Cape Town - The white shark population around the South African coastline has such a low level of genetic diversity that it may seriously jeopardizes their capability to survive into the future.
This is one of the findings from a major research project on white sharks and their DNA along the South African coastline, conducted by researchers from the evolutionary genomics group in the Department of Botany and Zoology at Stellenbosch University (SU).
'Over 302 genetic samples and 5 000 photographs'
Dr Sara Andreotti, who collected genetic samples as part of her doctoral research at SU, had to rely on the expertise of well-known shark conservationist, Mike Rutzen, to track down white sharks along the South African coast line. The field work kept them busy for four years, sometimes for up to two months at a time. By the end of 2014 they collected over 302 genetic samples and 5 000 photographs.
The results of the study have now been published in an article ‘New insights into the evolutionary history of white sharks, Carcharodon carcharias’ in the Journal of Biogeography.
'89% of all the sharks sharing the exact same gene sequence'
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Dr Andreotti says the genetic diversity of the South African white shark population is the lowest of all white sharks in the world: “We found only four maternal genetic lineages in the South African population, with 89% of all the sharks sharing the exact same gene sequence. When compared with other marine species, it is even lower than that of the highly endangered bottlenose dolphin (Tursiops truncates).”
Genetic diversity is generally regarded as an important indicator of the resilience of a population in the wild. The higher the diversity, the easier it is for a species to survive lethal diseases or unexpected changes in the environment. Among different individuals, there will always be some that are able to survive and breed to produce the next generation.
A completely different scenario emerges when all the individuals share the same genetic information. If genetic diversity is needed to survive, and does not exist, it can quickly lead to extinction.
Dr Andreotti says their research is ongoing: “The poor genepool could be the result of a severe bottleneck or historical local extinction and re-colonization processes. But our main and immediate concern now is to understand the potential negative effects the low levels of genetic diversity can have on South Africa’s white shark population.”(Elsa Hoffmann)
Global connections of white shark populations
The DNA of the South African white sharks was also compared with the DNA of 58 white sharks analysed in previous studies conducted elsewhere in the world. Surprisingly, the results revealed that a unique lineage exists along the South African coast line that does not connect closely to any other known lineage elsewhere in the world.
Dr Andreotti says, “It appears from this study that all white sharks originated from one common ancestral group in the Indo-Pacific Ocean around 14 million years ago. Based on the data we could predict a west to east migration pattern and an ancestral link between the white sharks of South Africa and Florida. But we also found a unique South African female lineage that does not connect to any other lineage in the world.
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'Unique South African female lineage in SA'
“This means that it is either a very old lineage that formed in South Africa millions of years ago, or it might be linked with a white shark population that has not been sampled yet.”
While the sharks from the common and newly-found lineage are interbreeding, the researchers still don’t know whether sharks from this lineage carry some morphological or physiological characteristics that differentiate them from the common group.
The findings have serious implication for the future management of the white shark population along the South African coastline.
“It is obvious that current conservation measures should take the low levels of genetic diversity into account, otherwise one of these days we will not have any white sharks left to worry about,” she warned.