26. 08. 2025
Reading time: 2 minutes
At first glance, the shape of sperm may seem like a minor detail without much significance. However, a new international study published in the iScience journal (IF 4.1) (open in a new window) by Cell Press shows that even this minor detail has a solid genetic basis and apparently also fundamental evolutionary significance. Reproductive biologist Helena Fulková from the Department of Cell Nucleus Plasticity of the IEM CAS also contributed significantly to the research.
The study focused on the protein protamine-1 (PRM1), which plays a key role in spermatogenesis. Together with protamine-2, it replaces histones in spermatozoa heads, ensuring high chromatin condensation, which is essential for DNA stability and proper sperm development. By comparing human and mouse PRM1 sequences, the researchers noticed that mice have two specific amino acids – cysteine 15 and 29 – that could be key to the “hooked” shape of mouse sperm nuclei.
The team decided to test this hypothesis and used genetic engineering to create an animal model in which these two amino acids were modified. The result? The mice remained fertile, and the number of sperm remained unchanged, but their shape changed—the sperm lost the characteristic hook on their heads, and their chromatin was significantly less compact under the microscope. The study suggests that small differences in PRM1, which vary between animal species, affect the shape of sperm and the amount of original proteins (histones) that remain in its nucleus.
An interesting finding was that the increased amount of histones in sperm nuclei did not affect male fertility. This is surprising, as previous research suggests that insufficient replacement of histones with protamines plays an essential role in some types of male infertility. This result thus raises new questions about the relationship between chromatin structure, fertility, and species-specific differences in sperm molecular equipment.