"Older sperm" produces healthier offspring

Spermatozoa differ markedly in their swimming performance. Our latest study shows that this difference not only determines their success in fertilizing an egg, but can have direct consequences on the resulting offspring. We have found that long-lived sperm selection in zebrafish ejaculate results in offspring that live longer and have more offspring than siblings from short-lived sperm of the same ejaculate.

For our study, we collected ejaculate from 26 male zebrafish and divided them into two batches. In one batch, we activated the sperm with water and added them immediately to half of a female's eggs, which allowed all the sperm to move and fertilize an egg. In the second batch, we activated the sperm but waited 25 seconds until half of the sperm could not move, allowing only the sperm of longer duration to fertilize eggs.

We reared the resulting offspring up to adulthood and measured their lifespan as well as the number and survival of offspring produced throughout life. This design allowed us to directly compare full siblings from shorter-lived and longer-lived sperms of the same man. "Older" sperm offspring not only lived much longer, but also produced larger and healthier offspring.

Interestingly, the effects differed between male and female offspring. Men from sperm who lived longer had healthier and healthier offspring than men from sperm with shorter lives throughout their lives.

The effect of sperm selection on women was more variable. Females from long-lived spermatozoa produced more offspring throughout life, but offspring survival rates decreased with age. Females spawned by shorter-lived spermatozoa produce fewer offspring throughout life, but the health of the offspring that they produce later is better than the offspring that they produce early.

Not all sperm are equal

Our research shows that selection, even among perfectly fertile spermatozoa in the ejaculate of a man, has direct consequences on the health of the resulting offspring. Our results challenge the idea that all sperm that can fertilize an egg are equivalent to producing healthy offspring.

Part of the variation in sperm characteristics in an ejaculate can be attributed to "errors" in sperm production. But the fact that the variation in swimming time between sperm cells in an ejaculate has effects over several generations suggests that other mechanisms are involved.

The spermatozoa contained in an ejaculate differ not only in swimming performance, but also in the combination of genes they carry. On average, two sperm produced by one man share 50% of their genes. This is due to the fact that the duplicate gene set of the male is distributed more or less randomly between the spermatozoa. In a previous study from our laboratory, we showed that sperm genes selected for longevity differ. This discovery establishes a direct link between the sperm phenotype (its appearance and behavior) and its genes and supports the idea that genetic variation between spermatozoa contributes to the variation in swimming duration.

The role of sperm genes in determining sperm swimming performance provides a plausible explanation for the differences we observed in offspring later in life. Our selection is not limited to older sperm, but to enduring spermatozoa.

barter

Biologists believe that, during aging, there is a trade-off between the number of offspring and the length of life, that is, the more offspring a creature has, the shorter the life of the offspring and vice versa. Our results suggest the opposite, that longer offspring also produce more offspring. Although this requires further research, it seems that our selection on sperm swim time results in the selection of better sperm. While waiting for half of the sperm to die, we get rid of the worst sperm.

If the benefits of long-lived sperm are so striking, why is there life-saving sperm? The conditions during fertilization vary and factors such as the delay between sperm and egg release or water temperature can promote different characteristics of the sperm. Spermatozoa with a longer life span will benefit if the female delays spawning for a few seconds or increases the distance between sperm and eggs.

We still know little about the processes that take place between the end of sperm production and the moment of fusion between sperm and egg. Our results highlight the need for further studies on this topic as they may have implications for fertilizer technologies used in fertility clinics. These technologies often ignore many, if not all, possible stages of selection before fertilization. Understanding which traits of sperm determine the survival and health of offspring can help improve the success rate of existing methods.