Rhino story revealed in DNA study

New research details the evolutionary history of rhinos, exposing a surprising lack of genetic diversity throughout their long history. Since all living rhino species are currently threatened and face their own genetic bottlenecks, the new research could improve conservation efforts.

At a scientific meeting held a few years ago in Copenhagen, paleogenetician Love Dalén of the Swedish Museum of Natural History met Tom Gilbert, an evolutionary biologist from the University of Copenhagen. They discussed a possible scientific collaboration, in which the subject of rhinos was brought up, as each independently studied these horned mammals. This kicked off a project in which Dalén and Gilbert, along with experts from around the world, used ancient and modern genomes to study the evolutionary history of the rhino family.

A collaboration that makes sense, given that scientists have struggled to reconstruct the rhino family tree. The biologist Charles Darwin even tried it, writing an essay on the subject 17 years before his seminal book, About the origin of species, published in 1859.

Studying the history of rhinos has been a challenge as all rhinos that exist today are highly endangered and are the subject of conservation efforts. In addition, the vast majority of rhinos became extinct before the Pleistocene epoch, which began some 2.58 million years ago. The rhino family emerged between 55 and 60 million years ago, after diverging from the tapirs. The rhinos were to be hugely successful, spawning over 100 different species and spreading across Africa, Eurasia, and North and Central America.

Some rhinos have gotten really big, like the woolly rhino (Old coelodonta). Weighing over 4,500 pounds (2,000 kg), these rhinos had a shaggy coat, a gigantic bump and a formidable 5ft (1.5-metre) Horn. At the end of the Pleistocene about 11,500 years ago, however, only nine rhino species remained on Earth.

To better understand rhinos based on their history and distant ancestors, the team mapped the genetic relationships of five living rhino species with three rhino species. which died out just before the end of the last ice age: the Siberian unicorn (Elasmotherium sibiricum), Merck’s rhino (Stephanorhinus kirchbergensis), and the aforementioned woolly rhinoceros. black rhinos (Diceros bicornis), white rhinos (ravelobensis), Sumatran rhino (Dicerorhinus sumatrensis), great one-horned rhino (Rhinoceros unicornis), and java rhino (R. sondaicus), were the living species included in the study.

The analysis that followed showed that an ancestral split occurred 16 million years ago in the early Miocene, creating two distinct rhino lineages, one in Africa and the other in Eurasia. This split was due to their geographical distribution and not the result of an emerging physical difference, namely the appearance of one-horned and two-horned rhinos.

The other key finding is that rhinos have a long history of low genetic diversity. A lack of genetic diversity is a sign of small populations, and it can lead to all kinds of genetic diseases as a result of deleterious mutations. It happened to woolly mammoths in their last days.

“All eight species have generally shown either a continuous but slow decrease in population size over the past 2 million years or continually small population sizes over extended periods of time,” Mick Westbury, co-author study and researcher at the University of Copenhagen, explained in an emailed press release.

As this research suggests, ancient rhinos have successfully coped with or adapt to small continuously populations. Scientists have a cool term to describe the process that makes this possible: the purging of the mutational load.

“Species are not adapted to low diversity, but in some ways you could say they can adapt to a small population size,” Dalén wrote in an email. “What the theory predicts is that natural selection can remove deleterious mutations from the population, even when the population size becomes small. This is not an adaptation to low diversity, but should rather in my opinion to be considered as an adaptation to inbreeding.

Such low genetic diversity, although an indelible part of rhino history, has not resulted in declining health as a result of inbreeding and damage mutations. Interestingly, rhinos are not alone in this regard. The cat family (Felidae) has an even lower genetic diversity, as explained by Dalén. He said this was not too surprising, “since carnivores generally have lower diversity than herbivores because their populations are generally smaller in size.”

But while “low genetic diversity is a long-term characteristic” of the rhino family, it has “been particularly exacerbated recently”, possibly because humans have driven these creatures to extinction., as biologists write in their to study, published today in Cell.

Indeed, while the historical purge of the mutational load could prevent genetic problems infiltrating, the extremely small population sizes of modern rhinos are a different story. As the article points out, the average genetic diversity observed in four modern rhino genomes was measured to be about half of what was observed in ancient genomes (the Java rhino was included as a historic species because its DNA has come of an individual who lived 200 years ago – before human influences on rhino populations).

Rhinos, as the study suggests, have been successful in eliminating unhealthy mutations over the past 100 years, but today’s rhinos now face lower levels of genetic variation and rates of inbreeding. higher than their ancestors. It is the result of morehunting and habitat destruction, and that puts these species in danger of extinction.

Fortunately, the new document can inform current conservation efforts. As research suggests, low genetic diversity doesn’t necessarily indicate rhinos are struggling. Rather, environmentalists should focus on increasing the size of their population, rather than increasing their individual genetic diversity. In practice, “this means that the main goal of conservation should be to prevent illegal poaching and the destruction of preferred rhino habitat,” said Dalén., and the approach should vary by species. African rhinos, for example, are threatened by illegal poaching, while Sumatran rhinos are threatened by destruction of their preferred habitat, he explained..

“Having said that, I also don’t think we can ignore the threat of low genetic diversity and inbreeding,” Dalén added. “All rhinos still have harmful mutations in their genome, although perhaps less than in ancient times. And given the small populations of most rhinos today, it is very likely that the Inbreeding will continue to increase in the future. If that happens, we will see an increase in genetic diseases. “

Dalén’s advice to conservation officials is to do what they can to prevent poaching and protect the remaining rhino habitat, “if there is any chance that future generations will be able to see these animals.”

Following: Unprecedented study of woolly mammoth shows where it roamed from birth to death.