How female fish choose the 'right' sperm

Washington, August 17 : A new study has revealed how females choose the right sperm to fertilize their eggs when they are faced with the risk of getting impregnated by wrong sperm from a different species.

Researchers investigated salmon and trout, which fertilize externally in river water.

The two species occasionally hybridize in the wild, but since hybrid offspring become reproductive dead-ends, females of both species are under selection to avoid hybrid fertilizations, and instead promote external fertilization by their own species' sperm.

The findings 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 favour their own species' sperm.

Lead researcher Prof Matt Gage, from UEA's School of Biological Sciences, said that 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.

Gage asserted that although they found almost 100 percent interfertility between salmon and trout sperm and eggs, when they mixed equal amounts of sperm from both species together, they found that sperm from their own species won 70 per cent of the fertilizations.

He said that since they are conducting in vitro fertilizations 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.

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.

Prof Gage said that they ran further sperm competition trials but this time they rinsed eggs of their ovarian fluid and then added back either their own fluid, or that from the other species.

He said that they found that the egg itself plays no significant role in promoting fertilization precedence by their own species' sperm and instead it is actually the ovarian fluid that controls which species' sperm wins the fertilizations, which was unexpected.

Gage said that if salmon ovarian fluid is put 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.

The researchers then used Video Tracking Analysis to analyse how salmon and trout sperm behave in ovarian fluid.

Gage said that they found that activating sperm in ovarian fluid makes them live about twice as long as in river water.

He said that importantly, both species' sperm also switch from swimming in tight elliptical circles in river water, to swimming in straightened trajectories in ovarian fluid

Gage added that this behaviour allows sperm to navigate towards the egg by following a chemical cue.

The findings have been published in the journal Evolution.

--ANI (Posted on 17-08-2013)

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