The biologists have used the model organism Caenorhabditis elegans to identify molecular mechanisms that produce DNA damage in sperm and contribute to male infertility following exposure to heat.
In humans, the optimal temperature for sperm production is just below body temperature, in a range of about 90-95 degrees F. Human studies have found that exposure to temperatures as little as 1 degree C (1.8 F) above this normal range adversely affects male fertility, said the senior author.
The phenomenon of heat-induced male infertility is well known, and the effects of modern exposures to heat such as hot tubs, tight clothing and excessive drive times have been extensively studied. The underlying mechanisms that damage sperm and impair fertilization are not completely understood.
"In both humans and C. elegans, relatively small increases in temperature are sufficient to reduce male fertility," said the author.
An increase of 2 C (3.6 F) above normal in C. elegans, a type of roundworm, led to a 25-fold increase in DNA damage in developing sperm compared to unexposed sperm. Eggs fertilized by these damaged sperm failed to produce offspring.
This basic research discovery is detailed in a paper published in the journal Current Biology.
The paper also helps to understand how meiosis, the process that produces sex cells, differs between sperm and eggs.
Sperm, the smallest cell in a person's body, form by the billions at temperatures below body temperature and are produced throughout the entire adult lifespan. Eggs, the largest cells in a person's body, are formed internally, where a consistent temperature is maintained, and are produced only for a limited time during fetal development.
"We know that sperm development is very sensitive to increased temperature, while egg development is not affected," the author said. "The data presented in this paper suggest that another way egg and sperm develop differently is in how tightly they control the ability of mobile DNA elements, which are also known as 'jumping genes' or transposons, to move in the genome, and how sensitive to heat stress those mechanisms are in preventing that movement."
Transposons are DNA segments that move around and alter genetic information by inserting themselves in new positions. They also leave DNA damage in their wake. Movement of these "jumping genes" is normally repressed in developing sperm and eggs. However, this study found that with exposure to heat transposons are moving specifically in developing sperm.
The research team used microscopy to observe developing sperm and eggs under both normal and heat-stressed conditions. In the latter, the researchers saw higher amounts of DNA damage in sperm, but not eggs. Using next-generation genome sequencing, they also identified the locations of transposons across the whole genome with and without exposure to heat.
"We found that after heat shock, certain transposons are found in new and more variable locations in the male genome," the author said.
The study not only shows that a small rise in temperature affects meiotic divisions but she also identifies a mechanism - not only where the error occurs but what the error is.
Why heat stress damages sperm?
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