Female fish can pick which sperm they want to father their babies

In a bizarre twist of evolution, some fish have been found to be able to control which sperm fertilizes their eggs in order to have the best offspring.

According to a study published in the journal Biology Letters on June 20, female zebrafish can select the best quality sperm using their female reproduction fluid (FRF)—the fluid that surrounds the eggs—in a process called within-ejaculate cryptic female choice.

“[This may provide a] potentially very large [advantage to the female],” Clelia Gasparini, an evolutionary biologist at the Universita degli Studi di Padova in Italy and co-author of the paper, told Newsweek.

Cryptic female choice is a strategy used by a variety of species, ranging from spiders to waterfowl, that allows the female to “choose” which male she wants to father her offspring by manipulating the sperm that fertilizes her eggs. This paper reveals an example of within-ejaculate cryptic female choice, which involves the female selecting the best sperm from a single male’s ejaculate, rather than that of multiple males.

In the paper, the authors describe how they studied how sperm was attracted by the zebrafish’s FRF, and found that they were of better quality than sperm not attracted by the FRF.

“We already know from previous research in this in other species (even it is still a very novel research field), this fluid (FRF) is able to differentially affect and attract sperm from different males to produce more and better quality offspring,” Gasparini said.

“Here, for the first time in any species, we have tested if the fluid is also able to affect and attract differentially sperm of the same male (so using only one male and one female), to test the hypothesis that this fluid can select the best sperm to improve fertilization outcome by attracting more and better quality sperm. The experimental design was fully balanced, so that sperm attracted by the fluid were analyzed and compared to sperm randomly swimming to the control solution (in this case only water).”

The sperm that was attracted by the FRF was found to be more viable and had higher DNA integrity.

“[It is] crucial to have more eggs fertilized, but also to have eggs fertilized by sperm with a better quality … we have found that sperm attracted by the fluid has a higher DNA integrity. Given that sperm with fragmented DNA can still fertilize eggs, but [have] short and long[-term] effects on the offspring’s survival and fitness, this fluid ability can, in principle, improve not only the quantity but also the quality of the offspring produced,” Gasparini said.

“Also, the fertilization rate is significantly higher with sperm FRF-selected compared to fertilization rate with non-selected sperm, albeit this difference can be due to the numerical advantage of FRF-selected sperm,” the authors wrote in the paper.

Exactly how this process—selecting the genetically-superior sperm inside the reproductive tract—works mechanistically is still unknown.

“We know that within an ejaculate, sperm differ in phenotype and genotype (our and other research), but what is really the underlying mechanical cause that translates into the results we saw, needs to be tested,” Gasparini said. “We are still in the infancy of this research field, just to give you an example, the relative male counterpart (the seminal fluid) has been studied for decades in terms of sperm selection and sexual selection, this female fluid only in the last few years.”

Often, strategies like this in females can trigger the males of the species to evolve some degree of counter-attack to maximize his reproductive success, and the female to again evolve another defense in response. One example of this conflict-driven coevolution includes many species of duck, which have evolved corkscrew-shaped vaginal cavities to prevent forced entry of the male’s corkscrew penises, and false pockets in which to store the sperm from unwanted sexual partners to prevent him fathering her young.

So, could male zebrafish evolve to combat their females’ choosiness?

“In the point of view of the male as an organism probably not, as the outcome is better (more and better offspring) for both the male and the female (keep in mind that here there are not other males involved), but we cannot exclude within-ejaculate competition, that is another interesting emerging field,” Gasparini said. “We’ll see where this parallel research will lead in the future.”

The authors hope to further research what the impacts of this female choice will be on the offspring themselves, and see if offspring born from the female’s chosen sperm are better “quality” than those born from those she did not choose.

“Next step we are exploring in my research group is what are the short and long consequences for the offspring (in this work we stopped at the stage of assessing fertilization rate) of this intra-ejaculate selection, and also whether this selection can also be based on compatibility between the partners,” Gasparini said.

“Last, we rely increasingly more and more as a society (in humans and other species) on assisted reproductive techniques to produce babies, but we still lack an effective way to screen sperm before being used for in vitro fertilization. This may pave the way one day to a better, easy and effective way to select sub-population of sperm and improve fertilization rate and offspring fitness.”

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