Centre for Snakebite Research & Interventions, Liverpool School of Tropical Medicine, Pembroke Place, Liverpool, L3 5QA, UK.
Centre for Advanced Imaging, University of Queensland, St Lucia, QLD, 4072, Australia.
BMC Biol. 2022 Jun 27;20(1):148. doi: 10.1186/s12915-022-01350-y.
Venoms are ecological innovations that have evolved numerous times, on each occasion accompanied by the co-evolution of specialised morphological and behavioural characters for venom production and delivery. The close evolutionary interdependence between these characters is exemplified by animals that control the composition of their secreted venom. This ability depends in part on the production of different toxins in different locations of the venom gland, which was recently documented in venomous snakes. Here, we test the hypothesis that the distinct spatial distributions of toxins in snake venom glands are an adaptation that enables the secretion of venoms with distinct ecological functions.
We show that the main defensive and predatory peptide toxins are produced in distinct regions of the venom glands of the black-necked spitting cobra (Naja nigricollis), but these distributions likely reflect developmental effects. Indeed, we detected no significant differences in venom collected via defensive 'spitting' or predatory 'biting' events from the same specimens representing multiple lineages of spitting cobra. We also found the same spatial distribution of toxins in a non-spitting cobra and show that heterogeneous toxin distribution is a feature shared with a viper with primarily predatory venom.
Our findings suggest that heterogeneous distributions of toxins are not an adaptation to controlling venom composition in snakes. Instead, it likely reflects physiological constraints on toxin production by the venom glands, opening avenues for future research on the mechanisms of functional differentiation of populations of protein-secreting cells within adaptive contexts.
毒液是生态创新的产物，已经多次进化，每一次进化都伴随着专门的形态和行为特征的共同进化，以产生和输送毒液。这些特征之间的密切进化相互依存关系体现在能够控制其分泌毒液组成的动物身上。这种能力部分取决于在毒液腺的不同部位产生不同的毒素，最近在毒蛇中已有记载。在这里，我们检验了这样一个假设，即蛇毒液腺中毒素的不同空间分布是一种适应，使具有不同生态功能的毒液得以分泌。
我们表明，主要的防御性和捕食性肽毒素在黑颈喷毒眼镜蛇（Naja nigricollis）的毒液腺的不同区域产生，但这些分布可能反映了发育的影响。事实上，我们没有检测到来自同一标本的通过防御性“喷吐”或捕食性“咬噬”事件收集的毒液之间有明显的毒素差异，这些标本代表了多个喷毒眼镜蛇的谱系。我们还在一种不喷毒的眼镜蛇中发现了相同的毒素空间分布，并表明毒素的不均匀分布是具有主要捕食性毒液的蝰蛇的一个特征。
我们的研究结果表明，毒素的不均匀分布不是控制蛇毒液组成的适应。相反，它可能反映了毒液腺对毒素产生的生理限制，为未来在适应性背景下研究蛋白分泌细胞群体功能分化的机制开辟了途径。
