October 19, 2016
The African clawed frog's ancestor inherited one set of chromosomes each from two different species and doubled its whole genome some 18 million years ago, according to an international research consortium led by Japanese and American scientists who sequenced the entire genome of the Xenopus laevis for the first time. Scientists hope that the finding will help our understanding of vertebrate evolution, as the vertebrate genome doubled twice 500 million years ago.
X. laevis is unusual in that it is a tetraploid species that has four sets of chromosomes, while many organisms, including humans, are diploid and have two sets of chromosomes. How and when this came about has been a topic of debate for some time.
One hypothesis is that the tetraploid X. laevis inherited one half of its genetic material from each parent when two diploid ancestral species mated, and the genome of this diploid offspring then doubled, giving rise to a tetraploid organism with twice the number of chromosomes as its ancestors.
"X. laevis is an essential organism for biological and biomedical research, but the sheer size and complexity of its genome made it difficult for scientists to sequence the genome in its entirety," says Associate Professor Masanori Taira, leader of the Japanese research team at the University of Tokyo. He continues, "He thought that sequencing the entire genome would not only be valuable for biological and biomedical research but also provide clues as to the origins of tetraploidy."
The researchers sequenced the entire genome of the J (Japan) strain of the frog inbred in Japan, in which the genetic codes inherited from the mother and father, respectively, are identical.