Originally, it was thought that they were just dust spots on a slide.
Now, a new study suggests that microchromosomes – a type of tiny chromosomes found in birds and reptiles – have a longer history and a larger role to play in mammals than we ever imagined.
By aligning the DNA sequence of microchromosomes across many different species, scientists have been able to show the consistency of these DNA molecules across bird and reptile families, a consistency that extends hundreds of millions of years back.
What’s more, the team found that these parts of genetic code have been distorted and placed on larger chromosomes in marsupials and placenta mammals, including humans. In other words, the human genome is not quite as ‘normal’ as previously thought.
“We compiled these sequences from birds, turtles, snakes and lizards, platypus and humans and compared them,” says geneticist Jenny Graves of La Trobe University in Australia. “Amazingly, the microchromosomes were the same across all bird and reptile species.
“Even more astonishingly, they were the same as the small chromosomes of Amphioxus – a small fish-like animal without a backbone that last shared a common ancestor with vertebrates 684 million years ago.”
By tracing these microchromosomes back to the ancient Amphioxus, scientists were able to establish genetic links to all of its descendants. These small ‘dust spots’ are actually important building blocks for vertebrates, not just abnormal extras.
It seems that most mammals have absorbed and mixed their microchromosomes as they have evolved, making them appear like normal pieces of DNA. The exception is the platypus, which has several chromosome sections in line with microchromosomes, suggesting that this method may well have acted as a ‘springboard’ for other mammals in this regard, according to the researchers.
The study also revealed that in addition to being similar across several species, the microchromosomes were also located in the same place inside cells.
“Not only are they identical in each species, but they crowd together in the middle of the nucleus, where they physically interact with each other, suggesting functional coherence,” said biologist Paul Waters, of the University of New South Wales (UNSW) in Australia.
“This strange behavior is not true for the large chromosomes in our genomes.”
The researchers credit the latest advances in DNA sequencing technology for the ability to sequence micro-chromosomes end-to-end and to better determine where these DNA fragments came from and what their purpose may be.
It is not clear whether there is an evolutionary advantage to encoding DNA in larger chromosomes or in microchromosomes, and the results outlined in this paper may help scientists calm the particular debate – although many questions remain. .
The study suggests that the large chromosome influx that has evolved in mammals is not actually the normal state and can be a drawback: genes are packed much more closely together in microchromosomes, for example.
“Instead of being ‘normal’, chromosomes in humans and other mammals were inflated with lots of ‘junk DNA’ and distorted in many different ways,” says Graves.
“The new knowledge helps explain why there are such a large number of mammals with vastly different genomes inhabiting every corner of our planet.”
The research is published in PNAS.