Conversely, in the teleost lineage, it is proposed that the metapterygium was lost while the propterygium and mesopterygium were retained, and no teleost exhibits end-on-end articulation of long bones in the pectoral fin. In the lineage leading to tetrapods, the propterygium and mesopterygium are postulated to have been lost along with the dermal fin rays, while new long bones that articulate end on end were added along the PD axis of the metapterygium, ultimately forming the limb.
The pectoral fin consisted of the anterior propterygium, the middle mesopterygium, and the posterior metapterygium. The last common ancestor of the bony fishes, the clade that encompasses the teleost and tetrapod lineages, had a polybasal condition in which the most proximal endochondral elements of the fin articulated with the shoulder girdle. Phylogenetic model of pectoral appendage articulation in bony fishes. The divergent architectures of teleost fins and tetrapod limbs have been maintained within each clade for over 250 ( 4, 5) and 350 million ( 6) years, respectively. No known teleost species exhibits more than a single long bone along the proximal-distal (PD) axis ( 3). The endoskeleton typically consists of four long bones, called proximal radials, arranged side by side along the anterior-posterior (AP) axis, followed distally by small nodular distal radials ( 1, 2). Teleost pectoral fins are composed of dermal fin rays supported at their base by a diminutive endoskeleton ( Fig. In contrast to limbs, the skeletal pattern of the teleost pectoral appendage is almost invariant, showing a consistent arrangement across diverse lineages within this group of ∼30,000 species. Diverse limb structures and functions have evolved in tetrapods through digit reduction, phalangeal addition, transformation of wrist and ankle bones, alteration in element proportion, and outright limb-loss. Tetrapod limbs permit mobility via articulation of multiple endochondral long bones facilitated by specialized synovial joints. Our findings reveal an inherent limb-like patterning ability in fins that can be activated by simple genetic perturbation, resulting in the elaboration of the endoskeleton.Ĭhanges in appendage structure underlie key transitions in vertebrate evolution.
PADDLEFISH SERVER SHIFT EXCHANGE. CODE
Concordantly, formation of supernumerary fin long bones requires the function of hoxall paralogs, indicating developmental homology with the forearm and the existence of a latent functional Hox code patterning the fin endoskeleton.
We find that this pathway functions in appendage development across vertebrates, and loss of Wasl in developing limbs results in patterning defects identical to those seen in Hoxall knockout mice. This phenotype is caused by activating mutations in previously unrecognized regulators of appendage development, vav2 and waslb, which we show function in a common pathway. These new skeletal elements are integrated into the fin, as they are connected to the musculature, form joints, and articulate with neighboring bones. Here we identify zebrafish mutants that form supernumerary long bones along the proximal-distal axis of pectoral fins with limb-like patterning. Fins and limbs share many core developmental processes, but how these programs were reshaped to produce limbs from fins during evolution remains enigmatic. Whereas limb skeletons are often elaborate and diverse, teleost pectoral fins retain a simple endoskeleton. A hallmark of this transition is the addition of multiple long bones to the proximal-distal axis of paired appendages. The evolution of fins into limbs was a key transition in vertebrate history.
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