RUI: Functional Convergence in the Venom Delivery System of Snakes

蛇是脊椎动物中最特殊的摄食机制之一。喂蛇的一个关键方面是在大约五分之一的活蛇身上存在毒液输送系统。分子和解剖学研究表明，毒液输送系统至少在四个蛇种谱系中独立进化。蟒蛇科(包括蛇科和真正的毒蛇)、眼镜蛇科(眼镜蛇和海蛇)、白头蛇科(来自非洲的Stiletto蛇)以及游蛇科内的几个分类群--如大腹蛇--都进化出了毒液输送系统。近年来，人们开发了一套技术来研究一种具有代表性的毒蛇--西部小菜蛾响尾蛇(Crotalus Trex)--的毒液注射机制。这笔赠款的目的是将这些相同的技术应用于典型伊拉普斯的毒液输送机制。这将是第一次对Elapid的毒液排出进行实验分析，与早期研究的比较将突出蛇毒输送系统之间的功能融合程度。毒液输送系统将通过毒液注射的运动学、神经肌肉控制、功能形态和行为生态学四个综合项目进行检测。将使用高速数字摄像和基于计算机的图像分析技术来研究毒液排出的运动学，以记录单层眼镜蛇(Naja Kaouthia)根据目标的大小和攻击的行为背景等因素调整其攻击的程度。红色眼镜蛇(N.pallida)吐毒液的运动学也将被记录在案。在蛇身上，毒液是通过骨骼肌收缩排出的；通过调节肌肉的收缩状态，蛇可以控制注入的毒液数量。将使用解剖学技术研究北草毒腺的外在肌肉结构，以记录肌肉及其神经的大体和微观结构。随后的实验分析将结合肌肉刺激与毒液流动和压力的测量来记录肌肉内的任何空间功能专门化，并将使用糖原耗竭来绘制肌肉内的运动单位图。眼镜蛇的毒液是通过肌肉收缩从毒液腺中排出的，通过毒液管道流入一系列软组织腔，这些软组织腔被牙鞘、结缔组织和上皮层所包围。将使用大体和显微解剖学、不透射线示踪剂(用于记录毒液屏障)、插管实验和毒液压力记录来研究该区域的功能形态。通过将跨音速流动探头长期植入到白纹伊蚊的毒液管道上，研究注射毒液的行为生态学。当眼镜蛇攻击各种防御和掠夺性目标时，毒液流动(用流动探头测量)将与高速数字摄像同步。这将记录打击过程中毒液流动的运动学，并揭示眼镜蛇根据目标大小或行为背景注入不同体积毒液的程度。

Snakes have one of the most specialized feeding mechanisms among vertebrate animals. One of the key aspects of snake feeding is the presence of a venom delivery system in roughly one-fifth of the living snakes. Molecular and anatomical studies suggest that a venom delivery system has evolved independently in at least four snake lineages. The Viperidae (including pit-vipers and true vipers), the Elapidae (cobras and sea snakes), the Atractaspidae (Stiletto snakes from Africa), and several taxa - such as the boomslang - within the Colubridae have all evolved venom delivery systems. In recent years a suite of techniques were developed to study the mechanics of venom injection in one representative viper, the western diamondback rattlesnake (Crotalus atrox). The purpose of this grant is to apply these same techniques to the mechanics of venom delivery in representative elapids. This will be the first experimental analysis of venom expulsion in an elapid, and a comparison with the earlier studies will highlight the degree of functional convergence among snake venom delivery systems. The venom delivery system will be examined through four integrated projects on the kinematics, neuro-muscular control, functional morphology, and behavioral ecology of venom injection. The kinematics of venom expulsion will be studied using high-speed digital videography and computer-based image analysis techniques to document the degree to which the monocled cobra (Naja kaouthia) modulates its strike depending on such factors as the size of the target and the behavioral context of the strike. The kinematics of venom "spitting" by the red spitting cobra (N. pallida) will also be documented. In snakes the venom is expulsed through skeletal muscle contraction; by regulating the contractile state of the muscle the snake could control the amount of venom injected. The extrinsic venom gland musculature will be studied in N. kaouthia using anatomical techniques to document the gross and microscopic structure of the muscle and its innervation. Subsequent experimental analyses will combine muscle stimulation with measurements of venom flow and pressure to document any spatial functional specialization within the muscle, and the motor units within the muscle will be mapped using glycogen depletion. In cobras venom forced from the venom gland by muscle contraction flows through the venom duct into a series of soft tissue chambers which are surrounded by the fang sheath, a drape of connective tissue and epithelium. The functional morphology of this region will be studied in both N. kaouthia and N. pallida using gross and microscopic anatomy, radiopaque tracers (used to document venom barriers), cannulation experiments, and recordings of venom pressure. The behavioral ecology of venom injection will be studied by chronically implanting transonic flow probes onto the venom duct of N. kaouthia. Venom flow (measured with flow probes) will be synchronized with high-speed digital videography while the cobra strikes a variety of defensive and predatory targets. This will document the kinematics of venom flow during the strike and reveal the extent to which cobras inject differential volumes of venom depending on target size or behavioral context.