'Selfish chromosomes' influence which sperm cells survive

A gene that normally ensures sperm quality in fruit flies can be hijacked by 'selfish chromosomes' to bias inheritance, showing how some genetic elements determine which sperm survive.

Geneticists have long studied how 'selfish' genetic elements influence gamete survival and inheritance. Previous work has explored how such sequences affect sperm competition in various species and can distort normal segregation patterns (see BioNews 1086). Building on this foundation, researchers at the University of Utah have identified a specific mechanism in fruit flies (Drosophila) – involving the Overdrive (Ovd) gene – by which selfish chromosomes exploit a quality control checkpoint during sperm development to increase their own chances of being inherited. By comparing multiple Drosophila species, the team showed how this hijacking alters which sperm survive.

'This is the first time that the same gene has been shown to be crucial for eliminating gametes by multiple independent selfish chromosomes,' said Jackson Ridges, PhD student and lead author of the study published in Nature Communications. 'It indicates that evolutionarily distant selfish chromosomes may often converge on shared cellular processes.'

The researchers identified spermatid individualisation – a late stage of sperm development when immature sperm cells are separated and shaped – as the checkpoint where elimination occurs. Normally, Ovd removes post‑meiotic spermatids that fail essential quality checks, preserving fertility.

Selfish chromosomes exploit this checkpoint by disrupting the histone‑to‑protamine transition, a protein swap essential for tightly packaging DNA. Spermatids without the selfish chromosome are marked and eliminated, allowing the selfish chromosome to dominate in the surviving sperm.

The researchers observed the same Ovd‑linked behaviour in evolutionarily distant Drosophila species that carry unrelated selfish chromosomes, demonstrating that separate genetic elements can exploit the same vulnerability in sperm development. Ovd acts as a gatekeeper that can be 'weaponised' to influence which sperm succeed.

'How selfish genes can cause sterility has been a long-standing mystery in the field of speciation,' said Dr Nitin Phadnis, associate professor at the University of Utah and senior author of the study. 'By going for a deep understanding of how Overdrive works, we inadvertently opened up entirely new directions of research into understanding the mechanisms of cellular quality control systems, and how sterility emerges between young species.'

The Ovd gene does not exist in humans, so the findings are not directly applicable to people, but they help explain how some genetic elements influence inheritance.

Next, the team plans to remove Ovd in additional fruit fly species to see how widespread this hijacking is and whether other selfish chromosomes exploit the same checkpoint. They also aim to explore whether similar pathways may contribute to sterility in other lineages, potentially including human ancestry.