This Moss’s Huge, 300 Million-Year-Old Sex Chromosomes Offer Clues About Our Own
At first glance, a short tuft of moss accented by thin stems might not seem like it would have much to tell us about human reproduction.
But new research into the genome of fire moss (Ceratodon purpureus) has biologists rethinking assumptions about the development of human sex chromosomes. The research, published in Science Advances, was led by Sarah Carey, who earned her PhD in Biology from the University of Florida in 2020, and Stuart McDaniel, PhD, Associate Professor of Biology and Carey’s graduate advisor.
Fire moss is a resilient species that can be found on all seven continents. It has separate male and female plants — but only has one sex chromosome for each sex (U and V), unlike the two (XX and XY) usually seen in humans.
Its sex chromosomes are also different from those of humans in their size: The researchers found that the U and V chromosomes are both physically huge, each containing 3,400 genes.
For humans and many other mammals, meanwhile, the Y chromosome is tiny and contains fewer than 100 genes, a small fraction of those seen on their X chromosomes. Some suggest that humans’ Y chromosome will continue to lose functional genes and could disappear entirely within 5 million years.
Many have attributed the diminutive size of the humans’ Y chromosome to the fact that it doesn’t recombine with the X chromosome during reproduction, unlike many other chromosomes that are able to mix. Carey and McDaniel’s research, however, challenges that idea that the lack of mixing is to blame. Fire moss’s male sex chromosome also doesn’t mix, but it has nonetheless maintained its enormous size for at least 300 million years — since before the age of dinosaurs.
“The lack of mixing and the loss of genes are not linked in the moss,” McDaniel said. “Our findings suggest that the degeneration of the human Y may not be so clearly linked to only the lack of mixing.”
To conduct their research, Carey and McDaniel worked with US Department of Energy (DOE) Joint Genome Institute (JGI) researchers at the HudsonAlpha Institute for Biotechnology, where Carey now works as a postdoctoral fellow. Enabled by the JGI’s Community Science Program, HudsonAlpha’s Genome Sequencing Center (GSC), led by co-directors Jane Grimwood, PhD, and Jeremy Schmutz, completely sequenced the fire moss genome over the course of a decade. Carey and McDaniel discuss this project in an episode of the JGI’s Genome Insider podcast, “The Big Deal about Short Plants.”
Along with the insights it offered about human sex chromosomes, a deeper understanding of the fire moss’s genome could also be useful in crop production: Moss sex chromosomes contain important genes that could be used to control fertility in flowering and crop plants.
“The moss genome provides a new tool for studying genes that might have very economically important sex specific functions,” McDaniel said.
Looking forward, UF researchers and the HudsonAlpha team are now looking to the genomes of other mosses to find genes responsible for symbiotic interactions between mosses and beneficial microbes or insects.
