Finding a sex partner is important and worth of a big sacrifice

"Deep-sea anglerfish life may question the very foundations of the immune system: in order to mate, they fuse their bodies and thus forego an important part of the immune system. 

Deep-sea anglerfish have a hard time finding a sexual partner in the lightless areas of the oceans. Some species solve this problem by having the tiny males permanently bound with the much larger females after finding the female. The connection goes so far that the two sexes even end up sharing a blood vessel system and the males are completely dependent on the females. Often even several males are attached to one female. Such a close connection is otherwise only known from Siamese twins, who are, however, genetically identical. This is not the case with male and female deep-sea anglerfish. They have different genomes. Therefore, with such an intimate connection, there should actually be a rejection reaction - similar to an unlucky organ transplant.

Why the rejection does not take place, and which adaptations in the immune system of the fish were necessary in order for such a connection to be established has occupied the researchers for a good hundred years. A firmly bonded couple was found for the first time in 1920. It was caught in the net of Icelandic fishermen. The anglerfish solution that a German an USA research team has now uncovered is a tangible surprise. Thomas Boehm from the Max Planck Institute for Immunobiology and Epigenetics in Freiburg and Theodore Pietsch from the University of Washington in Seattle and their colleagues show in the latest edition of “Science” that anglerfish completely forego the immune defense they have acquired during their life. They pay for their reproductive success in the vastness of the deep sea with a radical turning point in the immune system.

Instead of relying on a two-armed system of innate and acquired immune defenses, as all other vertebrates do, they only rely on their innate immune system. Deep-sea anglerfish that form a permanent link between the sexes no longer form tailor-made antibodies. They do without specific killer T cells with which infected cells are eliminated and foreign tissue attacked, and they no longer have broad MHC molecules that sound the alarm when they discover something foreign.

"In our, human, case, such a loss of immunological equipment would immediately lead to a fatal immune deficiency," comments Boehm on the results. What remains for the deep-sea anglerfish after this radical turning point is their innate system - a guard team made up of unspecific scavenger and immune cells that are not properly reformed and molecularly refined with every pathogen attack, as is the case with the acquired immune system.

Not all of the circa 160 deep-sea anglerfish species have made this radical cut in the immune system. Among the 168 known species there are also those that still have remnants of an adaptive immune system. They are those species in which the sexes combine only for the duration of the sexual act and then separate again. For example, these species lack specific antibodies. The caesura is most radical in the species in which several males combine simultaneously and permanently with a single female. The scientists were also able to show that these adaptations were developed several times and independently of one another during evolution. The temporary connection between the sexes and thus the less radical cut into the immune system seems to be the more ancient form.

But the results of Boehm and his colleagues are also of fundamental importance because they call into question a central dogma of immunology. This dogma states that the innate and acquired immune systems can no longer be decoupled without catastrophic consequences. The long, mutual co-evolution forbids such an unbundling, so the reasoning. “Because of this dogma, we initially had completely different explanations for the lack of rejection in mind,” says Boehm.

“For example, deep-sea anglerfish might smell which female suits them immunologically and which does not. They would then have compared the tissue characteristics with their noses, as is done with an organ transplant. But that is definitely not the case."

A glance at the timeline also shows how unexpected this unbundling actually is. Vertebrates formed 500 million years ago, deep-sea anglerfish - a hundred million years ago. After 400 million years of co-evolution of innate and acquired immune defenses, the deep-sea anglerfish have succeeded in separating the two systems and upgrading the innate immune system in such a way that they can cope with it on their own. "We just have to understand much better how that was possible," says Boehm, "because then we can perhaps try to help patients with immune deficiencies by strengthening their innate immune system."

The scientists base their conclusions on genome analyzes, i.e. on the presence or absence of the necessary genes. To do this, they examined the genetic make-up of 31 individuals belonging to different species of deep-sea anglerfish. The analyzes were made with frozen material because the scientists did not have access to fresh deep-sea anglerfish. That is one of the weaknesses of their work. In order to show that the innate immune system actually fully compensates for the loss of the other arm, they would have to show what equipment the immune cells of deep-sea anglerfish are equipped with. However, this is only possible with an RNA analysis in which the current templates for the synthesis of the immunologically active proteins are scrutinized. But for this you need fresh tissue material because RNA is not as stable as DNA. Boehm's team hopes that these investigations can be done in the near future."