Study Shows how Female Trouts and Salmons Find the Right Sperm

A new study has found how females select the right sperm to fertilize their eggs. The study was conducted on salmon and trout. Both the fish fertilize their eggs externally and have a high risk of hybridizing in the wild.

Researchers from University of East Anglia and colleagues found that salmon and trout allow hybridization when presented with sperms of either species. However, when given a choice, both fish fertilize only with the sperms of their own species.

"The salmon-trout system is ideal for studying sperm-egg compatibilities because we are able to conduct controlled fertilization experiments and measure sperm behaviour under conditions to which the gametes are naturally adapted. Although we found almost 100% 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," said Lead researcher Prof Matt Gage, from UEA's School of Biological Sciences.

The study strove to find the mechanism that allows the egg to identify the right sperm. Scientists looked at two main components of female reproduction system- the egg and the ovarian fluid. The ovarian fluid is rich in protein and coats the egg.

In this part of experiment, researchers removed the ovarian fluid from the eggs and then coated the eggs with either the ovarian fluid from other species or their own species.

"Remarkably, we found that the egg itself plays no significant role in promoting fertilization precedence by their own species' sperm. Instead, it is actually the ovarian fluid that controls which species' sperm wins the fertilizations, 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," Gage said in a news release.

The team also used Video Tracking Analysis to see how trout and salmon sperm behave in ovarian fluid.

Experts found that sperm in contact with the ovarian fluid lives twice as long as sperm swimming in the water. Also, chemicals from the fluid change the way the sperm travels- from tight elliptical circles in river water to swimming in straightened trajectories in the ovarian fluid. This shows that the ovarian fluid sends chemical cues to the sperms of its species.

Researchers also allowed sperms to migrate across a membrane that had tiny pores. The pores mimicked an entrance into the egg. They found that sperms were more likely to cross the membrane to reach the ovarian fluid of their own species than the ovarian fluid of other species, showing that there is a specific mechanism that allows sperms to differentiate between own and other species.

The study is published in the journal Evolution.

The salmon-trout hybrid is sterile. Other researchers have used this trait to increase the size of the fish. Hybrid salmon-trout fish have an extra chromosome that allows the shift of energy, which could be used for reproduction in fertile fish.