This illustration shows the reconstruction by a paleoartist of three extinct species of rhinoceros. Their genomes were sequenced in the course of the study. The Siberian unicorn (Elasmotherium Sibiricum) is in the foreground. Two Mercks rhinoceroses are close behind (Stephanorhinus Kirchbergensis) A woolly rhinoceros (Coelodonta antiquetatis) can be seen in the background. Credit: Beth Zaiken
The relationships between the five world's rhinoceros species has been a long-standing question since Darwin's day. Answers have been difficult to find because most rhinos died out before the Pleistocene. Researchers have now published in Cell, August 24, their findings. They have analyzed the genomes from all five rhino species and three extinct species.
These findings reveal that the oldest split of Eurasian and African lineages occurred 16 million years ago. The researchers also found that the present-day declining rhino populations have less genetic diversity and are more likely to inbreed than in the pastrhinoceroses. Pastrhinoceroses historically had low levels.
Love Daln, of the Centre for Palaeogenetics, and the Swedish Museum of Natural History, says, "We now know that the main branch of the rhinoceroses’ tree of life is between geographic regions, Africa and Eurasia, rather than between rhinos with one or two horns." The second important finding is that rhinoceroses, even extinct ones, have relatively low genetic diversity. This means that current-day rhinos have a low level of genetic diversity, even though they are all endangered.
Mick Westbury, University of Copenhagen, Denmark, stated that "All eight species had either a slow but steady decline in their population over the past 2 million years or a decrease in their population over a longer time period." "Continuously low populations may indicate that rhinoceroses are adaptable to low levels diversity."
This is consistent with the apparent absence of deleterious mutations in rhinos over recent decades. Westbury believes that rhinos might have cleared deleterious mutations over the past 100 years, which may explain why they are relatively healthy despite their low genetic diversity.
A scientific meeting inspired the new study. Tom Gilbert, University of Copenhagen had previously been working on different rhino species separately. They realized they could collaborate with colleagues from around the globe to do a comparative study on all rhino species together with those that died during the last Ice Age.
Shanlin Liu from China Agricultural University in Beijing said that there were many challenges. Liu explained that when we decided to compile all the rhinoceroses data and conduct a comparative genomes study, we also faced the "big data" problem.
Because of the inclusions of both ancient and modern DNA, the genome data presented different data types. To take these differences into consideration, the team needed to create new analysis tools. These tools and new approaches can be used to study other taxonomic groups.
Daln states that the findings are both "partly positive news" and "partly not." Low levels of genetic diversity in rhinos are part of their long-term history. This hasn't caused an increase in diseases-causing mutations and inbreeding.
He says, "But, we also found that the present-day rhinos have less genetic diversity and more inbreeding than our historical and prehistoric genomes." This suggests that habitat loss and hunting have had an effect on the genomes. This is not a good sign, as low genetic diversity and high levels of inbreeding could increase the likelihood of species extinction.
According to the re-searchers, there are some practical implications for rhino conservation.
Westbury states, "Now we know the low diversity seen in modern individuals may not indicate an inability or inability to heal. It could be indicative that there is a natural state for rhinoceros." We can guide recovery programs better to concentrate on increasing the population size and not individual genetic diversity.
They hope that the team's new findings will help in continuing research on rhinoceroses and conservation. Daln says that his team is currently conducting a deeper study on the extinct woolly rhinoceros. Westbury is currently comparing the genomes from African black rhinoceros collected before the decline in their population to the current individuals.
Westbury stated, "We hope this will provide a framework for better understanding where translocated population may have arisen, direct changes to genetic diversity, or whether any populations have been lost forever due to humans."
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More information: Cell, Liu et al. : "Ancient and modern genomes unravel the evolutionary history of the rhinoceros family" DOI: 10.1016/j.cell.2021.07.032 Journal information: Cell , Liu et al. : "Ancient and modern genomes unravel the evolutionary history of the rhinoceros family" www.cell.com/cell/fulltext/S0092-8674(21)00891-6
