‘Useless Specks of Dust’ Turn Out To Be Building Blocks of All Vertebrate Genomes

An anonymous reader quotes a report from ScienceAlert: Originally, they were thought to be just specks of dust on a microscope slide. Now, a new study suggests that microchromosomes — a type of tiny chromosome found in birds and reptiles — have a longer history, and a bigger role to play in mammals than we ever suspected. By lining up the DNA sequence of microchromosomes across many different species, researchers have been able to show the consistency of these DNA molecules across bird and reptile families, a consistency that stretches back hundreds of millions of years. What’s more, the team found that these bits of genetic code have been scrambled and placed on larger chromosomes in marsupial and placental mammals, including humans. In other words, the human genome isn’t quite as ‘normal’ as previously supposed. By tracing these microchromosomes back to the ancient Amphioxus, the scientists were able to establish genetic links to all of its descendants. These tiny ‘specks of dust’ are actually important building blocks for vertebrates, not just abnormal extras. It seems that most mammals have absorbed and jumbled up their microchromosomes as they’ve evolved, making them seem like normal pieces of DNA. The exception is the platypus, which has several chromosome sections line up with microchromosomes, suggesting that this method may well have acted as a ‘stepping stone’ for other mammals in this regard, according to the researchers. A tree chart outlining the presence of similar DNA in snakes, lizards, birds, crocodiles, and mammals. The study also revealed that as well as being similar across numerous species, the microchromosomes were also located in the same place inside cells. “It’s not clear whether there’s an evolutionary benefit to coding DNA in larger chromosomes or in microchromosomes, and the findings outlined in this paper might help scientists put that particular debate to rest — although a lot of questions remain,” adds ScienceAlert. “The study suggests that the large chromosome approach that has evolved in mammals isn’t actually the normal state, and might be a disadvantage: genes are packed together much more tightly in microchromosomes, for example.” The findings have been published in the journal PNAS. Read more of this story at Slashdot.