Ashwell Ken W S, Hardman Craig D
Department of Anatomy, School of Medical Sciences, University of New South Wales, Sydney, Australia.
Brain Behav Evol. 2012;79(4):261-74. doi: 10.1159/000338079. Epub 2012 Jun 18.
Both lineages of the modern monotremes have been reported to be capable of electroreception using the trigeminal pathways and it has been argued that electroreception arose in an aquatic platypus-like ancestor of both modern monotreme groups. On the other hand, the trigeminal sensory nuclear complex of the platypus is highly modified for processing tactile and electrosensory information from the bill, whereas the trigeminal sensory nuclear complex of the short-beaked echidna (Tachyglossus aculeatus) is not particularly specialized. If the common ancestor for both platypus and echidna were an electroreceptively and trigeminally specialized aquatic feeder, one would expect the early stages of development of the trigeminal sensory nuclei in both species to show evidence of structural specialization from the outset. To determine whether this is the case, we examined the development of the trigeminal sensory nuclei in the platypus and short-beaked echidna using the Hill and Hubrecht embryological collections. We found that the highly specialized features of the platypus trigeminal sensory nuclei (i.e. the large size of the principal nucleus and oral part of the spinal trigeminal nuclear complex, and the presence of a dorsolateral parvicellular segment in the principal nucleus) appear around the time of hatching in the platypus, but are never seen at any stage in the echidna. Our findings support the proposition that the modern echidna and platypus are derived from a common ancestor with only minimal trigeminal specialization and that the peculiar anatomy of the trigeminal sensory nuclei in the modern platypus emerged in the ornithorhynchids after divergence from the tachyglossids.
据报道，现代单孔目动物的两个谱系都能够通过三叉神经通路进行电感受，有人认为电感受起源于现代单孔目两个类群的类似鸭嘴兽的水生祖先。另一方面，鸭嘴兽的三叉神经感觉核复合体经过高度改造，用于处理来自喙部的触觉和电感觉信息，而短吻针鼹（Tachyglossus aculeatus）的三叉神经感觉核复合体则没有特别的特化。如果鸭嘴兽和针鼹的共同祖先为具有电感受和三叉神经特化的水生食者，那么人们会预期这两个物种三叉神经感觉核的早期发育阶段从一开始就显示出结构特化的证据。为了确定是否如此，我们利用希尔和胡布雷希特胚胎学收藏，研究了鸭嘴兽和短吻针鼹三叉神经感觉核的发育情况。我们发现，鸭嘴兽三叉神经感觉核的高度特化特征（即主核和三叉神经脊髓核复合体口部的大尺寸，以及主核中背外侧小细胞段的存在）在鸭嘴兽孵化时出现，但在针鼹的任何发育阶段都未观察到。我们的研究结果支持这样一种观点，即现代针鼹和鸭嘴兽起源于一个三叉神经特化程度极低的共同祖先，现代鸭嘴兽三叉神经感觉核的特殊解剖结构是在与针鼹科分化后在鸭嘴兽科中出现的。
