This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Scientists have identified an unexpected source of genetic variation in human sperm that occurs earlier in sperm development than previously thought. Published in Nature, researchers at the Wellcome Sanger Institute, the University of Cambridge and their collaborators found that one type of genetic recombination involving chromosomes, called non-crossover gene conversion, happens before the cell divisions that produce sperm.
The findings suggest that genetic recombination events in sperm happen in two stages, both before and after cell division. The team found that these DNA recombination patterns can vary between individuals, including identical twins. This study opens new opportunities to investigate biological mechanisms underlying fertility, genome evolution and inherited disease.
Every child inherits a unique combination of DNA from their parents. Much of this diversity is generated through recombination, where the two copies of a chromosome—one inherited from each parent—exchange or copy segments of DNA as sperm and eggs are formed. The cells that ultimately produce sperm first undergo many rounds of ordinary cell division, or mitosis, before entering meiosis—the specialized cell division that generates reproductive cells.
Scientists have generally assumed that most genetic recombination occurs during meiosis, through crossovers or non-crossover gene conversion. Crossovers are where large sections of chromosomes are exchanged in a two-way swap. Non-crossover gene conversion is a one-way copying of a short stretch of DNA from one chromosome to the other.
These short events have been much harder to study directly. In this new study, the researchers were investigating whether all non-crossover gene conversions found in sperm originate during meiosis or whether some arise earlier, during the premeiotic cell divisions. The team analyzed 15 sperm samples from 13 donors ages 24 to 74.
Using highly accurate long-read sequencing, they identified 7,143 crossover events and 2,382 non-crossover gene-conversion events directly from sperm DNA. They compared these patterns with long-read genomic data from blood and previously published data. For the first time, researchers discovered that a substantial proportion of gene conversion appears to originate before meiosis begins.
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