Use this URL to cite or link to this record in EThOS:
Title: A comparative analysis of the influence of defensive venom spitting on the composition and function of cobra venoms
Author: Kazandjian, Taline
ISNI:       0000 0004 9359 1005
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2020
Availability of Full Text:
Access from EThOS:
Access from Institution:
Snake venoms are a complex cocktail of toxin proteins, the evolution of which is driven foremostly by the need to swiftly incapacitate or immobilise prey. Spitting cobras possess a defensive use for their venom, unique to these snakes, in the ability to deploy their venoms as a spray projectile into the eyes of an assailant, and this ability has evolved three times in elapid snakes (venomous, bearing fixed, posteriorly-located fangs). Spitting cobras show differences to non-spitting cobras in fang morphology and in clinical pathology following bites to humans, suggesting that there are underlying toxin composition differences between spitting and non-spitting cobras. This work represents the first of its kind; a large-scale and multidisciplinary comparative analysis incorporating the true cobras (genus: Naja) and its spitting sister taxa, the rinkhals/ring-necked spitting cobra (Hemachatus haemachatus), with a focus on examining patterns of venom evolution associated with the ability to spit venom. To do this, analysis of 17 species (14 species of Naja, Hemachatus haemachatus, and the outgroup elapid snakes Walterinnesia aegyptia and Aspidelaps scutatus), covering all lineages of spitting and non-spitting cobra, was undertaken at the transcriptome, proteome and functional levels and these put into evolutionary context by using phylogenetic statistical analyses. Additionally, the composition of spat venom versus venom extracted manually (simulating venom injected via biting) was compared to identify any control mechanisms at the morphological level that may alter venom composition when spitting. The data here show that, while there are no differences in venom composition between spitting and non-spitting cobras at the transcriptome level, African spitters show evidence of a phospholipase A2 (PLA2) gene duplication. Additionally, spitting cobras have a significantly higher abundance of PLA2 toxins than non-spitting cobras at the proteome level (Phylogenetic Generalized Least Squares, t = 4.24, p = 0.0007). Furthermore, cobra venom PLA2 proteins group by lineage based on amino acid composition, with African spitting cobras and Hemachatus haemachatus forming one group, Asian cobras another, and African non-spitters a third. These findings suggest that spitting cobra lineages have convergently upregulated PLA2sin their venoms following divergent selection on PLA2 amino acid composition between African and Asian cobras. African spitting cobras have upregulated the abundance of PLA2 in the venom proteome through gene duplication, while Asian spitting cobras have upregulated PLA2 abundance through posttranslational means. These data provide the first evidence that defensive uses of venom can drive the evolution of snake venom composition, and that the convergent emergence of a unique behavioural adaptation can result in convergent molecular evolution.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral