Considerable intraspecific geographic variation has been described in snake venom composition, which can have serious implications for snakebite therapy. Conventional theory states that venom composition is the product of phylogenetic descent; however, recent studies point towards a form of selection. This study therefore aimed to elucidate the causes of geographic variation in venom composition using Trimeresurus stejnegeri in Taiwan as a model system.
Phylogenetic and nested clade analyses of mtDNA cytb sequences (n=201) revealed two geographically overlapping clades separated by up to 5.9% corrected sequence divergence. The monophyly of the Taiwanese populations was rejected. The distribution of the lineages is discussed in relation to biogeography, conservation and systematics.
A combination of isoelectric focusing and Matrix Assisted Laser Desorption IonisationTime of Flight-Mass Spectrometric (MALDI-TOF-MS) analyses were used to characterise 216 individual T. stejnegeri venoms. Matrix correspondence tests
performed on the IEF data showed no ontogenetic variation, but revealed slight sexual and seasonal variation. Principal component analysis of the IEF data and analysis of dominant mass peak data corresponding to phospholipase A2 isoforms (from the MALDI-TOF-MS analysis) showed pronounced geographic variation in venom composition, primarily from Pacific insular populations.
Dietary analyses of this euryphagous species (n=231) showed that 183 of the snakes contained prey, and of these incidents, 43% were unidentifiable. The paucity of prey items identified presumably reflects either rapid digestion of anuran prey and/or rapid venting of faeces as an evolutionary adaptation to arboreal life. No sexual variation in diet composition was evident; however variation in diet composition was observed between the main island of Taiwan and the Pacific insular islands.
The phylogenetic and dietary analyses (in addition to geographic proximity data) were incorporated into simultaneous hypothesis testing of venom composition using partial Mantel tests and independent contrasts. Mantel tests revealed significant relationships between individual venom bands and a combination of all competing hypotheses. Only a single venom band was shown to be associated with phylogeny and overall patterns of venom variation were not concordant with phylogeographical pattern. However, a relationship between venom variation and geographic variation in diet was observed in a number of statistical tests and in overall patterns. This association, between a small proportion of venom components and diet (primarily in Green Island), may be the result of natural selection. The implications of the work are discussed in relation to the field of
snakebite therapy, use of venoms as phylogenetic markers, and the emerging field of the study of venom evolution.