These gangly, tree-swinging apes living in the tropical forests of southeast Asia are quite peculiar: Individuals have different numbers of chromosomes and an unusually large number of chromosomal breaks and rearrangements. These sorts of disruptions can cause cancer in humans. Now, with the gibbon genome sequenced, researchers are closer to understanding how these unstable genomes may contribute to their diversity and remarkable ability to adapt to their jungle habitat. The study was published in Nature this week.
Gibbons are part of the same superfamily (Hominoidea) as humans and great apes, and they were the first apes to branch off from the common ancestor we share with monkeys. Gibbons exhibit tremendous dexterity and diversity. They can traverse over a dozen meters in a single swing, at speeds beyond 55 kilometers per hour, Nature reports. The 19 known gibbon species represent three-quarters of all apes.
And then there’s the massively reshuffled DNA. Chromosomal rearrangements -- the same DNA structural changes that cause cancer in humans -- seem to happen very frequently in gibbons. Furthermore, whereas humans have 23 pairs of chromosomes, and chimps, gorillas, and orangutans have 24 pairs, gibbons have diploid chromosome numbers ranging from 38 to 52.
To resolve gibbon genomics and evolutionary history, a huge international team led by Lucia Carbone from Oregon Health & Science University assembled and analyzed the genome of Asia, a northern white-cheeked gibbon (Nomascus leucogenys) living in the Virginia Zoo. They then sequenced the genome of another seven gibbons to represent all four genera.
To the right is a northern white-cheeked gibbon mother, Astriks, with her 4-week-old male infant, Rossy. Astriks is one of the gibbons whose DNA sequence was analyzed.
All gibbon genera, they found, diverged almost instantaneously about four million years ago. Around that time, there were major changes to forests and significant shifts in sea levels in the areas occupied by gibbons. These geographical fluctuations restricted mating between groups, likely leading to their rapid divergence and the rise of new species.
The team identified a new repetitive DNA sequence called LAVA elements, which insert themselves into genes that help separate chromosomes when cells divide. “The LAVA element is an evolutionary novelty that is only in the DNA of gibbon species,” Carbone says in an university statement. “We think that it played a major role by increasing the ‘errors’ during cell division and chance for chromosomal rearrangements.”
Together with multiple episodes of climate and environmental change, the spread of LAVA may have contributed to the variation of the chromosomes of gibbons. “Similar disruptions cause disease, which is why everything we learn from this helps us better understand human biology and chromosomal structures,” says study coauthor Jeffrey Rogers of Baylor in a press release.
The team also found that the genes involved in bone and cartilage development and in making collagen seem to have evolved faster in gibbons than in other apes, Nature reports. This may explain how gibbons developed longer arms and powerful shoulder and arm tendons -- important if your main mode of locomotion is swinging from tree to tree in dense canopies.
Here’s an acrobatic female northern white-cheeked gibbon leaping from one branch to another in Yexianggu, China, with her baby clinging on:
Images: Heather Angel/Natural Visions (top, bottom), Gabriella Skollar (middle)