An international team of researchers has sequenced the genomes of the giraffe and its closest living relative, the okapi, and — through comparative analysis with other mammals — identified 70 genes that exhibit multiple signs of adaptation in giraffe.
An adult female Masai giraffe (Giraffa camelopardalis tippelskirchi) in the Masaai Mara National Park, Kenya. Image credit: Bjørn Christian Tørrissen / CC BY-SA 3.0.
“The origin of giraffe’s iconic long neck and legs, which combine to elevate its stature to the tallest terrestrial animal, has intrigued mankind throughout recorded history,” said lead author Prof. Douglas Cavener of Penn State University and his colleagues from the United States, Tanzania, Kenya and the UK.
“Giraffe’s unique anatomy imposes considerable existential challenges and three systems bear the greatest burden: (i) the cardiovascular system to maintain blood pressure homeostasis; (ii) the musculoskeletal system to support a vertically elongated body mass; and (iii) the nervous system to rapidly relay signaling over long neural networks.”
To identify genetic changes likely to be responsible for the giraffe’s unique characteristics, Prof. Cavener and co-authors compared the gene-coding sequences of the Masai giraffe (Giraffa camelopardalis tippelskirchi) and the okapi (Okapia johnstoni) to more than forty other mammals including the cow, sheep, goat, camel, and human.
“The whole-genome sequence of two Masai giraffe from the Masai Mara in Kenya and the Nashville Zoo, and one fetal okapi from the White Oak Conservatory was determined by constructing paired-end libraries followed by sequencing using an Illumina HiSeq yielding ca. 30 x coverage,” they said.
“The okapi, the giraffe’s closest relative and the only other extant member of the Giraffidae family, provides a useful comparison, because it does not share these unique attributes seen in giraffe.”
“Okapi’s gene sequences are very similar to the giraffe’s because the okapi and giraffe diverged from a common ancestor only 11-to-12 million years ago – relatively recently on an evolution timescale,” Prof. Cavener added.
The scientists discovered 70 genes that showed multiple signs of adaptations.
“These adaptations include unique amino-acid-sequence substitutions that are predicted to alter protein function, protein-sequence divergence, and positive natural selection,” Prof. Cavener said.
Among the team’s discoveries are that several genes known to either to regulate the development of the cardiovascular system or to control blood pressure are among the genes showing multiple signs of adaptation in the giraffe.
Some of these genes control both cardiovascular development and skeletal development, suggesting the intriguing possibility that the giraffe’s stature and turbocharged cardiovascular system evolved in concert through changes in a small number of genes.
The team also discovered genetic clues to the evolution of the giraffe’s long neck and legs, which have the same number of bones as the neck and legs of humans and other mammals.
“To achieve their extraordinary length, giraffe cervical vertebrae and leg bones have evolved to be greatly extended. At least two genes are required – one gene to specify the region of the skeleton to grow more and another gene to stimulate increased growth,” Prof. Cavener said.
The researchers identified genes that are known to regulate both of these functions.
“The most intriguing of these genes is FGFRL1, which has a cluster of amino acid substitutions unique to giraffe that are located in the part of the protein that binds fibroblast growth factors – a family of regulators involved in regulating many processes including embryo development,” Prof. Cavener said.
This fibroblast-growth-factor pathway plays a crucial role in controlling development, beginning in early development of the embryo and extending through the bone-growth phase after the giraffe is born.
In humans and also in mice, severe skeletal and cardiovascular defects are associated with debilitating mutations in FGFRL1.
The team also identified four homeobox genes — the kind involved in the development of body structures — which are known to specify the regions of the spine and legs.
“The combination of changes in these homeobox genes and the FGFRL1 gene might provide two of the required ingredients for the evolution of the giraffe’s long neck and legs,” Prof. Cavener said.
The team also noticed a group of genes regulating metabolism and growth that were diverged in giraffe as compared to okapi. One of these genes encodes the receptor for folic acid, which is an essential B vitamin necessary for normal growth and development.
Other metabolic genes that they found to be significantly changed in giraffe are those involved in the metabolism of the volatile fatty acids that are generated by the fermentation of ingested plants.
According to scientists, the giraffe has an unusual diet of acacia leaves and seedpods, which are highly nutritious but also are toxic to other animals.
Prof. Cavener and his colleagues speculate that the genes responsible for metabolizing acacia leaves may have evolved in the giraffe in order to circumvent this toxicity.
The results were published in the May 17 issue of the journal Nature Communications.
_____
Morris Agaba et al. 2016. Giraffe genome sequence reveals clues to its unique morphology and physiology. Nature Communications 7, article number: 11519; doi: 10.1038/ncomms11519
