Ashwell Ken W S
Department of Anatomy, School of Medical Sciences, The University of New South Wales, Sydney, N.S.W., Australia.
Brain Behav Evol. 2012;79(1):45-56. doi: 10.1159/000332804. Epub 2011 Dec 6.
The two groups of living monotremes (platypus and echidnas) have remarkably different olfactory structures in the adult. The layers of the main olfactory bulb of the short-beaked echidna are extensively folded, whereas those of the platypus are not. Similarly, the surface area of the piriform cortex of the echidna is large and its lamination complex, whereas in the platypus it is small and simple. It has been argued that the modern echidnas are derived from a platypus-like ancestor, in which case the extensive olfactory specializations of the modern echidnas would have developed relatively recently in monotreme evolution. In this study, the development of the constituent structures of the olfactory pathway was studied in sectioned platypus and echidna embryos and post-hatchlings at the Museum für Naturkunde, Berlin, Germany. The aim was to determine whether the olfactory structures follow a similar maturational path in the two monotremes during embryonic and early post-hatching ages or whether they show very different developmental paths from the outset. The findings indicate that anatomical differences in the central olfactory system between the short-beaked echidna and the platypus begin to develop immediately before hatching, although details of differences in nasal cavity architecture emerge progressively during late post-hatching life. These findings are most consistent with the proposition that the two modern monotreme lineages have followed independent evolutionary paths from a less olfaction-specialized ancestor. The monotreme olfactory pathway does not appear to be sufficiently structurally mature at birth to allow olfaction-mediated behaviour, because central components of both the main and accessory olfactory system have not differentiated at the time of hatching.
现生的单孔目动物（鸭嘴兽和针鼹）成年后具有截然不同的嗅觉结构。短吻针鼹主嗅球的各层有广泛折叠，而鸭嘴兽的则没有。同样，针鼹梨状皮质的表面积大且分层复杂，而鸭嘴兽的则小且简单。有人认为现代针鼹起源于类似鸭嘴兽的祖先，若果真如此，现代针鼹广泛的嗅觉特化应是在单孔目动物进化过程中相对较晚才发展出来的。在本研究中，德国柏林自然博物馆对切片的鸭嘴兽和针鼹胚胎及孵化后的幼体的嗅觉通路组成结构的发育进行了研究。目的是确定这两种单孔目动物在胚胎期和孵化后早期，其嗅觉结构是否遵循相似的成熟路径，或者从一开始就表现出截然不同的发育路径。研究结果表明，短吻针鼹和鸭嘴兽中枢嗅觉系统的解剖学差异在孵化前就已开始显现，尽管鼻腔结构差异的细节在孵化后的后期才逐渐显现。这些发现与以下观点最为一致，即两个现代单孔目谱系从嗅觉不太特化的祖先开始就遵循了独立的进化路径。单孔目动物的嗅觉通路在出生时似乎结构上还不够成熟，无法支持嗅觉介导的行为，因为主嗅觉系统和副嗅觉系统的中枢成分在孵化时都尚未分化。
