Fascinating role reversal: With the seahorses and pipefish the males are pregnant and even supply the unborn with nutrients via a kind of placenta in the stomach. Researchers have now been able to show another parallel to female pregnancies: In the case of pregnant males, an adapted immune system prevents the rejection of the fetus growing up in the body. The immunological concept of the whimsical fish could even be of interest to medicine, say the scientists.
Normally, the following applies to live-bearing animals: males only provide the semen, after mating the females then have a big belly and give birth to the young after pregnancy. There is, however, a curious exception to this rule: the males give birth to the offspring of seahorses and the pipefish related to them. In some species, the eggs only sit open on the fathers belly or they are covered by skin flaps there. But for certain species, an astonishingly complex system has developed that resembles the female pregnancy of other animals.
Close contact as with the placenta
With these seahorses, the female hands her eggs to the partner in a special fanny pack where they are fertilized by the sperm. Then they then nest there in supply structures that correspond to the placenta in live-bearing female animals. There the embryos are supplied with nutrients from the male body. This allows the little seahorses to mature until they are finally born by their father.
The researchers led by Olivia Roth from the GEOMAR Helmholtz Center for Ocean Research Kiel have now devoted themselves to researching an interesting aspect of this system: What prevents rejection reactions of the father’s tissue against the genetically different embryos in the placenta-like system? In the case of pregnancies of female mammals, the processes for preventing the rejection of the child’s foreign tissue are now quite well understood. But the researchers wondered how this could be regulated in male pregnancy.
Adapted immune system
In order to track down possible genetic mechanisms, Roth and her colleagues carried out genetic analyzes on twelve selected sea needle and sea horse species. They were species with different forms of male gestation: from the outer egg-bearing to the complete inner gestation with placenta-like structures.
The comparison of the genetic characteristics of these species showed that the development of male pregnancy was accompanied by a change in hereditary systems that are linked to the adaptive immune system. “It is precisely the parts of the immune system that have changed that are most relevant for distinguishing between oneself and others,” explains Roth. “We also found that similar genes are involved in male pregnancy that also play essential functions in the pregnancy of a female mammal. So it seems that in pregnancy, whether male or female, similar molecular mechanisms are used and similar genes are changed in their function for the development of pregnancy, ”summarizes Roth.
Medically relevant
Specifically, genetic changes in the so-called main histocompatibility complex (MHC) came into focus. A distinction is made between two types: MHC I and MHC II. These are hereditary systems of the vertebrates that code for proteins that are responsible for the detection of foreign tissues, which could be invading pathogens. They also play an important role in humans, for example in organ transplants and possible rejection reactions. In the pipefish with a highly developed pregnancy system, the researchers have now been able to detect a loss of several genes in the MHC-II signaling pathway. In the seahorses, however, it is modified by genetic changes.
In addition, it was found that during male pregnancy – analogous to that in mammals – genes of the MHC I are down-regulated, which also serves to tolerate the embryo. “This is something amazing,” emphasizes co-author Thorsten Reusch from GEOMAR. “Compared to an organ, it would be like finding a new group of fish that can survive without a liver,” said the scientist.
As he and his colleagues point out, the results are not only interesting from a biological point of view, but from a medical point of view. “The fact that seahorses and pipefish can survive in a sea full of microbes without an important part of their adaptive immune defense indicates a high degree of immunological flexibility in vertebrates. This in turn could promote our understanding of immunodeficiency disorders, ”the researchers write. It is interesting, for example, that the lost genes in the sea needle immune system are important for those pathways that are attacked by the AIDS pathogen. “This is why pipefish, which can survive without these critical immune system functions, could become an important model system for researching natural and disease-related immune system deficits,” Roth concludes.
Source: GEOMAR Helmholtz Center for Ocean Research Kiel, Facchartikel: PNAS, doi: 10.1073 / pnas.1916251117