It’s a startling image that sums up a turning point in shark conservation research. The genomes of great hammerheads and shortfin makos, which include their complete genetic code, have been sequenced at the chromosomal level by Prof. Shivji, Prof. Michael Stanhope and associates. Their DNA timeline reveals that over 250,000 years, their populations have drastically declined. What the researchers also found, which is worrying, is that great hammerhead sharks have less genetic diversity, making them less able to adapt to the rapidly changing world. The species also exhibits inbreeding, which can make its populations less able to thrive.
The shortfin mako shark, however, showed greater diversity and limited inbreeding, a beacon of hope in the grim conservation climate. Understanding change over such a long timescale can put the current conservation status of these endangered animals into context. The results can help guide us towards much more nuanced management strategies for sharks. The findings are published in a paper published in iScience: “Genomes of endangered great hammerhead and shortfin mako sharks reveal historic population declines and high levels of inbreeding among great hammerheads,” led by Professor Stanhope of Cornell University and Professor Shivji, Director of the Save Our Seas Foundation Shark Research Center and Guy Harvey Research Institute, Nova Southeastern University, with collaborators from Cornell University, Nova Southeastern University, Temple University, Governors State University and San Diego Zoo Wildlife Alliance.
Scientists acquired and assembled whole genome sequences from great hammerheads and shortfin mako sharks and compared their genomes with available genomic information for the whale shark, white shark, brownbanded bamboo shark and clouded fruit bat. . Their methods resemble the construction of a complex puzzle by scientific detectives: assembling successively from tiny fragments of DNA different sequences like a large patchwork tapestry that details the blueprint of life. Reaching the chromosomal level represents the latest high-quality whole-genome sequence research — and a tricky feat to achieve for species like sharks that have huge genomes.
The application of advanced techniques comes amid grim reports for sharks and rays. “Technical advances in the study of genomes mean that DNA sequencing approaches are now much more powerful and efficient,” says Professor Stanhope. “We can apply these new technologies to better understand the organism, information that we hope can be used to protect sharks and rays.” While we don’t know exactly the effects of inbreeding in sharks, findings from wolves and cheetahs show that problematic traits can creep in over time. The result is often reduced survival of the species. The picture of great hammerhead sharks – overfished and traded for their fins – is worrying. But without this essential genetic knowledge, we would be unable to alter the way their vulnerable populations are currently managed.
The researchers are cautious about overstating the results. “Genetics have advanced so far that genomes at the chromosomal level are the expectation of a reference quality genome for species. However, conservation research presents its own challenges in achieving this consistently and at the resolution expected in other fields. Professor Shivji adds that: “Obtaining tissue samples from endangered marine vertebrates is a major hurdle.
You can assemble the genome with a single tissue sample from a single shark, but the ideal circumstance would be to sequence the genomes of multiple individuals from different parts of their oceanic range, an ethically difficult and expensive undertaking. Indeed, the researchers state this as a limitation of their current study. The ethical limits of working with endangered species mean that conservation geneticists must balance the latest advances with respect for the fragile populations they study. In addition to revealing the genetic diversity and fragile status of two endangered shark species, the researchers hope their results will provide what they call reference-quality genomes, from which future basic science can build to improve what we know about sharks.
Certainly, as new opportunities arise, our knowledge of the shark model will help strengthen our understanding of these ecologically important species and conserve their vulnerable populations. (ANI)
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