Sequencing Sphagnum Leads to Discovery of Sex Chromosomes

A nuanced understanding of peat moss drives insight into carbon storage

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The Science

Boggy peatlands, which hold much of the Earth’s carbon as well as material that can be converted to energy, are made up heavily of sphagnum mosses. New research identifies sex chromosomes in the plant and illuminates the significant role sex plays in how the moss grows, stores carbon and responds to stress.

The Impact

Despite constituting less than 5% of land on Earth, peatlands store approximately one-third of the world’s soil carbon. The process to harvest energy-generating peat from these boggy wetlands produces an estimated 5% of annual greenhouse gas emissions. Understanding how sphagnum, a primary component of peatlands, processes carbon can help maximize peatlands’ potential as carbon sinks and minimize their role as carbon sources.

Summary

Sphagnum, a genus of just under 400 mosses that can carry up to 25 times their dry weight in water, makes up the overwhelming majority of the ground cover in peatlands. These boggy ecosystems are acidic and have very little oxygen, which is tough on the type of microbes that break down plant matter. This means plants don’t fully decompose in peatlands, and so carbon within their tissues doesn’t return to the atmosphere.

Peat bogs also serve the ecosystem by minimizing flood risks and filtering water. Peat formed there over thousands of years sequesters more carbon than all other vegetation types in the world combined. Harvesting peat for energy has been increasingly discouraged due to associated greenhouse gas emissions as well as the need to protect these vital ecosystems.

Peat bogs have two distinct zones: Hummocks and hollows flow like hills and valleys, on a smaller scale. Depending on the species, some sphagnum may prefer the dryer, more elevated soil of the hummocks or the more water-logged hollows below. These preferences affect their growth rates, decomposition, and carbon sequestration. Researchers wanted to better understand how sphagnum adapts to these varying topographies which could ultimately help preserve peat bogs in the wake of climate change.

Researchers at the U.S. Department of Energy Joint Genome Institute, a DOE Office of Science User Facility at Lawrence Berkeley National Laboratory, and the HudsonAlpha Institute for Biotechnology sequenced sphagnum genomes from plants selected by a team from Duke University, then tested for heat and pH stress by Oak Ridge National Laboratory. They selected two types of species: ones that preferred growing in hummocks, and others that preferred the hollows.

Those genome sequences showed not only that the sex of a sphagnum plant informs how it weathers stress, but they ultimately yielded answers as to how sex is determined in the moss. Previously it was theorized that sex determination was possibly driven by genetics, or even temperature. By sequencing the genome, researchers were able to pinpoint specific U/V chromosomes (‘U’ for female and ‘V’ for male) that determine sex. They are also the smallest sex chromosomes ever identified in the plant kingdom.

Specifically, researchers found that female plants within a species tended to weather pH stress from peatlands’ acidic environment more consistently, whereas male plants showed dramatic variation when under those same environmental stressors — with some thriving and others struggling to survive.

Given the vital role sphagnum plays in carbon sequestration, understanding how it responds to higher temperatures and a changing environment is vital to preserving peatlands in the face of a warming climate. Identifying these sex chromosomes and their role in sphagnum’s survival opens up a new avenue to explore for protecting these ecosystems.