From Devo to Evo: patterning, fusion and evolution of the zebrafish terminal vertebra

Background

With more than 30,000 species, teleosts comprise about half of today’s living vertebrates, enriched with a wide set of adaptations to all aquatic systems. Their evolution was marked by modifications of their tail, that involved major rearrangements of the metameric organization of the axial skeleton. The most posterior or ural caudal skeleton, primitively included more than 10 vertebrae and, through a series of fusions and losses, became reduced to a single vertebra in modern ostariophysans, one of the largest clades of teleosts. The ontogeny of the ostariophysan Danio rerio recapitulates this process by forming two or three separate vertebrae that become a single vertebra in adults. We characterize the developmental sequence of this change by describing the processes of patterning, fusion and differential growth on each of the constitutive elements that sculpt the adult terminal vertebra.

Results

The ontogenetic changes of the terminal vertebra were characterized, highlighting their shared and derived characters in comparison with other teleosts. In zebrafish, there is: i) a loss of the preural centrum 1, ii) the formation of an hourglass-shaped autocentrum only in the anterior but not the posterior border of the compound centrum, iii) the formation of a vestigial posterior centrum that does not form an autocentrum and becomes incorporated beneath the compound centrum during development, and iv) the elongated dorso-posterior process of the compound centrum or pleurostyle appears as an independent element posterior to the compound centrum, before fusing to the ural neural arches and the anterior portion of the compound centrum.

Conclusions

The unique features of the formation of the terminal vertebra in Danio rerio reflect the remarkable changes that occurred during the evolution of teleosts, with potential shared derived characteristics for some of the major lineages of modern teleosts. A new ontogenetic model is proposed to illustrate the development of the terminal vertebra, and the phylogenetic implications for the evolution of caudal skeleton consolidation in ostariophysans are discussed.

The osteology of the golden grey mullet Liza aurata (Teleostei: Mugiliformes: Mugilidae) including interactive three-dimensional reconstructions

Grey mullets are remarkably characterized by their overall uniform external morphology. Identifying species as well as positioning the Mugiliformes in a phylogenetic context is rather difficult. Most recently they were placed in the newly erected Ovalentaria, but more detailed relationships to potential sister taxa were not resolved. Studying the internal morphology, especially the osteology, might provide new insights into the evolution of the Mugiliformes as well as help clarify their phylogenetic position within the Ovalentaria. A detailed osteology of the golden grey mullet Liza aurata is presented. The use of cleared and stained specimens allowed for a complete examination of bony and cartilaginous structures, and a 3D reconstruction from a μCT data set provided additional information on the positional relationships of the bones. Following this, the data obtained were compared with different mugilid species, particularly with the flathead grey mullet Mugil cephalus. Several differences between these species could be identified, such as the position of the basisphenoid, the shape of the hyomandibular and the composition of the branchial arches. These characters might help in understanding the evolutionary changes happening within the mugiliforms and will provide the basis to study this taxon in detail, finally allowing the reconstruction of the body plan of grey mullets.

The caudal skeleton of Batrachoidiformes (Teleostei: Percomorphacea): a study of morphological diversity, intraspecific variation, and phylogenetic inferences

The caudal-fin skeleton is a primary data source for systematics of fishes, with characters from this complex being proposed as synapomorphies at many taxonomic levels. Batrachoidiformes is recognized as monophyletic, although intraordinal relationships are unclear. Likewise, interrelationships of Batrachoidiformes to other percomorphs are not well established. The caudal skeleton of Batrachoidiformes has not been thoroughly studied and is poorly represented in recent phylogenetic analyses. In this study, we examined the caudal-fin skeleton of 55 of the 82 species and 22 of the 23 genera of Batrachoidiformes, emphasizing the detection of intraspecific variation to recognize morphological characters with phylogenetic significance. Intraspecific variation is high, especially in the shape of epurals and the parhypural flange. A dorsal prezygapophysis on the first ural centrum and the acute articular edge of the parhypural flange are interpreted as putative synapomorphies of Porichthyinae. The anterior epural supporting two procurrent fin rays is found only in some Halophryninae, but is absent in Allenbatrachus, Batrachomeus, Batrichthys and Halophryne. Among Batrachoidiformes, a hypurapophysis-like process on the first ural centrum is found in Thalassophryninae and Barchatus, Batrichthys, Bifax, Chatrabus, Colletteichthys, Halobatrachus, Perulibatrachus and Riekertia. Caudal-fin ray counts are phylogenetically informative at several taxonomic levels. Distal caudal cartilages are described for Batrachoidiformes for the first time.

The Caudal Skeleton of the Zebrafish, Danio rerio, from a Phylogenetic Perspective: A Polyural Interpretation of Homologous Structures

The structure of the caudal skeleton of extant teleost fishes has been interpreted in two different ways. In a diural interpretation, a caudal skeleton is composed of two centra articulated with one to six hypurals. Most subsequent authors have followed this interpretation. In contrast, a polyural interpretation considers the teleost fin to be derived from a fully metameristic ancestral bauplan originally composed of a one-to-one relationship between neural arches, centra (when present), and hypurals. Three different interpretations of the identity and homology of skeletal components of the caudal skeleton of the teleost fish Danio rerio have been proposed, two from a diural perspective and one from a polyural perspective. We examine each caudal skeletal component of Danio rerio from both a developmental and phylogenetic perspective. We propose that a polyural interpretation of structures is consistent with the current interpretation of the basal neopterygian caudal fin for this model organism rather than the older diural interpretation that does not take into account the metamerism observed in caudal structures during development. The polyural interpretation suggests several shared evolutionary innovations of major clades that would remain undiscovered under the older diural naming paradigm and makes the terminology of the parts of the caudal fin of Danio rerio strictly comparable to more basal fishes.