Skates are cartilaginous fish that can flourish in benthic ecosystems thanks to their wing-like pectoral fins. This unique trait, however, remains unknown on a molecular level. Where did these wing-like fins come from on an evolutionary scale? This is what University College London researcher Ferdinand Marlétaz set out to study.
Vertebrates evolved and diversified alongside key developmental innovations. As well as those paired appendages we have seen displayed in tetrapods, the Chondrichthyans (all cartilaginous fish) have different fin structures that also exhibit an exquisite diversity of uses. A fascinating example is the wing-like appendages on batoid fishes (skates and rays) in which the pectoral fins are fused with the head and extend anteriorly. Due to this unique structure, skates and rays are able to not only propel themselves forward along the seafloor but become fearsome predators of the ocean floor. Research published by scientists at the University of Chicago in 2015 found that skates and rays evolved their striking, wing-like pectoral fins using repurposed genes. “What is surprising is that the extraordinary anatomy of skate fins comes about by simple tweaks to the processes that make the more normal-looking fins of other fish,” senior study author Neil Shubin, PhD, Robert R. Bensley Professor of Organismal Biology and Anatomy at the University of Chicago said in a press release at the time.
Chondrichthyans (all cartilaginous fish) have different fin structures that also exhibit an ... [+]
In the early stages of embryonic skate development, pectoral fins mirrored those of other fish and tetrapods; however, instead of their genes creating one apical ectodermal ridge (AER) per appendage, they develop two - one that directs growth of the pectoral fin toward the tail, while the other directs growth towards the head. The apical ectodermal ridge (AER) is one of the main signaling centers during limb development and is an important structure of the developing limb bud. The researchers also found that embryonic skates develop the rear portion of the fin using limb-development genes, while the front portion develops using shoulder genes. In other words, even though the posterior and anterior of a skate fin look similar, they are completely different in terms of gene expression. “These changes arose ~286–221 million years ago after the divergence between sharks and skates. Nevertheless, the genomic and regulatory changes underlying these novel expression domains have remained elusive,” the Marlétaz’s team reports in their latest study.
The scientists investigate the origin of this phenotypic innovation by developing the little skate (Leucoraja erinacea) as a genomically enabled model. “[Co-author] José-Luis Gómez-Skarmeta was particularly interested in applying molecular techniques relying on short-read sequencing that shed light on the regulatory processes (which enhancer or 'cis-regulatory element' controls which genes) to understand the particular skate morphology (the wing-like fins),” says Marlétaz. Annotating 26,715 protein-coding genes using extensive transcriptome resources, with 23,870 possessing homologues in other species, the team completed a comparative analysis with 20 other sequenced vertebrates to reconstructed the complete set of skate gene evolutionary histories (the phylome) and used it to infer patterns of gene duplication and loss, as well as orthology and paralogy relationships. “The analysis of a high-quality chromosome-scale genome sequence for the little skate shows that it preserves many ancestral jawed vertebrate features compared with other sequenced genomes, including numerous ancient microchromosomes.”
"The analysis of a high-quality chromosome-scale genome sequence for the little skate shows that it ... [+]
Batoid fin morphology was significantly affected by functional inhibition of planar cell polarity signaling, confirming that this pathway is a major contributor to batoid fin morphology. But why is it important we know about the morphology and evolution of these animals? Says Marlétaz: “One of the big questions in evolution is to understand how novelties (novel morphologies, properties, etc.) emerge. The example of the skate fin is interesting because it is a striking body plan but we also know it did appear by altering the shape of the existing structure, the anterior fins and extending them forward. What we tried to understand is what type of mechanism was involved in this 'tinkering' and we show it is not some drastic like an entirely new set of genes, but some subtle differences in the way they are regulated.”
