Abstract
Robber flies are an understudied family of venomous, predatory Diptera. With the recent characterization of venom from three asilid species, it is possible, for the first time, to study the molecular evolution of venom genes in this unique lineage. To accomplish this, a novel whole-body transcriptome of Eudioctria media was combined with 10 other publicly available asiloid thoracic or salivary gland transcriptomes to identify putative venom gene families and assess evidence of pervasive positive selection. A total of 348 gene families of sufficient size were analyzed, and 33 of these were predicted to contain venom genes. We recovered 151 families containing homologs to previously described venom proteins, and 40 of these were uniquely gained in Asilidae. Our gene family clustering suggests that many asilidin venom gene families are not natural groupings, as delimited by previous authors, but instead form multiple discrete gene families. Additionally, robber fly venoms have relatively few sites under positive selection, consistent with the hypothesis that the venoms of older lineages are dominated by negative selection acting to maintain toxic function.
Highlights
Venoms are typically a composition of various neurotoxins, enzymes, ions, and small organic molecules [1,2,3,4]
The Eudioctria media transcriptome was assembled into 103,352 contigs
We used Families Under Selection in Transcriptomes (FUSTr) to generate putative gene families derived from thoracic and whole-body transcriptomes and to identify which of these families had evidence of positive selection
Summary
Venoms are typically a composition of various neurotoxins, enzymes, ions, and small organic molecules [1,2,3,4]. Venom proteins generally originate via gene duplication of non-toxic proteins that are selectively expressed in a venom gland (recruitment) and undergo subsequent neofunctionalization [3,5] Alternative processes such as single gene co-option or de novo protein evolution are believed to be significant drivers of venom evolution in some taxa [6]. Venom proteins are often recruited from secretory proteins involved in rapid physiological processes and from those that have stable tertiary structures due to multiple disulfide bonds [7] Among these polypeptide components, many neurotoxins have abundant inhibitory cystine knots (ICKs) while venom enzymes are larger and, owing to a different role in the venom cocktail, typically lack this motif [7].
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