There are few animals stranger than the fisherman, a species that has so much trouble finding a gear that when the male and female connect underwater, the males actually fuse their tissue with the females of a lifetime. After the merger, the two share a unique respiratory and digestive system.
Now scientists have discovered that the fisherman gets this sexual parasitism because it has lost a key part of its immune system, which allows two bodies to become one without rejecting the tissue. (Remember Symbiont’s Jionia Dax Deep Space Nine?)
All vertebrates, including humans, have two types of immune systems. The first is the innate system, which responds quickly to attacks by microscopic invaders with a variety of chemicals such as mucous physical barriers such as hair and skin and cells that cause diseases called macrophages. The second line of defense is an adaptive system that produces “killer”; T cells to attack the pathogen and antibodies tailored to fight specific bacteria or viruses. The two systems work together to fight infections and prevent disease.
But in a study published Thursday in the journal Science, researchers at the Max Planck Institute in Germany and the University of Washington found that many species of anglerfish (there are more than 300) have evolved over time to lose the genes that control their adaptive immune systems, which which means they can’t create antibodies and they don’t have them. T cells.
“Fishery fish have traded in their immune faculties, which we believe are essential, for this reproductive behavior,” says Thomas Boehm, a professor in the institute’s department of immunobiology and epigenetics in Freiburg, Germany, and lead author of the paper.
To reach this conclusion, Boehm and his collaborators spent the past six years conducting genetic testing on samples of fishery fish tissue taken from around the world. They tried to catch them by using deep-bottom trawls that collect specimens 1,000 feet below the surface, but because the fishery fish is rare and evasive, they were unable to collect any live specimens. Thus, to obtain enough tissue for their genetic analysis, the researchers searched the museum’s collections and other laboratories that had canned fish in preservatives, some of them for decades.
Within the fish family, there are several breeding methods. Females of some species fuse with a male; others fuse with multiple males; and yet another group has only a temporary merger. After shredding 31 tissue samples from 10 species, the team performed genetic testing and found that species that temporarily fuse with their mates do not have the genes responsible for the maturation of the antibodies. Species that create a permanent link to their mates had also lost a set of additional genes that are responsible for the assembly of T-cell receptors and antibody genes that are the foundation of the innate immune system in all vertebrates.
“It was intuitive to think that there is a certain genetic propensity to allow this to happen,” Boehm says of the fisherman’s unusual immune systems. “This is the first evidence that these animals have this inability to reject a part of themselves and allow these matings to take place.”