The Gibbon Genome and Natural Genetic Engineering in Primate Evolution

Last week Nature published the draft genome of a Gibbon species (Nomascus leucogenys) in an important article, "Gibbon genome and the fast karyotype evolution of small apes." Gibbons are near relatives. They are intermediate between Old World Monkeys and the Great Apes, to which we belong. The four Gibbon genera "experienced a near-instantaneous radiation ~5 million years ago."

Gibbon genomes show high variability in chromosome number and structure, unlike their monkey, ape and human relatives. Gibbons have chromosome numbers that range from 38-52. The apes all have 48 chromosomes; we have 46 due to a chromosome fusion. Moreover, Gibbon chromosomes are massively rearranged compared to ape chromosomes.

How could such dramatic evolutionary changes in chromosome structure have taken place so rapidly? The genome sequence provides clues to three distinct phases of natural genetic engineering (NGE) at work.

The first phase was the appearance of LAVA, a non-LTR retrotransposon mobile genetic element unique to Gibbons. LAVA was named for its component parts derived from L1 (mammal-wide), Alu (primate-specific), and SVA (hominid-specific) retrotransposons.

The second phase involved the mobilization of LAVA to insert into and modify the expression of genetic loci encoding cell division functions. These modifications then led to the third phase of disrupted divisions and chromosome breakage. Chromosome fragments were then stitched together by "Non-homologous end-joining" (NHEJ).

All three steps involve NGE: construction of LAVA from other retrotransposons, LAVA mobilization to modify cell division functions, and NHEJ to repair chromosome damage. There could hardly be a better example than our cousins the Gibbons of rapid evolutionary change mediated by NGE.