Catania K C, Northcutt R G, Kaas J H
Department of Psychology, Vanderbilt University, Nashville, TN 37240, USA.
J Exp Biol. 1999 Oct;202(Pt 20):2719-26. doi: 10.1242/jeb.202.20.2719.
The nose of the star-nosed mole Condylura cristata is a complex biological novelty consisting of 22 epidermal appendages. How did this new set of facial appendages arise? Recent studies find remarkable conservation of the genes expressed during appendage formation across phyla, suggesting that the basic mechanisms for appendage development are ancient. In the nose of these moles, however, we find a unique pattern of appendage morphogenesis, showing that evolution is capable of constructing appendages in different ways. During development, the nasal appendages of the mole begin as a series of waves in the epidermis. A second deep layer of epidermis then grows under these superficial epidermal waves to produce 22 separate, elongated epidermal cylinders embedded in the side of the mole's face. The caudal end of each cylinder later erupts from the face and rotates forward to project rostrally, remaining attached only at the tip of the snout. As a result of this unique 'unfolding' formation, the rostral end of each adult appendage is derived from caudal embryonic facial tissue, while the caudal end of each appendage is derived from rostral facial tissue. This developmental process has essentially no outgrowth phase and results in the reversal of the original embryonic orientation of each appendage. This differs from the development of other known appendages, which originate either as outgrowths of the body wall or from subdivisions of outgrowths (e.g. tetrapod digits). Adults of a different mole species (Scapanus townsendii) exhibit a star-like pattern that resembles an embryonic stage of the star-nosed mole, suggesting that the development of the star recapitulates stages of its evolution.
星鼻鼹（Condylura cristata）的鼻子是一个复杂的生物新特征，由22个表皮附属物组成。这组新的面部附属物是如何产生的呢？最近的研究发现，在不同门的生物中，附属物形成过程中表达的基因具有显著的保守性，这表明附属物发育的基本机制是古老的。然而，在这些鼹鼠的鼻子中，我们发现了一种独特的附属物形态发生模式，这表明进化能够以不同的方式构建附属物。在发育过程中，鼹鼠的鼻附属物最初表现为表皮中的一系列波状结构。然后，第二层深层表皮在这些表层表皮波之下生长，形成22个独立的、细长的表皮圆柱体，嵌入鼹鼠脸部两侧。每个圆柱体的尾端随后从脸部穿出并向前旋转，向吻部前方突出，仅在鼻尖处相连。由于这种独特的“展开”形成方式，每个成年附属物的吻端源自胚胎面部组织的尾端，而每个附属物的尾端则源自面部组织的吻端。这个发育过程基本上没有生长阶段，导致每个附属物的原始胚胎方向发生了反转。这与其他已知附属物的发育不同，其他附属物要么起源于体壁的生长，要么起源于生长物的细分（如四足动物的趾）。另一种鼹鼠物种（汤氏鼹鼠，Scapanus townsendii）的成年个体呈现出一种类似星鼻鼹胚胎阶段的星状图案，这表明星鼻的发育重演了其进化阶段。
