Bt Research 2024, Vol.15, No.1, 42-52 http://microbescipublisher.com/index.php/bt 47 larvae via the oral route was used. Significant physical deterioration of the larvae after toxin injection was highlighted, including melanization of the hemolymph and extended abdominal feet, while control larvae appeared normal. Dose-response curves (C and D) showed that the survival of both insects decreased with increasing doses, with Cry1AcF being more effective than TcaB. Statistical analysis confirmed the significance of these findings, highlighting the potential of Cry1AcF as an effective biological control agent for these agricultural pests. 6.2 Studies in coleoptera Research on coleopteran insects, such as the cottonwood leaf beetle (Chrysomela scripta), has revealed that Bt toxins like Cry1Ba1 exhibit significant toxicity against these pests. The interaction between Cry toxins and midgut receptors in coleopterans is less understood compared to lepidopterans, but it is known that similar receptor families, including cadherin, APN, and ALP, are involved (Zhong et al., 2000; Domínguez-Arrizabalaga et al., 2020). For example, Cry1Ba1 has been shown to bind specifically to receptors in the midgut of coleopteran larvae, leading to mortality at low to moderate concentrations (Zhong et al., 2000). This indicates that while the specific interactions may vary, the overall mechanism of receptor-mediated toxicity is conserved across different insect orders. 6.3 Comparative analysis of different insect orders Comparative studies across different insect orders, including Lepidoptera, Coleoptera, and Diptera, have shown that Bt toxins interact with a conserved set of midgut receptors, such as cadherin, APN, and ALP (Bravo et al., 2011; Likitvivatanavong et al., 2011; Li et al., 2021). For instance, a genome-wide analysis of Cry toxin receptor families in seven insect species revealed that ALPs and APNs are divided into multiple clades, with certain clades containing multiple paralogs within each species. This suggests a conserved evolutionary mechanism for Cry toxin susceptibility. Additionally, the identification of multiple receptors in mosquitoes, such as Aedes aegypti, further supports the idea that Cry toxin-receptor interactions are a common theme across diverse insect taxa (Likitvivatanavong et al., 2011). These comparative analyses provide valuable insights into the broad-spectrum efficacy of Bt toxins and the potential for cross-order resistance management strategies. 7 Implications for Insect Specificity 7.1. Determinants of insect specificity The specificity of Bacillus thuringiensis (Bt) toxins towards different insect species is primarily determined by the interaction between the toxins and specific receptors in the insect midgut. For instance, the Cry1Ab toxin interacts with the cadherin-like receptor Bt-R1 in Manduca sexta through specific epitopes, such as the (865) NITIHITDTNN (875) sequence and loop 2 of domain II in the toxin (Gómez et al., 2003). Additionally, the role of ATP-binding cassette (ABC) transporters, particularly ABCC2 and ABCC3, has been highlighted in various studies. These transporters are crucial for the binding and subsequent toxicity of Cry1 toxins in insects like Bombyx mori and Plutella xylostella (Guo et al., 2019; Wang et al., 2021). The presence or absence of these receptors and transporters in different insect species determines the specificity and effectiveness of Bt toxins. 7.2 Evolution of receptor specificity The evolution of receptor specificity is a dynamic process influenced by the continuous interaction between Bt toxins and insect receptors. Insects can develop resistance through mutations in these receptors, which alter the binding affinity of the toxins. For example, mutations in the cadherin gene in Manduca sexta and other insects have been linked to reduced susceptibility to Cry1Ab toxin (Figure 3) (Soberón et al., 2007). Moreover, the evolution of Bt toxins themselves, such as the development of Cry1Ac variants with higher affinity for non-native receptors, demonstrates an adaptive response to overcome insect resistance (Badran et al., 2016). This co-evolutionary arms race between Bt toxins and insect receptors underscores the importance of understanding receptor specificity to develop more effective pest control strategies.
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