The primitive-looking coelacanth has long been regarded as a ‘living fossil,’ with extant specimens looking very similar to fossils dating back to the Cretaceous period. But while the coelacanth’s body may have changed little, its genome tells another story.
Latimeria chalumnae off Pumula on the KwaZulu-Natal South Coast, South Africa, on November 22, 2019. Image credit: Bruce Henderson, doi: 10.17159/sajs.2020/7806.
Coelacanths are lobe-finned fish that were thought to be extinct for 65 million years, until a first living specimen was discovered fortuitously in South Africa in 1938 by a South African museum curator on a local fishing trawler.
There are two extant species of coelacanths: Latimeria chalumnae form the Comoros Islands off the east coast of Africa, and Latimeria menadoensis from the waters off Sulawesi, Indonesia.
Coelacanths present several unique and intriguing features such as unpaired lobbed-fins looking much like paired fins and highly modified lungs/swim bladder.
Together with lungfish, they are the closest relatives to tetrapods and share with them several morpho-anatomical features that are not found in more distantly related vertebrates such as ray-finned fishes.
When the first extant coelacanth was discovered, it reminded so much fossil forms from the Cretaceous period that it was designated as a ‘living fossil,’ i.e. a species with a morphology that did not evolve much over a long period of time.
To account for this morphological stasis, it has often been suggested that coelacanths possess a slowly or even not evolving genome.
“Coelacanths may have evolved a bit more slowly but it is certainly not a fossil,” said Isaac Yellan, a graduate student in the Department of Molecular Genetics at the University of Toronto.
In their new study, Yellan and colleagues found that Latimeria chalumnae gained 62 new genes through encounters with other species 10 million years ago.
What’s even more fascinating is how these genes came about. Their sequences suggest they arose from transposons, also known as ‘selfish genes.’
These are parasitic DNA elements whose sole purpose is to make more copies of themselves, which they sometimes achieve by moving between species.
“Our findings provide a rather striking example of this phenomenon of transposons contributing to the host genome,” said Professor Tim Hughes, a researcher in the Donnelly Centre for Cellular and Biomolecular Research at the University of Toronto.
“We don’t know what these 62 genes are doing, but many of them encode DNA binding proteins and probably have a role in gene regulation, where even subtle changes are important in evolution.”
The findings were published in the journal Molecular Biology and Evolution.
Isaac Yellan et al. Diverse Eukaryotic CGG Binding Proteins Produced by Independent Domestications of hAT Transposons. Molecular Biology and Evolution, published online February 9, 2021; doi: 10.1093/molbev/msab007