Fish Can Control Which Sperm Fertilizes Their Eggs To Produce the Best Offspring [Study]

Female fish can pick which sperm to use to fertilize their eggs. They can reportedly do so to help them produce the best offspring.

Female Reproduction Fluid in Fish Reproduction

In a process known as a within-ejaculate cryptic female choice, female zebrafish use their female reproductive fluid (FRF), the fluid surrounding the eggs, to choose the highest quality sperm.

Clelia Gasparini, an evolutionary biologist at the Universita degli Studi di Padova in Italy and a co-author of the study, told Newsweek that it might provide the female potentially very big advantage.

Cryptic female choice is an evolutionary technique that allows the female to "choose" the male she wants to father her progeny by controlling the sperm that fertilizes her eggs. It is utilized by a variety of animals, from waterfowl to spiders.

The new study illustrates cryptic female choice, choosing the best sperm from a single male's ejaculate rather than from several males.

The authors of the research explain how they investigated the sperm that was drawn to the zebrafish's FRF and discovered that it was of higher quality than sperm that was not drawn to the FRF.

The fluid (FRF) can differentially alter and attract sperm from different males to generate more and better quality offspring, as they already know from prior studies in this area in other species, Gasparini noted.

To test the theory that the fluid can select the best sperm to improve fertilization outcomes by attracting more and better quality sperm, they tried for the first time in any species whether the fluid is also able to affect and attract differentially sperm of the same male by using only one male and one female).

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Sperm Drawn to The FRF Are More Viable And Have. Higher Integrity

Sperm drawn to the fluid was examined and compared to sperm moving randomly to the control solution. In this case, only water, as part of an equally balanced experimental design.

It was discovered that the sperm drawn to the FRF was more viable and had higher DNA integrity. More fertilized eggs are necessary, but so are more fertilized eggs with higher-quality sperm.

They have discovered that sperm drawn to the fluid has more intact DNA. This fluid capacity can, in theory, increase both the quantity and quality of the children generated.

Sperm with fragmented DNA can still fertilize eggs but has short and long[-term] impacts on the offspring's survival and fitness.

The author also noted that although this difference may result from the numerical advantage of FRF-selected sperm, the fertilization rate with FRF-selected sperm is numerically superior to the fertilization rate with non-selected sperm.

The new study and other research have shown that sperm vary in phenotype and genotype within an ejaculate. However, according to Gasparini, it is still necessary to investigate the underlying mechanical factors contributing to the observed outcomes.

They are still in the early stages of this research field; as an illustration, the relative male counterpart (the seminal fluid) has been extensively researched regarding sperm selection and sexual selection over many years. However, the female fluid has only recently been the subject of such research.

The study was published in the journal Biology Letters.