Britz R, Conway K W
Department of Zoology, The Natural History Museum, London, UK.
J Morphol. 2009 Apr;270(4):389-412. doi: 10.1002/jmor.10698.
Species of the cyprinid genus Paedocypris are among the smallest and most developmentally truncated fishes and vertebrates. Our analysis of their skeletal structure reveals a puzzling combination of extreme developmental truncation and an increased morphological complexity in sexually dimorphic characters. The skeleton of Paedocypris is characterized by reduction and loss and resembles in many aspects that of a larval/early juvenile stage of its close relatives. We found 61 characters that have been affected by developmental truncation. A comparison with the skeletal development of a close relative, the zebrafish Danio rerio, demonstrates that the majority of the absent bones or skeletal structures in Paedocypris are those that appear late in the ossification trajectory of the zebrafish. Thus, their absence in Paedocypris seems to be due to the simple developmental truncation of terminal stages in the ossification sequence. Our study of the sexually dimorphic structures in Paedocypris demonstrates that predominantly the male exhibits the more complex state. In relation to the female, male Paedocypris uniquely possess a cleithrum with a pointed posterior process that covers the scapula laterally, and a more medially situated posterior flange that contacts the dorsal area of the coracoid; a massive and heavily ossified uppermost pectoral radial tightly bound to the scapula; thickened and enlarged three uppermost pectoral-fin rays; a large triangular, dorsolaterally directed process on the outer arm of the massive os suspensorium; and a enlarged and shovel-like anterodorsally directed basipterygium; and a hypertrophied first pelvic-fin ray with additional anterior flanges that support keratinized pads of skin. Female Paedocypris show only one structure that is better developed than in males: the first proximal-middle radial and the anteriormost fin ray of the dorsal fin are more massive and more heavily ossified. Although the function and biological role of these dimorphisms is still unknown, we hypothesize that they are related to a special reproductive behavior. Paedocypris is a prime example for the recent claim that miniaturization among cyprinids is associated with evolutionary novelty only in developmentally truncated miniatures and not in proportioned dwarfs. Paedocypris offers a strong challenge to Schindleria as the most extreme example of developmental truncation known among fishes. We highlight the difficulties that developmentally truncated taxa frequently pose to the resolution of their phylogenetic position and propose an approach to overcome this problem. Our phylogenetic comparison to determine the systematic position of Paedocypris among cyprinids reveals that it shares not only a number of unique absences, but also highly unusual progressive characters with Sundadanio and Danionella, two other Asian miniature cyprinids. We hypothesize that the three genera form a monophyletic group. We further found that Paedocypris and Danionella share a number of uniquely derived characters pointing to a sister group relationship of the two.
鲤科原鲤属的物种是最小且发育最为简化的鱼类和脊椎动物之一。我们对它们骨骼结构的分析揭示了极端发育简化与性二态特征中形态复杂性增加这一令人费解的组合。原鲤的骨骼以退化和缺失为特征，在许多方面类似于其近亲幼体/幼年期早期的骨骼。我们发现61个特征受到了发育简化的影响。与近亲斑马鱼的骨骼发育进行比较表明，原鲤中缺失的大多数骨骼或骨骼结构是那些在斑马鱼骨化轨迹后期出现的结构。因此，它们在原鲤中的缺失似乎是由于骨化序列末期阶段的简单发育简化。我们对原鲤性二态结构的研究表明，主要是雄性表现出更复杂的状态。与雌性相比，雄性原鲤独特地拥有一块具尖后突的匙骨，该后突从侧面覆盖肩胛骨，以及一个位置更靠内侧的后凸缘，其与喙骨的背侧区域接触；一块紧密连接到肩胛骨的巨大且高度骨化的最上方胸鳍辐状骨；加厚且增大的最上方三根胸鳍鳍条；在巨大的悬骨外侧臂上有一个大的三角形、背外侧指向的突起；以及一个增大且呈铲状的向前背侧指向的基鳍骨；还有一条肥大的第一腹鳍鳍条，带有额外的前凸缘，支撑着角质化的皮肤垫。雌性原鲤仅有一种结构比雄性发育得更好：第一近端 - 中间辐状骨和背鳍最前方的鳍条更粗壮且骨化程度更高。尽管这些二态性的功能和生物学作用仍然未知，但我们推测它们与一种特殊的繁殖行为有关。原鲤是近期一种观点的典型例子，即鲤科鱼类中的小型化仅与发育简化的小型鱼类相关，而与比例缩小的侏儒鱼类无关。原鲤对辛氏微体鱼构成了强有力的挑战，辛氏微体鱼是鱼类中已知发育简化最极端的例子。我们强调了发育简化的分类群在确定其系统发育位置时经常带来的困难，并提出了一种克服这一问题的方法。我们通过系统发育比较来确定原鲤在鲤科中的系统位置，结果表明它不仅与另外两种亚洲小型鲤科鱼类——巽他丹鱼属和细鲫属——有许多独特的缺失特征相同，还具有非常不寻常的进步特征。我们推测这三个属构成一个单系群。我们进一步发现原鲤和细鲫属共有许多独特的衍生特征，表明这两个属是姐妹群关系。
