German anatomist Wilhelm von Waldeyer-Hartz named these structures chromosomes in 1888, for the ease with which they took up dye.Īround the same time, cell biologists observed chunks of chromosomes being discarded in a parasitic roundworm called Parascaris univalens that infects horses - a much-studied worm because its pair of huge chromosomes were easy to view under a microscope. Starting in the late 1800s, well before scientists nailed down the link between DNA and heredity, biologists peering down microscopes used dyes to study tiny, twig-like bodies inside dividing cells, watching as the twigs grouped together and then separated. “It’s a fundamental difference between the DNA that’s going on to the next generation and the DNA that’s in all the other cell types,” says Jeramiah Smith, a geneticist at the University of Kentucky who studies the phenomenon in lampreys and described it in the 2020 Annual Review of Animal Biosciences. DAVIS / CURRENT OPINION IN GENETICS & DEVELOPMENT 2014īeyond their fascinating oddness, these quirks may hold broader lessons on how genomes work the way they do, scientists think, and how and why the DNA in germ cells is treated differently from the DNA in the rest of the developing critter’s body. (Ciliates are noted in blue to indicate the tree’s root: the most recent common ancestor of all the creatures shown.)ĬREDIT: J. Animal groups in which cases have been documented are noted in red, as are dates when the discoveries in a given group were first reported. The standout stars of this odd show are songbirds, which all appear to display the phenomenon. But what is it?įrom copepods and worms to lampreys and bandicoots, programmed DNA elimination crops up all over the multicellular tree of life, with more cases likely waiting to be found.
“It’s not just one freak event.” Presumably, then, there must be some selective advantage to the creatures that go down that evolutionary route. “A lot of these weird genomic features tend to be fairly rare, but they do evolve repeatedly,” Ross says. They’ve seen it in rat-like marsupials called bandicoots and in songbirds - probably all songbirds, according to recent work. To date, scientists have observed the phenomenon in various insects, in lampreys and hagfish, in hairy one-celled life forms called ciliates, in parasitic roundworms and tiny crustaceans called copepods. A surprisingly wide array of creatures, all the way up to some vertebrates, dump significant stretches of DNA during early development, so the stretches don’t end up in most of their body cells. And black-winged fungus gnats aren’t the only genetic screwballs. How and why this feature evolved remains largely mysterious, though biologists first spotted it a century ago. Only the cells that become germ cells retain the bonus DNA and pass it on to the next generation. Early in the embryo’s development, most of the cells jettison two specific chromosomes - enormous ones, compared with the others - so the pair never ends up in the lion’s share of the gnat’s body. Health & Disease Genetic tricks of the longest-lived animalsīut the fungus gnat does something bizarre.