Ancient Elephant Genes Tell New Story
Fall/Winter 2006
As the largest creatures living on land, elephants can stand up to 13 feet at the shoulder and weigh more than eight tons. But their population is less than elephantine — poaching and loss of habitat have halved their numbers in the last two decades to about 600,000.
However, instead of having one African species with a dwindling population, there are two smaller species that may be at even greater risk. This is because the African elephant — distinguished from its Asian cousin by large, fan-like ears and long tusks — was categorized five years ago as two distinct species.
In fact, genetic differences between the two African species — the evolutionary split occurred about 2.6 million years ago — were first confirmed by SAIC-Frederick researcher Alfred Roca (and colleagues) in 2001. To better understand the two species, Roca and colleagues recently published an ESTC Award-winning article examining gene flow between African savanna elephants and African forest elephants.
According to the research, many savanna elephants far from forest habitats have certain genetic components that suggest forest elephant ancestry. These genetic components — mitochondrial DNA — are passed on only through female elephants.
According to Roca, the apparent disconnect between the lack of forest nuclear DNA in savanna elephants and the presence of forest mitochondrial DNA implies crossbreeding between forest females and savanna males. In fact, the research indicates that larger savanna males — which are almost twice as massive as forest males — have for generations out-reproduced both forest males and smaller hybrid males. This has led to nuclear genome replacement in savanna herds that retained the maternal forest elephant DNA. (The authors also note that female elephants, in contrast to males, remain with their natal herd for life.)
According to the authors, there have been recent proposals to use mitochondrial DNA as the sole marker to identify elephants. However, Roca cautions against using a single mitochondrial DNA sequence to characterize a species, as it may be misleading. Also, their research shows that mitochondrial DNA preserves a record of ancient habitat change. In fact, ongoing deforestation may foster genetic replacement of forest elephants by opening their habitat to reproductive competition by larger savanna males in those regions where both species exist.
The article by Roca (and colleagues), "Cytonuclear Genomic Dissociation in African Elephant Species", was published in Nature Genetics. Their research was supported by the African Elephant Conservation Fund of the U.S. Fish and Wildlife Service. Roca conducted research at the Laboratory of Genomic Diversity, which SAIC operates for the National Cancer Institute. The lab investigates genetic factors involved in human and feline diseases, as well as genetic variability in wildlife.
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