The relationship between the five living rhino species in the world is a matter that dates back to time Darwin. One reason it’s hard to find answers is that most rhinos became extinct before the Ice Age.
Now, the researchers publish a report in the journal ‘prison cellwhich helped fill in the gaps in this family tree by analyzing the genomes of the five living species along with those in Three ancient and extinct species.
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The results show that the oldest split separated the African and Eurasian lineages at about 16 million years ago. They also found that while today’s dwindling rhino populations have lower genetic diversity and greater inbreeding than in the past, rhinos have historically had lower levels of genetic diversity.
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“We can now show that the main branch in the rhino tree of life is between geographic regions, Africa versus Eurasia, and not between rhinos with horns versus two horns,” says Loew Dallen of the Center for Paleogenetics and Swedish Museum of Natural History.
“The second important finding is that all rhinos, even extinct organisms, have relatively low genetic diversity,” he continues. The extinction is partly a result of its biology. “
“All eight species have generally shown either a steady but slow decline in population size over the past two million years, or the persistence of consistently small numbers over long periods of time,” adds Mick Westbury of University of Copenhagen (Denmark) –. Consistently low population sizes may indicate that rhinos in general are adapting to low levels of diversity.”
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This idea is consistent with the apparent lack of accumulated deleterious mutations in rhinos in recent decades. Westbury claims that rhinos may have shed harmful mutations in the past 100 years, allowing them to remain relatively healthy, despite their low genetic diversity.
The new study was inspired by a scientific meeting. Dalin and Tom Gilbert of the University of CopenhagenThey were working separately on different species of rhino. They realized that if they joined forces, along with colleagues from around the world, they could conduct a comparative study of all living rhinos with the three species that became extinct during the last Ice Age.
There were some challenges to overcome, says Shanlin Liu of China Agricultural University in Beijing. “When we decided to collect all the rhino data and do a comparative genomic study, we also faced the problem of ‘big data'”Liu explains.
Genome data represent different types of data, due in part to the inclusion of both modern and ancient DNA. The team had to develop new analysis tools to account for these differences. The new methods and tools they developed can now be applied to studies in other taxonomic groups.
The results are “partly good news, partly not,” Dalin says. Low levels of genetic diversity in rhinos appear to be part of their long history and have not led to increased health problems related to inbreeding and disease-causing mutations.
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“However, we also found that today’s rhinos have lower genetic diversity, and higher levels of inbreeding, than the genomes of historical and prehistoric rhinos. This suggests that recent population declines caused by hunting and habitat destruction have had an impact on the genome. This is not good, as lower genetic diversity and higher rates of inbreeding could increase the risk of extinction in current species.”
The researchers say the findings have some practical implications for rhino conservation.
Westbury says: “We now know that the low diversity we see in modern individuals may not be indicative of an inability to recover, but rather indicates a natural state of rhinos. We can better target recovery programs to focus on increasing the size of the population rather than individual genetic diversity. “.
The team hopes that the new findings will be useful for further study of the rhinoceros and its conservation. Dalén’s team is now working on a deeper study of the extinct woolly rhinoceros. Meanwhile, Westbury is involved in comparing the genomes of African black rhinos sampled before the recent decline in population size with those of modern individuals.
“We hope this will provide a framework for a better understanding of where transferred populations arose, direct changes in genetic diversity, and whether humans are lost forever,” Westbury says.
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