Identification and characterization of <i>Spätzle</i> in <i>Myzus persicae</i> and its role during microbial infection

As a Toll receptor ligand, Spätzle (Spz) plays a crucial role in activating the Toll pathway and participating in the innate immune response of insects. However, the immune function of Spz in Myzus persicae remains poorly understood. In this study, we identified and cloned 7 Spz genes from M. persicae, all containing a Spz domain (cystine-knot domain). Phylogenetic analysis revealed that the 7 different MpSpzs were divided into 6 groups within a single cluster with each Spz of Drosophila melanogaster and Acyrthosiphon pisum. These genes were mainly expressed in 1st-instar nymphs, hemolymph, and embryos and showed varying levels of positive response to infection with Escherichia coli, Staphylococcus aureus, and Beauveria bassiana. After gene-silencing of MpSpzs by RNA interference with injection of target gene-specific double-stranded RNA, microbial infection significantly increased the mortality of M. persicae compared to control groups. Further studies revealed that the suppression of MpSpz resulted in a significant reduction in lysozyme expression. The present study offers novel insights into the role of Spätzle in the innate immune response against microbial infection in M. persicae.