Evolution of venom, one of nature’s most complex cocktails, has underpinned the predatory success of venomous animals. Until today, venom research has mostly focused on how genes coding venom proteins evolve, and our knowledge regarding the evolutionary origin of cells that produce venom and the mechanisms of injecting venom have been very limited. Understanding how venom and venom-secreting cells (VSCs) evolve in Cnidaria – which are amongst the first venomous animals, is not only fascinating from an evolutionary perspective but could also prove to be instrumental in the development of efficient nematocyte (or stinging cell) based drug-delivery tools. Identifying variation in venom composition and sites of expression across various developmental stages and between the sexes (gender-specific expression) has also been a much neglected area of research. The former is particularly intriguing in sea anemones, as their larval stages do not feed, and yet, synthesize toxins.
Therefore, the major aims of this project were to 1) understand the origin and evolution of venom producing cells in Cnidaria; 2) characterize their expression profiles; and 3) identify variability in venoms across developmental stages, and between the sexes.
To address aims 1 and 2, we generated transgenic lines of the sea anemone Nematostella vectensis by modifying their genomes in such a way that VSCs produce a fluorescent protein. We then developed proteolytic cocktails that dissociate tissues and liberate individual cells. Following dissociation, the fluorescent VSCs were separated from all other types of cells using a Fluorescence Assisted Cell Sorter (FACS). We then sequenced the transcriptomes of these separated cell populations and employed bioinformatic analyses to identify genes that are differentially expressed (over- and underexpressed) within nematocytes, in comparison to the other types of cells. Our innovative strategy provided fascinating insights into the evolution of VSCs in Cnidaria. We have generated for the first time, the transcriptomic profiles of nematocytes in these animals. We have demonstrated that these evolutionarily unique cells develop through a number of transitionary stages, during which, they express different sets of toxins and physiological genes. We have made several other fascinating discoveries that will soon be submitted for publication in high-profile journals.
Citation: Sunagar K*, Columbus-Shenkar Y*, Fridrich A, Gutkovich N, Aharoni N, and Moran Y. Cell type-specific expression profiling sheds light on the development of a peculiar neuron, housing a complex organelle (in preparation).
* Equal contribution
For understanding the variability in venoms across different developmental stages, and between the sexes of N. vectensis (aim 2), we employed NanoString nCounter gene expression assay. Here, transcripts encoded by a large number of selected genes in various developmental stages and sexes were counted in a high-throughput manner. Our analyses revealed that toxin proteins are expressed throughout the development of this animal and that at least one class of toxin is maternally deposited in the eggs. Our results unravel a much more complex and dynamic venom landscape than initially appreciated, and highlight the importance of studying ontogenetic variability in venoms.
Citation: Columbus-Shenkar Y*, Sachkova M*, Fridrich A, Modepalli V, Sunagar K, and Moran Y. Dynamics of venom composition across a complex life cycle (under review).
* Equal contribution
Other research highlights
1. Widespread convergence in toxin resistance by predictable molecular evolution: “Convergence has a strong bearing on the fundamental debate about whether evolution is stochastic and unpredictable or subject to constraints. Here we show that, in certain circumstances, evolution can be highly predictable. We demonstrate that several lineages of insects, amphibians, reptiles, and mammals have utilized the same molecular solution, via the process of convergence, to evolve resistance to toxic cardiac glycosides produced defensively by plants and bufonid toads.”
Citation: Ujvari B*, Casewell NR*, Sunagar K*, Arbuckle K, Wuster W, Lo N, O'Meally D, Beckmann C, King GF, Deplazes E, et al. 2015. Widespread convergence in toxin resistance by predictable molecular evolution. Proc Natl Acad Sci U S A 112:11911-11916.
* Equal contribution
2. The Rise and Fall of an Evolutionary Innovation - contrasting Strategies of Venom Evolution in Ancient and Young Animals: “While the influence of positive selection in diversifying animal venoms is widely recognized, the role of purifying selection that conserves the amino acid sequence of venom components such as peptide toxins has never been considered. Here, we highlight the significant role of purifying selection in shaping the composition of animal venoms. Analysis of numerous toxin families, spanning the breadth of the animal kingdom, has revealed a striking contrast between the evolution of venom in ancient and evolutionarily young animal groups. Our findings enable the postulation of a new theory of venom evolution.”
Citation: Sunagar K, Moran Y. 2015. The Rise and Fall of an Evolutionary Innovation: Contrasting Strategies of Venom Evolution in Ancient and Young Animals. PLoS Genet 11:e1005596.
