Genome “dust” turns out to be tiny chromosomes from ancient ancestors

Scientists in Australia have made some intriguing discoveries in the genomes of a wide range of animals. What was initially mistaken for dust on the slides turned out to be tiny “microchromosomes” from an ancient fish-like invertebrate, which were present in some form in a variety of birds, reptiles and mammals.

An organism’s genome is a complex tapestry woven over hundreds of millions of years of evolution. Nature is constantly shuffling and remixing DNA, but some sequences stay remarkably intact for long periods of time, and may be conserved across distantly related species.

A new study, led by Professor Jenny Graves at La Trobe University, has now identified an intriguing example of this phenomenon. Among the larger chromosomes that make up a genome, the team found what were thought to be specks of dust that, on closer inspection, were found to be tiny chromosomes of their own. When the researchers sequenced these microchromosomes, they found the unexpected fingerprint of an animal separated by more than half a billion years of evolution.

“We lined up these sequences from birds, turtles, snakes and lizards, platypus and humans and compared them,” says Professor Graves. “Astonishingly, the microchromosomes were the same across all bird and reptile species. Even more astonishingly, they were the same as the tiny chromosomes of Amphioxus – a little fish-like animal with no backbone that last shared a common ancestor with vertebrates 684 million years ago.”

Although these microchromosomes could still be found in the mammals studied, the sequences had been divided into smaller patches across the larger, “normal” chromosomes. In the platypus, however, the microchromosomes had instead fused together into a few larger blocks.

The researchers say that the new work improves our understanding of evolution, and how we might view and study the human genome.

The research was published in the journal PNAS.

Source: La Trobe University

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