Long term ionizing radiation exposure reduces fertility by damaging spermatocyte meiosis

Ionizing radiation has been part of our universe since long before life appeared on Earth. Long before the first microbes emerged in ancient oceans, the planet was already bathed in high energy particles from space and radiation from naturally occurring radioactive elements in rocks and soil.

“Every organism, from tiny bacteria to humans, is continuously exposed to low levels of background ionizing radiation,” PhD candidate Hengyi Zhu says.

The radiation comes from cosmic rays, terrestrial minerals, and modern human-made sources such as medical imaging and air travel. Living organisms also contain small amounts of naturally occurring radionuclides.

“Understanding how ionizing radiation affects living organisms is essential for protecting both humans and ecosystems.”

In her PhD, Zhu has examined how long-term exposure to different types of ionizing radiation harms cells. She has also studied the natural defence mechanisms that help protect against this damage.

“Because many of these repair pathways are shared across species, understanding these mechanisms helps us assess environmental risks and protect both human health and wildlife,” she explains.

Reduced sperm counts and fertility

Previous studies have shown that chronic exposure to ionizing radiation impairs sperm production in the nematode Caenorhabditis elegans. C elegans is a microscopic worm commonly found in soil, compost, and decaying plant matter. However, until now, it remained unclear which specific stages of sperm development are affected.

The results show that in hermaphrodites chronically exposed chronic gamma rays, the critical damage occurs during spermatocyte meiosis. This is the stage where sperm precursor cells divide into spermatids, which are the immature sperm cells that will later mature and become capable of fertilization.

“Disruption at this stage leads directly to reduced sperm counts and decreased fertility,” Zhu says.

Effects were also observed during earlier phases of sperm development. However, these appear to be either reversible or not severe enough to impact overall fertility.

Gamma rays and alpha radiation

To study the effects of chronic radiation, C. elegans populations were exposed over several days to either:

• gamma rays from a cobalt-60 source, or
• alpha particles from radium224 chloride solutions

The worms were studied at different life stages. Zhu measured:

• fertility (brood size)
• sperm counts
• embryo survival
• germ cell death
• residual body counts
• oxidative stress
• transcriptomic changes (genes upregulated or downregulated)

She also used genetic tools to “turn off” specific DNA repair genes. This helped reveal which repair mechanisms protect against radiation-induced damage.

Differences between sex

Zhu also found that the adverse effects on spermatogenesis may be sex-dimorphic, based on differences in post-meiotic phenotypes and transcriptomic alterations. Hermaphrodites showed clear adverse effects during the post-meiotic stage of spermatogenesis, along with downregulation of sperm-related genes, whereas males did not.

This means that males and hermaphrodites do not respond to radiation in the same way and that the underlying genetic activity that supports sperm development can differ between the two sexes.

DNA repair makes a crucial difference

Ionizing radiation can break DNA strands. The research identified two DNA repair pathways that help protect against this:

• RAD51-mediated homologous recombination repair (HRR)
• POLQ1-mediated end joining (TMEJ)

Worms lacking these repair systems suffered far more severe fertility loss. RAD‑51 was especially important, not only for the exposed parents but also for the survival of their offspring.

Effects can carry over to the next generation

Most embryos born to irradiated parents survived and grew into fertile adults. But they did not escape unscathed. The offspring showed shorter gonads, fewer egg cells and changes in gene expression related to development and stress. These effects were linked mainly to damage inherited through the oocytes. Sperm exposure alone did not reduce offspring fertility.

“This means chronic ionizing radiation-induced changes can pass across generations, but mostly through the maternal line,” Zhu says.

Similar effects despite huge dose differences

She also compared chronic exposure to two types of ionizing radiation:

• Gamma radiation (low linear energy transfer)
• Alpha radiation from radium‑224 (high linear energy transfer)

Alpha particles usually cause more severe damage because they deposit energy densely in tissues.

“The expectation was that the effects following the gamma radiation would be less severe than the ones following alpha radiation,” she says.

That, however, turned out not to be the case.

Worms exposed to alpha particles showed similar levels of fertility decline, sperm reduction, and oxidative stress as those exposed to gamma rays, even though their internal dose was estimated to be about 1000 times higher.

At the molecular level, the two radiation types triggered different gene expression profiles. Alpha radiation caused a much broader transcriptomic response, especially in genes linked to stress and chromatin organization.

Radiation risks for reproduction

“The results show that the most radiosensitive step in sperm formation is meiosis. When this stage is disrupted, fertility declines, even when the overall radiation dose is low,” Zhu summarizes.

Longterm radiation exposure is relevant to human activities in many settings, such as radioactive waste management, contaminated environments, medical exposure, and even space travel. These findings help identify which biological processes are most at risk and can guide better protection strategies for both humans and ecosystems.