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.
Highlights
Programmed cell death results from the unleashed activity of caspases, a deeply conserved gene family of cysteinyl aspartate proteases
Initial searches for RHG homologs outside Diptera and Lepidoptera were conducted using the silkworm RHG homolog inhibitor of apoptosis (IAP)-binding motif 1 (IBM1) (NP_001159813.1) as a query in BLASTp searches against the National Center for Biotechnology Information (NCBI) nr database with Diptera and Lepidoptera excluded from the taxonomic search range [24]
This effort yielded low confidence hits against candidate homologs in the hemipteran species Bemisia tabaci (LOC109029550; e-value = 0.021), Nilaparvata lugens (LOC111048366; e-value = 0.005), and Laodelphax striatellus (RZF36208.1; 0.005). All of these sequences started with the RHG homology-defining IAP-binding motif (IBM) [13], were less than 300 amino acids long, and returned IBM1 as the single best hit when reBLASTed against the silkworm protein sequence database
Summary
Programmed cell death results from the unleashed activity of caspases, a deeply conserved gene family of cysteinyl aspartate proteases. First characterized for their executive role in programmed cell death in the nematode Caenorhabditis elegans [1], subsequent studies in other model organisms, i.e., Drosophila and mice, uncovered the functional conservation of caspases as executive forces in the programmed cell death pathway [2]. For instance, mitochondrial signals and members of the Bcl gene family are in control of caspase activation [3]. In Drosophila, caspases are constitutively expressed but blocked by default through physical interventions by members of the inhibitor of apoptosis (IAP) gene family [5]. Pending developmental cues or cellular stress conditions, this block is relieved by the small protein products of the RHG gene family [6], which includes the name-giving paralogs reaper (rpr), head involution defective (hid), and grim, besides sickle (skl) [7,8]
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