An ancient, conserved gene regulatory network led to the rise of oral venom systems

Abstract

Oral venom systems evolved multiple times in numerous vertebrates enabling the exploitation of unique predatory niches. Yet how and when they evolved remains poorly understood. Up to now, most research on venom evolution has focused strictly on the toxins. However, using toxins present in modern day animals to trace the origin of the venom system is difficult, since they tend to evolve rapidly, show complex patterns of expression, and were incorporated into the venom arsenal relatively recently. Here we focus on gene regulatory networks associated with the production of toxins in snakes, rather than the toxins themselves. We found that overall venom gland gene expression was surprisingly well conserved when compared to salivary glands of other amniotes. We characterized the “metavenom network,” a network of ∼3,000 nonsecreted housekeeping genes that are strongly coexpressed with the toxins, and are primarily involved in protein folding and modification. Conserved across amniotes, this network was coopted for venom evolution by exaptation of existing members and the recruitment of new toxin genes. For instance, starting from this common molecular foundation, Heloderma lizards, shrews, and solenodon, evolved venoms in parallel by overexpression of kallikreins, which were common in ancestral saliva and induce vasodilation when injected, causing circulatory shock. Derived venoms, such as those of snakes, incorporated novel toxins, though still rely on hypotension for prey immobilization. These similarities suggest repeated cooption of shared molecular machinery for the evolution of oral venom in mammals and reptiles, blurring the line between truly venomous animals and their ancestors.

The parallel origins of vertebrate venoms

“…Such exaptation has led to the origin of vertebrate oral venoms on at least two levels. Recent work has shown that the ancestral salivary gland gene regulatory mechanisms were exapted in snake venom glands (Barua and Mikheyev 2021). We now show that individual serine protease based toxins used by diverse lineages also evolved from homologous genes.…”

“…In highly venomous snakes like vipers, the expansion of SVSP genes is linked to the diversification of the venom phenotype (Barua andMikheyev 2019, 2020) . Expansions of serine protease genes (KLK1) were observed in Solenodon as well.…”

“…So far genomes of venomous mammals and heloderma lizards are insufficiently well characterized to see whether this has been the case as well. Barua and Mikheyev (2021) proposed a unified model of early venom evolution in mammals and reptiles, suggesting that venoms evolved when kallikreins already present in saliva increased (via higher copy number) and became more effective (via sequence level changes).…”

“…Exceptionally, reptile and mammalian oral venoms are proteinaceous cocktails where each constituent toxin can be traced to a specific locus, providing an unprecedented level of genetic tractability (Rokyta et al 2012;Sunagar and Moran 2015;Casewell et al 2019;Walker 2020) . Furthermore, venoms mainly evolve through sequence and gene expression changes of their constituent toxins, providing a strong link between genetic variation and phenotypic change (Sunagar and Moran 2015;Rodríguez de la Vega and Giraud 2016;Safavi-Hemami et al 2016;Margres et al 2017;Barua and Mikheyev 2019) . Intriguingly, recent work has shown the secretion of diverse and derived toxins by snakes is powered by an ancient conserved gene regulatory network that dates back to amniote ancestors, suggesting a common model for early venom evolution (Barua and Mikheyev 2020) .…”

“…Furthermore, venoms mainly evolve through sequence and gene expression changes of their constituent toxins, providing a strong link between genetic variation and phenotypic change (Sunagar and Moran 2015;Rodríguez de la Vega and Giraud 2016;Safavi-Hemami et al 2016;Margres et al 2017;Barua and Mikheyev 2019) . Intriguingly, recent work has shown the secretion of diverse and derived toxins by snakes is powered by an ancient conserved gene regulatory network that dates back to amniote ancestors, suggesting a common model for early venom evolution (Barua and Mikheyev 2020) . Yet, it is generally believed that individual toxins are convergently recruited, particularly during the evolution of mammalian and reptilian venoms (Schachter 1969;Amine ach et al 2009;Casewell et al 2019) .…”