Spice up your life!

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Cryptic female choice is the ability of a polyandrous mating female to favour - or hinder - the sperm of a particular male from fertilizing her ova, under sperm competition. It operates under varied mechanisms and occurs in both internal and external fertilizers. In fishes, a key process is driven by the effect of ovarian fluid - the fluid released with the ova at spawning - on the swimming characteristics of sperm. Due to the limited functioning life-span of fish gametes, researchers have previously had to use fresh ovarian fluid and sperm, thus limiting the geographical and temporal range of their studies, or freeze one, or the other, or both. Much work has been done on the cryo-preservation of fish sperm; however, it was unclear - until now - whether the freezing of ovarian fluid affects its sperm-influencing properties. This is where Craig Purchase and Anna Rooke of Memorial University of Newfoundland, Canada come in (Purchase & Rooke, 2020). They took ovarian fluid and semen from three species of salmonids - lake tout, brown trout and Atlantic salmon - and tested both frozen and fresh ovarian fluid on the swimming characteristics of sperm. They found quite clearly that freezing did not reduce the effect of ovarian fluid on the motility, swimming speed or linearity of conspecific sperm. This is good news for researchers; it allows them to build up collections of ovarian fluids from different females - the “spice-rack” of the title - in the knowledge that freezing them will not alter the very properties that they are investigating. Now, studies can include different populations, even different years, to investigate how ovarian fluids influence the sperm of different males, and thus the extent and importance of this effect in reproductive isolation, for example. It will also help in the ultimate identification of the proximate factor(s) involved, but this is still some way ahead.