Evolution of a Cytoplasmic Determinant: Evidence for the Biochemical Basis of Functional Evolution of the Novel Germ Line Regulator Oskar

Abstract

Germ line specification is essential in sexually reproducing organisms. Despite their critical role, the evolutionary history of the genes that specify animal germ cells is heterogeneous and dynamic. In many insects, the gene oskar is required for the specification of the germ line. However, the germ line role of oskar is thought to be a derived role resulting from co-option from an ancestral somatic role. To address how evolutionary changes in protein sequence could have led to changes in the function of Oskar protein that enabled it to regulate germ line specification, we searched for oskar orthologs in 1565 publicly available insect genomic and transcriptomic datasets. The earliest-diverging lineage in which we identified an oskar ortholog was the order Zygentoma (silverfish and firebrats), suggesting that oskar originated before the origin of winged insects. We noted some order-specific trends in oskar sequence evolution, including whole gene duplications, clade-specific losses, and rapid divergence. An alignment of all known 379 Oskar sequences revealed new highly conserved residues as candidates that promote dimerization of the LOTUS domain. Moreover, we identified regions of the OSK domain with conserved predicted RNA binding potential. Furthermore, we show that despite a low overall amino acid conservation, the LOTUS domain shows higher conservation of predicted secondary structure than the OSK domain. Finally, we suggest new key amino acids in the LOTUS domain that may be involved in the previously reported Oskar-Vasa physical interaction that is required for its germ line role.

Gene Editing and Genetic Control of Hemipteran Pests: Progress, Challenges and Perspectives

“…The Drosophila oskar gene encodes a germ granule protein that is synthesized at the posterior pole of the oocyte. Oskar initiates germplasm assembly and germ cell determination; this gene appears to have been lost in the Hemiptera ( Ephrussi and Lehmann, 1992 ; Lynch et al, 2011 ; Quan and Lynch, 2016 ; Blondel et al, 2021 ). For this reason, it is expected that there should be fundamental differences in the transcriptomes of embryos during oogenesis and early embryogenesis.…”

oskaracts with the transcription factor Creb to regulate long-term memory in crickets

“…This is consistent with the hypothesis that osk played an ancient neural role of some kind in insects. We previously showed evidence supporting the hypothesis that evolved changes in the biophysical characteristics of Oskar protein may have driven the evolution of a novel mechanism of germ line specification in the holometabolous insects (16). A broader understanding of the putative ancestral and derived function(s) of osk thus requires additional studies of phylogenetically diverse insects, as well as further detailed biochemical analysis in the context of Drosophila germ cells and neurons.…”

“…Indeed, many different highly conserved “germ line genes” including vasa , nanos , and piwi are found in a variety of multipotent cells in diverse animals(14, 53), raising the possibility that such genes were involved in establishing multipotency rather than specifying germ cell fate per se . In our previous examination of the distribution of osk orthologs across insects, we observed that crickets are not the only insects reported to express osk in the brain, as osk transcripts are detected in transcriptomes from the brains of cockroach, wasp and beetle species as well (16). This is consistent with the hypothesis that osk played an ancient neural role of some kind in insects.…”

“…We assessed expression of these genes in brains and gonads using previously published RNA-seq libraries (55) available at NCBI (PRJNA564136). We analyzed the RNA-seq data as in (16), including removing adapters and reads shorter than 20 nucleotides with Cutadapt v3.4 (65) and quantifying the gene expression in transcripts per million (TPM) with RSEM v1.2.29 (66), using STAR v2.7.0e1 (67) as read mapper against the G. bimaculatus genome (36) (Suppl. Table S8).…”

“…Furthermore, osk plays a role in the embryonic nervous system, but not in the germ line, in a basally branching hemimetabolous insect, the cricket Gryllus bimaculatus , where it is important for proper neuroblast divisions and subsequent axonal patterning 15 . Our recent analysis of hundreds of previously unidentified osk orthologs across insects showed that osk is expressed in at least a dozen somatic tissues in species across the insect tree 16 . This suggests that a somatic function of osk may be ancestral.…”