Graduate School of Science and Engineering, University of Toyama, Toyama, 930-8555, Japan.
Evol Dev. 2012 May-Jun;14(3):286-95. doi: 10.1111/j.1525-142X.2012.00545.x.
One of the major foci in evolutionary developmental biology is to understand developmental mechanisms that underlie the acquisition of morphological novelties. Termite soldiers, the highly specialized defensive caste, show exaggerated species-specific morphologies, mostly enlarged mandibles. Soldiers of the subfamily Nasutitermitinae (Termitidae), however, possess a novel structure for defense in their heads, that is a horn-like frontal projection (nasus) from which defensive chemicals are discharged. Just prior to the molt into presoldiers (the preceding stage to soldiers) from workers, a nasus disc, or a nasus primordium, is observed under the worker head cuticle. In order to understand the developmental underpinnings of this evolutionarily novel structure, the role of a homeobox gene Distal-less (Dll) during nasus development was examined in this study, using a nasute termite Nasutitermes takasagoensis. Histological observations showed that complex developmental processes comprising epidermal evagination and invagination through changes in cell shape and cell proliferation formed the projection and the gland. Immunohistochemistry showed that Dll was localized in the developing nasus disc, but not in the frontal-gland primordium. Consistent with this finding, Dll RNA interference only repressed nasus growth not the frontal-gland formation. Taken together, the co-option of Dll is suggested to contribute to the acquisition of a novel defensive structure in a termite lineage, coupled with the acquisition of adaptive defensive behaviors.
进化发育生物学的一个主要焦点是理解导致形态新颖性获得的发育机制。白蚁兵蚁是高度特化的防御阶层，具有夸张的物种特异性形态，主要是增大的下颚。然而，在鼻白蚁亚科（白蚁科）的兵蚁中，它们的头部具有一种用于防御的新型结构，即从头部排出防御性化学物质的角状额突（鼻）。在从工蚁向预兵蚁（兵蚁的前一个阶段）蜕皮之前，可以在工蚁头部表皮下观察到一个鼻盘或鼻原基。为了了解这种进化新颖结构的发育基础，本研究使用一种鼻白蚁 Nasutitermes takasagoensis 研究了同源盒基因 Distal-less（Dll）在鼻发育过程中的作用。组织学观察表明，通过细胞形状和细胞增殖的变化，包括表皮外生和内陷的复杂发育过程形成了突起和腺体。免疫组织化学显示 Dll 定位于发育中的鼻盘，但不在额腺原基中。与这一发现一致，Dll RNA 干扰仅抑制了鼻的生长，而不影响额腺的形成。综上所述，Dll 的共适应被认为有助于白蚁谱系中获得一种新的防御结构，并伴随着适应性防御行为的获得。
