They were initially thought to be specks on a microscope slide.
A new study has shown that microchromosomes, a small chromosome that is found in reptiles and birds, have a more long history and play a greater role in mammals than previously thought.
Researchers have been able show consistency in DNA molecules between bird and reptile species by comparing the sequences of microchromosomes from many species. This consistency can be traced back hundreds of thousands of years.
The team also discovered that these bits and pieces of genetic code were placed on larger chromosomes by marsupial or placental mammals (including humans). This means that the human genome may not be as normal as it was thought.
Jenny Graves, a geneticist at La Trobe University in Australia, says that she compared sequences of birds, turtles and snakes with lizards, humans, and humans. Surprisingly, all species of reptile and bird had the same microchromosomes.
"Even more amazing, they were identical to the tiny chromosomes from Amphioxus, a small fish-like animal that had no backbone and last shared a common ancestor with vertebrates 684 millions years ago."
Scientists were able establish genetic connections to all its descendants by tracing microchromosomes back at the ancient Amphioxus. These tiny "specks" of dust are not abnormal extras but important building blocks for vertebrates.
Most mammals seem to have absorbed and jumbled their microchromosomes over time, making them appear like normal DNA. Researchers found that the platypus is an exception. It has multiple chromosome segments that line up with microchromosomes. This suggests that this method may have been a stepping stone for other mammals.
The microchromosomes of birds and reptiles are constant, but they can be mixed up with larger chromosomes for mammals. (Paul Waters).
The study revealed that microchromosomes are also found in the same spot inside cells as they are across many species.
Paul Waters, a biologist at the University of New South Wales (UNSW), in Australia, says that they are not only the same in every species but that they also crowd together in central nucleus, where they interact with one another, suggesting functional coherence.
"This is not the case for large chromosomes within our genomes.
Recent advances in DNA sequencing technology have allowed researchers to sequence microchromosomes from end to end, as well as to determine where and why these DNA fragments were created.
Although it is not clear whether coding DNA in larger or microchromosomes has an evolutionary advantage, the paper may help scientists to put this debate to rest. However, there are many questions.
This study shows that large chromosomes that evolved in mammals may not be the norm. It could actually be detrimental. For example, microchromosomes pack genes together more tightly.
Graves says that chromosomes from humans and other mammals were not 'normal'. They were instead stuffed with lots of junk DNA and scrambled in many ways.
"The new knowledge helps to explain why there are so many mammals with vastly differing genomes that inhabit every corner of the planet."
The research was published in PNAS.
