Females select the 'right' sperm to fertilize their eggs when faced with the risk of being fertilized by wrong sperm from a different species and researchers recently set out to investigate salmon and trout, which fertilize externally in river water, because the two species occasionally hybridize in the wild.


Their results show that when eggs from each species are presented with either salmon or trout, they happily allow complete fertilization by either species' sperm. However, if eggs are given a simultaneous choice of both species' sperm, they clearly favor their own species' sperm. 

 The team then went on to investigate what mechanisms allow female eggs to encourage the right sperm to fertilize by examining two key components of reproduction in female fish – the egg, and the ovarian fluid that coats the egg. Ovarian fluid is a protein-rich solution that bathes the eggs and released at spawning – but little has been known about its function.

Scientists have revealed how females select the 'right' sperm to fertilize their eggs when faced with the risk of being fertilized by wrong sperm from a different species. Credit: University of East Anglia

Lead researcher Professor Matt Gage, from University of East Anglia's School of Biological Sciences, said, "The salmon-trout system is ideal for studying sperm-egg compatibilities because we are able to conduct controlled fertilization experiments and measure sperm behavior under conditions to which the gametes are naturally adapted. Although we found almost 100 per cent interfertility between salmon and trout sperm and eggs, when we mixed equal amounts of sperm from both species together, we found that sperm from their own species won 70 per cent of the fertilizations.

"Since we are conducting in vitro fertilizations without interference or control from males or females, this provides clear evidence that eggs favor the sperm of their own species, but only when given a choice. 







Citation: Sarah E. Yeates, Sian E. Diamond, Sigurd Einum, Brent C. Emerson, William V. Holt and Matthew J. G. Gage, 'CRYPTIC CHOICE OF CONSPECIFIC SPERM CONTROLLED BY THE IMPACT OF OVARIAN FLUID ON SPERM SWIMMING BEHAVIOR', Evolution 12 AUG 2013 DOI: 10.1111/evo.12208