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Female zebrafish shown to be able to choose which sperm to use to fertilize their eggs

A team of biologists at the University of Padova, in Italy, has found that female zebrafish use their female reproductive fluid (FRF) to choose which sperm to use for fertilizing their eggs. In their study, published in the journal Biology Letters, the group used a specially designed sperm selection tool to collect and test sperm samples.

Prior research has shown that females of many species have means to improve their chances of giving birth to the healthiest offspring with the highest chances of survival. One approach is called cryptic female choice, where females choose which male fertilizes her eggs by manipulating the sperm once it enters her body. In this new effort, the researchers found an example of such choice in female zebrafish.

Zebrafish are native to South Asia, where they mate and spawn in shallow water. Upon successful mating, females produce from 200 to 300 eggs over the course of a single spawning season. Prior research has shown that due to egg and sperm quality factors, just 58% to 78% of eggs are successfully fertilized. In this new effort, the research team found that FRF, a fluid produced by the females that surrounds the eggs as they develop, is a previously unknown factor that may play a role in successful fertilization.

To learn more about the role FRF plays in reproduction success in zebrafish, the researchers used a recently developed tool called a sperm selection chamber. It allows for collecting sperm samples from inside of a female fish after mating has occurred. After collection of multiple sperm samples, each was tested for their number, fertilizing ability, DNA integrity and viability—all factors that play a role in determining the success of offspring.

In making comparisons between sperm samples, the research team found that some of them were more attracted to the FRF than others. Those sperm that were the most attracted to the FRF were also the ones that were the best choice for producing the most viable offspring. Such sperm wound up producing the most offspring in their lab. Thus, by producing FRF of the right type, the female was able to choose which sperm were the best for use as a fertilization source.