Prehistoric Genome: Secrets of Coelacanth

Latimeria chalumnae

Since in 1938, a South African fisherman extracted a zoological find of the century - 1.5-meter coelacanth from his nets - scientists have discovered two species of these "living minerals" (one of them lives off the coast of Africa, the other - Indonesia). Coelacanths with their fleshy, large fins and round tails look like coelacanths living in the Cretaceous period, contemporaries of dinosaurs.

Along with double-breathing fish, coelacanths are biologically much closer to humans and other mammals than to ray-creatures, which include the vast majority of modern fish. The ancient brushworms were the first vertebrates to venture to conquer land, so the data on the genome of African latimeria (Latimeria chalumnae), sequenced and analyzed by an international team of scientists, can tell a lot about the origin and evolution of tetrapods. In particular, the results of the work allowed us to put an end to a long-standing dispute: who are our closer “relatives” - coelacanths or lonely-breathing? The issue was resolved in favor of the latter, however, the genome of any of the double-breathing (much more complex than latimeria) is unlikely to be sequenced in the near future.

Although coelacanths are often called “living fossils”, in fact, they did not freeze in time, stopping in their evolution: analysis of genes encoding proteins showed that changes in the DNA of coelacanths slowly but continuously accumulated. Evidence of the slow evolution of coelacanth is also provided by DNA analysis of African and Indonesian species. In the homeosis genes of these species, separated from each other 6 million years ago, there are 11 times fewer differences than in the homeosis genes of humans and chimpanzees, which differentiated 6-8 million years ago.

Probably, such a slow development of coelacanths is due to a decrease in the influence of natural selection: they live at great depths, where life flows quite measuredly and calmly. However, analysis of the coelacanth genome showed that far from everything, the coelacanth evolution is so unhurried: a significant number of mobile elements (the non-coding part of the genome, which plays an important role in the regulation of genes) moved quite quickly inside the genome. Non-coding DNA can be an important source of evolutionary changes, but at the moment their role in the development of latimeria has not been studied.

As expected, the evolutionary steps are encrypted in the coelacanth genome on the way from the fin-brush to the paws of terrestrial vertebrates. For example, coelacanths and modern tetrapods have a common regulatory sequence responsible for limb development. There were unexpected discoveries. Coelacanth was the first known vertebrate that lacked genes encoding immunoglobulin M - an almost universal tool of the immune system. A pair of remotely resembling immunoglobulin M proteins, genes for which are present in the fish genome, is not the best substitute for it.

Scientists are sure that further analysis of the coelacanth genome will allow us to learn much more about our own distant past and the process of land development.

Based on materials from Nature News


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