Genetic variation in human sperm can begin years earlier than previously recognized. Researchers have discovered that non-crossover gene conversion occurs during routine mitotic cell divisions before meiosis begins, challenging long-standing assumptions about how inherited differences are generated and introducing new complexities to reproductive biology.
The Discovery of Early Genetic Shuffling
Some of the genetic differences that make every child unique may arise in a father’s sperm much earlier in development than scientists previously thought. A study published in Nature reveals that certain genetic reshuffling in human sperm occurs before meiosis, the specialized cell division that creates reproductive cells. Conducted by researchers at the Wellcome Sanger Institute, the University of Cambridge, and collaborating institutions, the work focuses on non-crossover gene conversion.
Gene conversion is a subtle process in which one chromosome copies a short DNA sequence from its matching partner. While it does not necessarily create a brand-new DNA sequence, it can change which inherited version is present at a specific location.
Distinguishing Mitosis from Meiosis in Sperm Development
Every child inherits one set of chromosomes from each parent, and recombination rearranges this genetic material during the production of sperm and eggs to create novel combinations. Traditionally, science has pointed to two main processes: crossovers, which exchange large chromosome segments in a two-way swap, and non-crossover gene conversion, which copies a much shorter stretch of DNA in one direction.
Historically, both processes were placed squarely within meiosis. However, precursor cells that eventually produce sperm must undergo repeated rounds of ordinary cell division, or mitosis, to maintain sperm production throughout adulthood. To investigate whether gene conversion begins during this earlier mitotic phase, the research team examined 15 sperm samples sourced from 13 donors ranging in age from 24 to 74 years old.
Utilizing high-accuracy long-read sequencing, the investigators analyzed extended sections of individual DNA molecules. This approach allowed them to identify small recombination events that are typically difficult to detect using conventional methods.
Uncovering Thousands of Early Events
The team successfully identified 7,143 crossovers and 2,382 non-crossover gene conversion events directly in sperm DNA. Comparing these patterns against long-read genomic data from blood and information from earlier studies revealed a critical distinction: a substantial proportion of the gene conversions appeared to form before meiosis even began.
These early events displayed molecular signatures that differed markedly from those produced during meiosis. Instead, they closely resembled DNA repair activity typically observed in ordinary body tissues.
Gene conversion patterns also varied in both frequency and location from person to person. Differences emerged even between identical twins despite their nearly identical inherited genomes. This variance indicates that sperm diversity is driven not just by genetic instructions, but also by biological events unfolding independently within each individual.
Challenging Long-Standing Assumptions
These findings support a two-stage model of recombination in sperm. While meiotic division later introduces further reshuffling, the initial copying of DNA takes place during standard cell divisions that maintain the pool of sperm-producing cells.
“We found that gene conversion in sperm does not appear to come from a single process. Alongside the familiar meiotic pathway, there is a substantial component with features consistent with DNA repair before meiosis. Being able to see thousands of these events directly in sperm gives us a new window into how genetic diversity is generated,” stated Dr. Regev Schweiger, first author at the Gray Faculty of Medical and Health Sciences, Tel Aviv University.
Professor Richard Durbin, co-senior author at the University of Cambridge and associate faculty at the Wellcome Sanger Institute, highlighted that achieving this breakthrough required a fundamental technical advancement in methodology. “High-accuracy long-read sequencing lets us observe both crossovers and the much harder-to-detect non-crossovers directly in sperm. This gives us far greater power to study recombination within individuals and has revealed biology that would be very difficult to resolve from previous data.”
Implications for Genomic Stability and Inherited Disease
The research uncovered many of these DNA copying events in genomic regions that are naturally vulnerable to breakage. While repairing these unstable regions protects the overall genome, mistakes made during the repair process can produce changes that are subsequently transmitted to the next generation.
Studying how these changes develop provides new clues regarding inherited diseases, reproductive health, and the underlying mechanisms that maintain DNA integrity.
