and functional analysis demonstrate importance of nematocidal toxins and other genetic elements during adaptation (Adopted from Masri et al., 2015) Image caption: A, Workflow: Genomic variation of BT-679 populations was contrasted between treatments or correlated with phenotypic variation. B, mviN gene deletion and plasmid with cry toxins. C, Pathogen killing ability correlates negatively with mviN deletion frequency (left axis, filled circles) and positively with toxin plasmid copy number (right axis, open diamonds). The two most deviating values in all three considered traits were recorded for the same two populations (coevolved populations one and five, both from transfer 20, as indicated adjacent to the measured values), strongly indicating a link between reduced plasmid copy number, increased deletion frequency, and loss of virulence. D, Significant variation among the evolution treatments in population genomic statistics for the plasmid Bti_GWDALJX04I0LJH_51-405_fm319.5 (its structure is given in the outer circle) (Adopted from Masri et al., 2015) 7.2 Impact on host range and virulence Plasmid-encoded toxins significantly expand Bt's host range and enhance its virulence. The Cry toxins, for instance, are responsible for the bacterium's ability to infect a wide variety of invertebrates, including insects and nematodes (Stein et al., 2006; Malovichko et al., 2019). The presence of multiple virulence factors on plasmids allows Bt to overcome host resistance and adapt to different insect orders, thereby broadening its host range (Zheng et al., 2017). Moreover, the instability of certain toxin-encoding plasmids, such as BTI_23p, highlights the dynamic nature of Bt's virulence factors, which can be lost and reacquired, influencing the bacterium's pathogenicity (Sheppard et al., 2016; Tetreau, 2021). 7.3 Environmental persistence and spread The environmental persistence and spread of Bt are closely linked to the stability and mobility of its plasmid-encoded toxins. Studies have shown that Cry proteins from transgenic crops expressing Bt toxins can persist in soil for varying durations, affecting non-target organisms and potentially leading to ecological imbalances (Clark et al., 2005). The ability of Bt to maintain low levels of unstable plasmids, such as BTI_23p, in the absence of a host suggests a mechanism for long-term environmental persistence, allowing the bacterium to remain viable and infectious in diverse environments (Sheppard et al., 2016). Furthermore, the horizontal transfer of plasmids among Bt populations facilitates the spread of virulence traits, contributing to the bacterium's ecological success and adaptability (Méric et al., 2018).
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