Analysis of expression patterns of SiMLs in Solenopsis invicta in immune response to different pathogens

Related lipid recognition proteins, which compose of a class of proteins featuring a single ML domain, play a crucial role in lipid recognition and innate immune signaling pathways. Numerous ML protein family members exist in arthropods, exhibiting complex functions. These ML proteins researches primarily focus on the interactions between hosts and viruses. However, studies investigating the interaction mechanisms of ML proteins in invasive insects and pathogens remain scarce. Specifically, the function of ML protein in invasive insects have not been documented. Five ML genes (SiML1~5) were identified based on the genome of S. invicta and laboratory transcriptome data. Bioinformatics analyses were conducted on the S. invicta ML family, which included a signal peptide and a typical ML domain. The ML domain encompasseed nearly the entire coding region of SiML1 (25~151 aa), SiML2 (23~150 aa), SiML3 (24~145 aa), SiML4 (21~150 aa) and SiML5 (58~175 aa), and contained six conserved cysteine residues. Phylogenetic analysis revealed that SiML1, SiML2, SiML3 and SiML4 were grouped in the same clade, showing close relation to Macrosteles quadrilineatus (MqML). SiML5 and Microplitis mediator (MmML3) were grouped in the same clade. Results from fluorescence quantitative PCR (RT-qPCR) indicated that SiMLs family genes were transcribed in six tissues of S. invicta, with the highest expression in the fat body. These genes were expressed throughout the developmental stages and exhibited differential expression which mainly up-regulated during egg, larva, pupa and adult metamorphosis, suggesting that ML proteins played a role in the metamorphic development of S. invicta. Immune induction with bacterial and fungal pathogens was done by spraying or soaking the major worker ants. RT-qPCR results demonstrated that the expression of ML family members in major worker ants was up-regulated significantly 3 h to 48 h after induction by Escherichia coli (P<0.05). Following induction by Metarhizium anisopliae and Beauveria bassiana, the expression of ML family members increased in the early stage (3 h to 12 h) but inhibited in the later stage (24 h to 48 h). These results indicated that the SiMLs family in major worker ants could respond to pathogenic bacterial infections, with distinct immune response modes observed among different pathogens. Furthermore, the findings suggest that S. invicta SiMLs can effectively respond to pathogen invasion, exhibiting varied expression patterns in response to different pathogens. This study lays the groundwork for future functional investigations of SiMLs proteins.