Genomic basis of adaptation to cardiac glycosides in three insect orders

Insects across different orders have convergently evolved adaptations to toxic cardiac glycosides (CGs), which are derived either from their diet or via endogenous synthesis. Previous studies on CG-resistance focused on changes in ATPα that is the direct inhibition target of CGs. Adaptation of whole organisms to toxic CGs could involve orchestrated changes at multiple genes and at multiple biological levels. Here, we explore this possibility by using whole genome sequences to identify several signatures of molecular convergence across multiple CG-adapted species. We identify gene families that changed convergently in CG-adapted species, including one member of stable fatty acyl-CoA reductase, CG5065, carboxylesterases and gustatory receptors that expanded in two of the three species. We find a number of candidate genes under positive selection in all CG-adapted species. We also identify convergent amino acid substitutions that have independently evolved in CG-adapted insects, including a conserved gene involved in the septate junction, Gliotactin (Gli). We used CRISPR-Cas9 to generate viable, homozygous Gli knock-in Drosophila lines with the convergent substitution. Through egg-larva and larva-adult survival experiments, we found that mutant flies consistently exhibit a lower survival rate compared to wild-type lines. Transmission electron microscopy (TEM) analysis of stage 17 embryos showed that in Gli mutants, the dihedral angles of bicellular membranes near the tricellular junction (TCJ) were unequal, and electron-dense materials were absent in the TCJ center. We propose that this convergently evolved Gli variant may contribute to CG adaptation by modulating epithelial permeability, potentially facilitating the sequestration of toxic CGs.