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Selfish genetic elements identified in sperm race

8 March 2021
Appeared in BioNews 1086

Genes associated with a 'selfish' survival advantage in sperm have been identified, providing evidence of sperm-level natural selection.

The study describes variation between sperm dependent on the specific allele they are carrying. These findings contradict the established scientific consensus about Mendelian inheritance, which relies on sperm being functionally equivalent to ensure equal probability of alleles being taken up by offspring.

'Since the time of Mendel, our understanding of genetics has relied on the fact that alleles […] are randomly distributed with a 50/50 chance to each of my offspring' co-author Dr Robin Friedman, of Ohana Biosciences in Cambridge, Massechussetts told Inverse.

He said that whilst this assumption, referred to as Mendel's first law, still remains generally true, outliers were 'far more frequent than previously thought.'

Initial analysis was conducted using single-cell sequencing of haploid cells obtained from mice. However, the authors also demonstrated that these effects are conserved between individuals and across species, including cows and humans.

The scientists found that, for thousands of genes, their expression is influenced by the particular allele carried within the cell. These genes are known as 'genoinformative markers' and are associated with variation in features between individual sperm.

In particular, they found that these markers are enriched for testis-specific gene expression, implying response to an evolutionary pressure that may influence survival of individual sperm cells.

'[Variable expression] can lead to functional differences between sperm more often than previously thought', said Dr Friedman, 'allowing competition between sperm which violates Mendel's first law.'

The researchers predict that, beyond challenging the assumption of random allelic distribution, these findings will have significant impact on research surrounding fertility and reproductive medicine.

In addition to investigating a potential role in fertility treatments, future work will further study the mechanism by which sperm-level natural selection occurs, as well as the extent to which these findings challenge the assumptions surrounding classical Mendelian genetics.

The research was published in Science.

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