Kaji Tomonari, Keiler Jonas, Bourguignon Thomas, Miura Toru
Bamfield Marine Sciences Centre, 100 Pachena Rd, Bamfield, BC, Canada, V0R 1B0.
Allgemeine und Spezielle Zoologie, Institut für Biowissenschaften, Universität Rostock, Rostock, Mecklenburg-Vorpommern, Germany.
Evol Dev. 2016 Mar-Apr;18(2):78-88. doi: 10.1111/ede.12179. Epub 2016 Jan 13.
The origins of evolutionary novelties are often deeply puzzling. They are generally associated with new functions that were absent in ancestors. The new functional configuration should arise via intermediate stages without any loss of function or impediment to the whole organism during the transitions. Therefore, understanding of the functional configurations of transitional states can shed light on how novel forms arise. Here we infer the evolutionary origin of a highly specialized termite defensive organ "nasus" where different functions overlap in different structural configurations at intermediate evolutionary stages to ensure that each phase is functional. Soldiers of a nasutitermitine termite use reconfigured mandibular muscles to squirt a viscous secretion from a nozzle-like head projection (the nasus). This contrasts sharply with the primitive defensive strategy where mandibles are used to bite. MicroCT observations of soldiers of Nasutitermes takasagoensis and of species with the ancestral state (Hodotermopsis sjostedti, Embiratermes neotenicus) revealed three different yet fully functional configurations in the transition from ancestral to novel state: (i) elevated hydrostatic pressure induced by contraction of mandibular muscles when biting gently oozes secretion from a gland; (ii) direct pressure on an enlarged gland arises from expansion of the mandibular muscles when biting; (iii) squirting in a piston-like manner by an inflated gland enveloped by highly modified mandibular muscles. Even a structure as exotic as the nasus therefore appears to have evolved with no loss of function at any stage. Such a functional approach, holds much promise for understanding the evolutionary origin of seemingly preposterous novel forms.
进化新特征的起源常常令人深感困惑。它们通常与祖先所没有的新功能相关联。新的功能结构应该通过中间阶段产生，在转变过程中不会出现任何功能丧失或对整个生物体造成阻碍的情况。因此，了解过渡状态的功能结构有助于揭示新形态是如何产生的。在这里，我们推断一种高度特化的白蚁防御器官“鼻”的进化起源，在中间进化阶段，不同的功能在不同的结构配置中重叠，以确保每个阶段都是有功能的。鼻白蚁科白蚁的兵蚁利用重新配置的下颌肌肉从喷嘴状的头部突出物（鼻）中喷出粘性分泌物。这与使用下颌进行咬噬的原始防御策略形成了鲜明对比。对高砂鼻白蚁以及具有原始状态的物种（霍氏缺齿蚁、新白蚁）的兵蚁进行显微CT观察，揭示了从原始状态到新状态转变过程中的三种不同但功能完全正常的结构配置：（i）轻轻咬噬时下颌肌肉收缩产生的静水压力升高，使腺体分泌物流出；（ii）咬噬时下颌肌肉扩张，对扩大的腺体产生直接压力；（iii）由高度特化的下颌肌肉包裹的膨胀腺体以活塞式方式喷射。因此，即使是像鼻这样奇特的结构，在任何阶段似乎也是在功能无损失的情况下进化而来的。这种功能研究方法，对于理解看似荒谬的新形态的进化起源很有前景。
