Deep Conservation of Hid-Like RHG Gene Family Homologs in Winged Insects Revealed by “Taxon Hopping” BLAST

SIMPLE SUMMARY: Programmed cell death is a universal mechanism in animal development and tissue maintenance, which facilitates the elimination of surplus or poorly functioning cells. Many conserved regulators of programmed cell death have been identified in model organisms including the fruit fly Drosophila melanogaster. In the latter, the four members of the RHG gene family function as critical inducers of programmed cell death. Despite this important role, RHG genes had thus far only been found in a surprisingly small number of insect groups, i.e., other flies and butterflies. This study reports the much deeper conservation of RHG genes in winged insects, ranging from cockroaches to beetles. In addition to opening new opportunities to study programmed cell death in a wide range of insects, the bioinformatic search strategy developed for this work will be of general use for studying gene families with challenging sequence evolution dynamics. ABSTRACT: Together with sickle (skl), the Drosophila paralogs reaper (rpr), head involution defective (hid), and grim (RHG) control a critical switch in the induction of programmed cell death. RHG homologs have been identified in other dipteran and lepidopteran species but not beyond. Revisiting this issue with a “taxon hopping” BLAST search strategy in current genome and transcriptome resources, I detected high confidence RHG homologs in Coleoptera, Hymenoptera, Hemiptera, and Dictyoptera. Analyses of gene structure and protein sequence conservation revealed aconserved splicing pattern and highly conserved amino acid residues at both the N- and C-terminal ends that identify hid as the most ancestrally organized RHG gene family member in Drosophila. hid-like RHG homologs were also detected in mosquitoes, redefining their michelob_x (mx) genes as an expansion of derived RHG homologs. Only singleton homologs were detected in the large majority of other insect clades. Lepidopteran RHG homologs, however, stand out by producing an evolutionarily-derived splice isoform, identified in previous work, in addition to the newly detected hid-like isoform. Exceptional sequence diversification of select RHG homologs at the family- and genus-level explain their previous elusiveness in important insect genome model species like the red flour beetle Tribolium castaneum and the pea aphid Acyrthosiphon pisum. Combined, these findings expand the minimal age of the RHG gene family by about 100 million years and open new avenues for molecular cell death studies in insects.