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 g...

The evolutionary relationships of species of Danio and the monophyly and phylogenetic placement of the genus within the family Cyprinidae and subfamily Rasborinae provide fundamentally important phyloinformatics necessary for direct evaluations of an array of pertinent questions in modern comparative biology. Although the genus Danio is not one of the most diverse within the family, Danio rerio is one of the most important model species in biology. Many investigations have used this species or presumed close relatives to address specific questions that have lasting impact on the hypothesis and theory of development in vertebrates. Largely lacking from this approach has been a holistic picture of the exact phylogenetic or evolutionary relationships of this species and its close relatives. One thing that has been learned over the previous century is that many organismal attributes (e.g., developmental pathways, ecologies, behaviors, speciation) are historically constrained and their origins and functions are best explained via a phylogenetic approach. Herein, we provide a molecular evaluation of the phylogenetic placement of the model species Danio rerio within the genus Danio and among hypothesized closely related species and genera. Our analysis is derived from data using two nuclear genes (RAG1, rhodopsin) and five mitochondrial genes (ND4, ND4L, ND5, COI, cyt b) evaluated using parsimony, maximum likelihood, and Bayesian analyses. The family Cyprinidae is resolved as monophyletic but the subfamily Rasborinae (priority over Danioinae) is an unnatural assemblage. Danio is identified as a monophyletic group sister to a clade inclusive of the genera Chela, Microrasbora, Devario, and Inlecypris, not Devario nor Esomus as hypothesized in previous studies. Danio rerio is sister

Danionella dracula is a new species of sexually dimorphic, miniature and highly developmentally truncated cyprinid fish. Compared with its close relative, the zebrafish Danio rerio , it lacks 44 bones or parts thereof and represents one of the most developmentally truncated vertebrates. Absence of the majority of bones appears to be due to developmental truncation via terminal deletion. In contrast to these larval-like features, D. dracula also shows several hyperossifications. Uniquely, among carp-like fishes, male D. dracula have a series of long, pointed odontoid processes on the jaws greatly resembling the jaw dentition of teleosts with true teeth. The anterior-most process in each jaw is extended as a canine-like fang projecting through the epithelium. True jaw teeth are absent from all 3700 species of cypriniforms and were lost at least in the Upper Eocene. It remains to be investigated, however, whether the conserved pathways to regulate tooth development in cypriniforms have been used in D. dracula to form and pattern the odontoid processes. This new species represents a remarkable example linking progenetic paedomorphosis via heterochronic change in developmental timing to the evolution of morphological novelties.

The osteology of the miniature cyprinid Celestichthys margaritatus Roberts, type species of Celestichthys Roberts, is described in detail and briefly compared with that of other members of the Rasborinae (notably Danio (s.s), Danionella, Devario, Esomus, Microrasbora, Paedocypris and Sundadanio). Celestichthys margaritatus possesses an “A” stripe on the anal fin and two pigment stripes on the caudal fin (apomorphic features of Danio sensu Fang, 2003). In addition, C. margaritatus exhibits a median projection on the outer arm of the os suspensorium, a derived feature, present only in species of Danio (including D. erythromicron) amongst the Cyprinidae, and a lateral projection on the lateral face of the dentary (present only in Danio, Sundadanio and Paedocypris). Phylogenetic analysis of 1,494 bp of the RAG1 nuclear gene for 31 rasborine taxa, including 5 species of Danio, places C. margaritatus as the sister group to D. erythromicron, and part of a larger monophyletic group including all other species of Danio included for analysis. Based on characters of morphology and its position in a molecular phylogeny of the Rasborinae it is proposed that Celestichthys be placed in the synonymy of Danio, its only member referred to as Danio margaritatus new combination.

We provide a detailed account of the osteology of the miniature Asian freshwater cyprinid fish Danionella dracula. The skeleton of D. dracula shows a high degree of developmental truncation when compared to most other cyprinids, including its close relative the zebrafish Danio rerio. Sixty-one bones, parts thereof or cartilages present in most other cyprinids are missing in D. dracula. This impressive organism-wide case of progenesis renders it one of the most developmentally truncated bony fishes or even vertebrates. Danionella dracula lacks six of the eight unique synapomorphies that define the order Cypriniformes and has, thus, departed from the cypriniform Bauplan more dramatically than any other member of this group. This escape from one of the most successful Baupläne among bony fishes may have been facilitated by the organism-wide progenesis encountered in D. dracula. By returning in its skeletal structure to the early developmental condition of other cypriniforms, D. dracula may have managed to overcome the evolutionary constraints associated with this Bauplan and opened up new evolutionary avenues that enabled it to evolve a number of striking morphological novelties, including its tooth-like odontoid processes and a complex drumming apparatus.