The evolution of gut defense

How animal guts evolved to defend themselves from microbial attack?

Red microbeads indicate the flow of food particles into the digestive tract of the tunicate Ciona intestinalis Type A
Red microbeads indicate the flow of food particles into the digestive tract of the tunicate Ciona intestinalis Type A. Credit: Keisuke Nakashima

Scientists at the Okinawa Institute of Science and Technology Graduate University (OIST) have mapped the transformative journey of how animal guts have advanced to shield themselves from microbial attack.

Most of the organisms produce an abundant biological material, Chitin. Chitin acts as a defensive barrier against potentially harmful microbes. But instead of using this chitin-lined gut, they protect themselves with a layer of mucous that bacteria are able to colonize but not penetrate.

Scientists noticed a group of animals that had both chitin and mucous coexisting in its gut: Tunicates. Tunicates are known simple animals that live by filtering food particles from seawater. Weel, Tunicates don’t have a spine, but they are closely related to the vertebrates, with a similar genomic background that makes them ideal for studying their evolution.

Dr. Keisuke Nakashima of OIST’s Marine Genomics Unit said, “We noticed that tunicates seemed to have an intermediate kind of gut lining that could show that mammalian and invertebrate gut linings shared an evolutionary link.”

From left to right: Ms. Sakura Kikuchi, Dr. Keisuke Nakashima & Prof. Noriyuki Satoh (leader of OIST’s Marine Genomics Unit)
From left to right: Ms. Sakura Kikuchi, Dr. Keisuke Nakashima & Prof. Noriyuki Satoh (leader of OIST’s Marine Genomics Unit)
Credit:
OIST

During the study on Ciona intestinalis Type A, scientists observed the surface of their rudimentary gut, composed of hair-like cilia behind a barrier of nanoscopic fibers made almost entirely of chitin. This mesh of chitin nanofibers is embedded in a surface matrix, which happens to have gel-forming mucin as a major component, which also happens to be a building block of the mucous layers found in the mammalian gut.

Some tunicates died while chemically preventing some of them from synthesizing chitin, But, some of them survived due to antibiotics diagnosis. This experiment suggests that chitin layer had an antimicrobial effect, preventing infection by physically blocking pathogenic bacteria and other harmful microbes from reaching the gut surface.

In any case, the presence of mucus in a similar space was fundamentally the same as more intricate vertebrates. It looked as though the tunicates were a sort of missing connection between spineless creatures like bugs that have just chitin as a microbial guard, and mammals that use a mucous layer as a kind of decoy for microorganisms to colonize, leaving helpless cell walls to play out their imperative capacities untouched.

Running several years’ worth of comparative analyses on a variety of chordates, Dr. Nakashima’s team was able to trace an evolutionary path from one model of the gut to the other, cataloging intermediate gut structures that utilized varying quantities of chitin and mucous along the way. It’s an unprecedented look into the evolution of one of the most critical body structures that are keeping us all safe from microbes to this very day.

The study is published in the journal Nature Communications.