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In many fish species, males are the primary if not sole parent. But male seahorses, sea dragons, and pipefish in the family Syngnathidae take fatherhood to a whole new level.

They have embraced the role of incubator, too. And in some species, the males actually experience a full blown pregnancy, complete with giving birth to live young. This feat has required some interesting modifications, including alterations to a really important part of their immune systems.

And learning more about how male pregnancy is even possible in these fishes could help us better understand our own immune systems and how to help when they falter— like when they're attacked by HIV. Now, you might think of pregnancies as pretty run-of-the-mill, considering they happen in our species all the time. But there are a number of challenges to overcome for a pregnancy to be successful.

Arguably one of the biggest is keeping the parent's immune system from rejecting the fetus, since from the body's perspective, a fetus is half-foreign. And that's basically what immune systems are for—getting rid of foreign material. The immune systems in vertebrates like us and fish are especially good at this because of something called adaptive immunity.

That's a whole arm of the immune system that protects us against specific pathogens. It also creates a memory of our experiences with them, to help protect against potential future encounters. But it can't really discriminate between “good” and “bad” encounters—so in theory, a fetus should trigger the same alarm as a deadly virus.

But most of the time, it doesn't. It wasn't until 2012 that researchers figured out why. Turns out that pregnant mammals' bodies tamp down on the activity of genes that control adaptive immunity to ensure they don't eject the partial-foreigners from their wombs.

And in a 2020 study, it was discovered that pregnant seahorses and pipefish have adopted a similar strategy, with a slightly dramatic twist. Now, before I reveal that twist, it's important to understand that some of the most important genes in the adaptive immune system are found in a region of the genome called the Major. Histocompatibility Complex, or MHC for short.

The MHC is made up of three classes of genes, which are super creatively named MHC I, II, and III. Each of these codes for important proteins, but MHC II proteins are especially important:. They go on the outside of many cells, and are responsible for sounding the alarm when an invader is spotted.

Specifically, they bind to proteins called CD4 receptors on other immune cells that come to help, passing along insider knowledge about the invader. That way, the body can launch an effective assault and remember the foe in case it ever dares to show its face again. Now, during a mammalian pregnancy, the MHC genes are down-regulated, taking the immune system off high alert so it doesn't attack the fetus.

The fetus is also surrounded by a layer of specialized cells called trophoblasts, which don't have MHC IIs, so they help shield the embryo from the parent's immune cells. Pregnant seahorses and pipefish also down-regulate MHC genes in general, but they don't stop there. They've completely ditched MHC II genes altogether.

Living without these genes is kind of like the immune system equivalent of living without a vital organ. Based on our current understanding of the adaptive immune system, without MHC II, these fish should not be able to remember pathogens and protect themselves against them. Which is kind of a head scratcher, considering these fish spend their entire lives in water where viruses are the most abundant thing around.

The discovery that they're able to survive in this environment without a vital part of their immune system was downright shocking to researchers. Though, they quickly realized that the adaptive immune system is probably more flexible than they'd originally thought and can still be effective without MHC II. This newfound flexibility could give us a better understanding of our own immune system and provide new ways of treating a variety of diseases.

But especially, it could help us tackle HIV infections. You see, human immunodeficiency virus replicates in cells that have CD4 receptors on their surface. And it replicates at significantly higher levels when those cell's CD4 receptors are attached to MHC II proteins.

So, basically, the virus has figured out a way to make one of the most important parts of our immune system work against us. But, if we really understood how seahorses can ditch MHC II genes without compromising overall immunity, maybe we could find a way to deprive HIV of what it needs most. And that's just one example.

There are other kinds of immune deficiencies that involve MHC II, so maybe, studying seahorses will help us develop better treatments for them. Plus, MHC II proteins are major players in a number of different diseases, especially autoimmune conditions. So, by understanding seahorse fatherhood better, we can better equip ourselves to tackle things like diabetes, celiac disease, and lupus.

Who knew all of that could come from studying a little fishy father? Thanks for watching this episode of SciShow! If you want to learn more awesome things about fish, you may want to check out our episode on three fish with unexpected abilities.

And before I go, I want to give a quick shout out to our patrons! We think you're all fintastic, and we really appreciate your continued support. If you're not a patron but want to learn more about the community, you can head on over to Patreon.com/SciShow. [♪ outro ].