Scientists have decoded how females select the 'right' sperm to fertilise their eggs when faced with the risk of being fertilised by wrong sperm from a different species.
Researchers from the University of East Anglia investigated salmon and trout, which fertilise externally in river water.
The two species occasionally hybridise in the wild, but since hybrid offspring become reproductive dead-ends, females of both species are under selection to avoid hybrid fertilisations, and instead promote external fertilisation by their own species' sperm.
The study shows that when eggs from each species are presented with either salmon or trout, they happily allow complete fertilisation by either species' sperm.
However, if eggs are given a simultaneous choice of both species' sperm, they clearly favour their own species' sperm.
"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 fertilisations," lead researcher Prof Matt Gage, from UEA's School of Biological Sciences, said.
"Since we are conducting in vitro fertilisations without interference or control from males or females, this provides clear evidence that eggs favour the sperm of their own species, but only when given a choice," Gage said.
The team then investigated what mechanisms allow female eggs to encourage the right sperm to fertilise by examining two key components of reproduction in female fish - the egg, and the ovarian fluid that coats the egg.
"It is actually the ovarian fluid that controls which species' sperm wins the fertilisations, which was very unexpected. If we put salmon ovarian fluid onto salmon eggs, then salmon sperm win, but if we put trout ovarian fluid onto eggs from that same salmon female, trout sperm now win," researchers said.
"So what we're seeing is that ovarian fluid gives a specific chemical signal to the sperm of its own species, causing changes in the way their tails beat, so that they swim in a straighter trajectory, and therefore guided more effectively towards the site of fertilisation.
"These findings allow us to establish that females have indeed evolved mechanisms of 'cryptic choice' at the intimate level of the sperm and egg. The results also give us a valuable insight into why female salmon mate so promiscuously - typically being fertilised by eight, and up to 16, males in one nest, said researchers.
The study was published in the journal Evolution.
