Abstract
The transformation of paired fins into tetrapod limbs is one of the most intensively scrutinized events in animal evolution. Early anatomical and embryological datasets identified distinctive morphological regions within the appendage and posed hypotheses about how the loss, gain, and transformation of these regions could explain the observed patterns of both extant and fossil appendage diversity. These hypotheses have been put to the test by our growing understanding of patterning mechanisms that regulate formation of the appendage axes, comparisons of gene expression data from an array of phylogenetically informative taxa, and increasingly sophisticated and elegant experiments leveraging the latest molecular approaches. Together, these data demonstrate the remarkable conservation of developmental mechanisms, even across phylogenetically and morphologically disparate taxa, as well as raising new questions about the way we view homology, evolutionary novelty, and the often non-linear connection between morphology and gene expression. In this review, we present historical hypotheses regarding paired fin evolution and limb origins, summarize key aspects of central appendage patterning mechanisms in model and non-model species, address how modern comparative developmental data interface with our understanding of appendage anatomy, and highlight new approaches that promise to provide new insight into these well-traveled questions.
Highlights
From the fins of fishes to the wings of birds to human arms, paired appendage diversity has profoundly shaped the ecology and evolution of vertebrate animals
We focus on comparative developmental studies that inform our understanding of paired-fin diversification and the fin-to-limb transition, and what these insights may tell us about the evolution of novelty
These same studies further probed the components of the sonic hedgehog (Shh)/fibroblast growth factors (Fgfs) transcriptional network required for distal limb outgrowth and polarization, revealing striking similarities between the developing paddlefish fin fold and the mouse autopod (Figure 3C)
Summary
From the fins of fishes to the wings of birds to human arms, paired appendage diversity has profoundly shaped the ecology and evolution of vertebrate animals. For this reason, appendage anatomy and evolution have been the focus of intensive study for centuries [1,2,3,4]. Embryological studies went in search of both homology and mechanism These studies, combined with modern molecular and developmental datasets, have made great strides in demonstrating the processes underlying appendage development and diversification and inform how we view the concept of homology. Of paired-fin diversification and the fin-to-limb transition, and what these insights may tell us about the evolution of novelty
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